Guidance on residential premise plumbing pathogens and water quality

Document for public consultation

Consultation period ends: November 24, 2026

On this page

• Purpose of consultation
• Drinking water safety in Canada
• Background on guidance document
• Purpose of this document and intended audience
• How to read this document
• Executive summary
• Part A. Guidance on residential premise plumbing pathogens and water quality: For public consultation
  • A.1 Introduction
  • A.2 Causes of deterioration in premise plumbing
  • A.3 Opportunistic premise plumbing pathogens
  • A.4 Chemical contaminants
  • A.5 Impacts of climate change
  • A.6 Management strategies
  • A.7 Monitoring
• Part B. Supporting information
  • B. 1 Premise plumbing
  • B. 2 Causes of water quality deterioration
  • B. 3 Opportunistic premise plumbing pathogens
  • B. 4 Chemical contaminants
  • B. 5 Impact of climate change
  • B. 6 Management strategies
  • B. 7 Monitoring
  • B. 8 International considerations
• Part C. References and abbreviations
  • C. 1 References
  • C. 2 Abbreviations
• Part D. Summaries of studies on the occurrence of opportunistic premise plumbing pathogens in residential dwellings
• Part E. Summary of international recommendations for residential buildings: hot water temperature and Legionella action levels

Purpose of consultation

This document has been developed with the intent to provide regulatory authorities and decision-makers with guidance on residential premise plumbing pathogens and water quality in Canadian drinking water supplies.

The document is being made available for a 60-day public consultation period. The purpose of this comment period is to solicit comments on the proposed guidance document. Comments are appreciated, with accompanying rationale, where required. Comments for this document can be sent to Health Canada via email at water-consultations-eau@hc-sc.gc.ca. All comments must be received before November 24, 2026.

Comments received as part of this comment period will be shared with members of the Federal-Provincial-Territorial Committee on Drinking Water (CDW), along with the name and affiliation of their author. Authors who do not want their name and affiliation to be shared with CDW should provide a statement to this effect along with their comments.

It should be noted that this guidance document on residential premise plumbing pathogens and water quality will be revised following evaluation of comments received, and a final guidance document will be posted. This document should be considered as a draft for comment only.

Drinking water safety in Canada

Canadian drinking water supplies are generally of excellent quality. However, water in nature is never “pure”. It picks up some of everything it comes into contact with, including minerals, silt, vegetation, fertilizers and agricultural run-off. While most of these substances are harmless, some may pose a human health risk. 

Provincial, territorial, municipal, First Nations and federal governments share the responsibility for making sure that drinking water supplies are safe. The federal government has or shares responsibility for ensuring the safety of drinking water supplies on federal lands, in federal facilities and in First Nations communities. Provincial and territorial governments are responsible for setting drinking water standards and regulations, monitoring performance of municipal water systems and enforcing compliance. Municipalities are responsible for the delivery of drinking water from public water supplies. This includes the day-to-day operation and maintenance of public drinking water systems, in accordance with provincial or territorial standards. Private property owners who draw drinking water from a source on their property (usually a groundwater well) are responsible for their own water safety.

Health Canada reviews the available scientific literature and assesses health risks associated with specific substances in drinking water. As part of this assessment, it evaluates new studies and approaches and takes into consideration the methodological limitations for the detection of substances in drinking water. These assessments serve as the basis for Health Canada’s Guidelines for Canadian Drinking Water Quality (GCDWQ), which are developed in partnership with the provinces, territories and other federal departments through the CDW. The Guidelines set out the basic parameters that every water system should strive to achieve in order to provide safe drinking water. Every jurisdiction in Canada uses these guidelines to establish drinking water quality requirements based on their need and context. 

Background on guidance document

While the main responsibility of the CDW is to collaborate with Health Canada to develop and update the GCDWQ, its role has evolved over the years; Health Canada and the CDW also develop guidance documents. Guidance documents provide advice on issues related to drinking water quality for parameters that do not require a formal GCDWQ.

There are two instances when Health Canada in collaboration with the CDW may choose to develop guidance documents. The first is to provide operational or management guidance related to specific drinking water-related issues (such as boil water advisories or corrosion control), in which case the document would provide only limited scientific information or a health risk assessment.

The second instance is to make risk assessment information available when a guideline is not deemed necessary. The GCDWQ are developed specifically for parameters that meet all of the following criteria:

1. exposure to the parameter could lead to adverse health effects;
2. the parameter is frequently detected or could be expected to be found in a large number of drinking water supplies throughout Canada; and
3. the parameter is detected, or could be expected to be detected, at a level that is of possible health significance.

If a parameter of interest does not meet all these criteria, Health Canada in collaboration with the CDW may choose not to establish a numerical guideline or develop a guideline technical document. In that case, a guidance document may be developed.

Guidance documents undergo a similar process as guideline technical documents, including public consultations through the Health Canada website. They are offered as information for drinking water authorities and, in some cases, to help provide guidance in spill or other emergency situations.

Purpose of this document and intended audience

This document addresses the issue of water quality deterioration in plumbing within buildings, referred to as premise plumbing, and possible health impacts on consumers. The document focuses on residential premise plumbing, that is, plumbing in places of residence, also referred to as dwellings, which are supplied by municipal and small-scale drinking water systems and private wells. This document does not provide guidance on premise plumbing for hospitals, long‑term care facilities, retirement residences, or office buildings. It also does not cover domestic water reuse systems.

The document provides an overview of:

1. causes of water quality deterioration in premise plumbing, with particular emphasis on opportunistic premise plumbing pathogens (OPPPs) and lead;
2. impacts of deterioration;
3. management strategies; and
4. important aspects of monitoring within drinking water quality management approaches and in building water management programs.

The document pertains to routine dwelling premise plumbing use and conditions.

The document builds on information provided in the Guidance on Monitoring the Biological Stability of Drinking Water in Distribution Systems (Health Canada 2022a).

The document is intended for responsible authorities, such as municipalities and water system operators, as well as building owners and managers. 

How to read this document

Part A of this document is a summary of Part B, outlining key information, including guidance on water quality monitoring and possible management strategies. Part B provides the scientific and technical information, including citations, to support this guidance. Part C provides the references and the list of acronyms, and Part D and Part E provide supplementary information.

Some terms are used repeatedly throughout this document. It is important that the reader familiarize themselves with their meaning. While they are defined in Part B, they are also described in this section, to facilitate the understanding of Part A. 

Deterioration in water quality:

changes in water quality that support microbial (re)growth, development of biofilm, and/or precipitation, leaching or release of inorganics (typically metals such as arsenic, copper, iron and/or lead) in the distributed water. 

Drinking water supplier:

entity that supplies drinking water to a community and is responsible for operating and managing a drinking water supply/system. Most people in Canada get their drinking water from public water systems, in which case the drinking water supplier is the municipality.

Dwelling:

building where a person or people live or reside. Dwellings can vary significantly in their characteristics and configuration. They include private buildings, such as detached houses, and collective buildings, such as multi-storey apartment complexes (including those that house older adults).

Fitting:

device in a plumbing system that is designed to control and guide the flow of water, such as faucets, shower heads, water heaters, shutoff valves, shower valves, and tub spouts. Fittings may also be (incorrectly) described as “fixtures” (this is especially common in the United States). 

Fixture:

a plumbing device that receives water, waste matter, or both, such as toilets, sinks, bathtubs, and shower bases. 

Opportunistic premise plumbing pathogens (OPPPs):

microorganisms naturally present in soils and aquatic environments that have adapted to grow and persist in built environments, particularly, premise plumbing. They typically do not cause illness in healthy individuals; infections occur predominantly in susceptible individuals -- thus, they are referred to as “opportunistic” pathogens. 

Premise plumbing:

plumbing within buildings. For buildings serviced by a municipal drinking water supply, it is the piping within the property line of the building that is connected from the drinking water distribution system via the service line. For those relying on a private well, it is the portion beyond the service line, leading into the building. Premise plumbing consists of pipes, tanks, fittings, fixtures and other devices. The building owner or homeowner or well owner is responsible for their premise plumbing. Also referred to as building plumbing, residential plumbing, plumbing in dwellings, and plumbing systems of residences.

Resident:

person who occupies a dwelling.

Service line:

pipe connecting the drinking water distribution system to premise plumbing. In the case of private wells, the pipe connecting the well to the premise plumbing is called a water or supply line. It is possible for a service line to serve more than one dwelling or for a multi-unit dwelling to be supplied by more than one service line (typically branched at the curb stop). 

Executive summary

After leaving a treatment plant, water enters the drinking water distribution system, where it may encounter conditions that diminish its quality. Likewise, when water from the distribution system enters buildings, via premise plumbing, it is exposed to an environment that can further compromise its quality. If this water degradation is significant, it may pose health risks to consumers.

Health Canada completed its review of residential premise plumbing pathogens and water quality in Canadian drinking water supplies. This guidance document was prepared in collaboration with the CDW and assesses all available information on this topic in the context of drinking water quality and safety.

The unique characteristics and operating conditions of premise plumbing, such as increased temperatures, increased time since treatment (also known as high water age) and lack of disinfectant residual, can create conditions ideal for microbial growth and/or leaching of chemical contaminants, including metals. OPPPs, such as Legionella, are environmental microorganisms that have adapted to grow in premise plumbing, including that of residences. These microorganisms do not typically cause illness in healthy individuals; however, they pose a significant risk to sensitive populations possessing one or more specific risk factors (for example, older age, vulnerable immune systems, underlying disease). Like OPPPs, metals leached from premise plumbing materials can pose a significant health risk to consumers. Lead is one of the primary metals of concern in drinking water since no level of exposure is considered safe.

Mitigating premise plumbing risks is challenging because the responsibility for premise plumbing lies with the building owner rather than the drinking water supplier. In the context of health risks associated with premise plumbing conditions, risk management requires a partnership. This partnership includes drinking water suppliers striving to manage distribution system conditions to limit OPPPs and chemical contaminants from entering residential plumbing, and building owners and managers of residential premise plumbing taking steps to prevent conditions favouring OPPP growth and spread, accumulation, and chemical leaching.

The intent of this document is to outline actions that drinking water suppliers, building owners and managers, and homeowners and tenants can take to reduce the risks of consumer exposure to OPPPs and lead from residential premise plumbing systems. Actions can include optimizing treatments, maintaining good water quality during distribution and storage and limiting exposures in dwellings through various maintenance and cleaning actions.

Part A. Guidance on residential premise plumbing pathogens and water quality: For public consultation

A.1 Introduction

Drinking water suppliers aim to provide consumers with safe and high-quality water by protecting source waters, using the most appropriate water treatment processes, and conducting routine water quality testing. However, once water leaves a treatment plant, it enters the drinking water distribution system (DWDS), where it is exposed to conditions that can lead to deterioration of water quality. This may lead to increased levels of microorganisms and metals in the distributed water. Similarly, when water from the distribution system, or a well, enters plumbing within buildings, referred to as premise plumbing, it is subjected to an environment that can result in further water quality degradation. When this degradation is significant, it can lead to issues that impact human health. Management of these issues is complicated by the fact that premise plumbing is the responsibility of the building owner rather than the drinking water supplier. Thus, a comprehensive approach to risk control requires:

1. drinking water suppliers to understand and manage conditions in the distribution system to minimize the frequency of detection and concentration of opportunistic premise plumbing pathogens (OPPPs) and chemical contaminants entering residential dwelling plumbing; and
2. owners and managers of residential premise plumbing to prevent conditions that promote: a) OPPP growth or accumulation in biofilms and subsequent transmission to consumers, and b) leaching and accumulation of metals.

This represents a fundamental shift in the current drinking water risk management paradigm, which is primarily focused on enteric pathogens (see A.6 Management strategies).

A.2 Causes of deterioration in premise plumbing

Many of the causes of worsening water quality in the DWDS also apply to premise plumbing. However, the unique characteristics of premise plumbing (List 1) can exacerbate water quality degradation.

A.3 Opportunistic premise plumbing pathogens

Unlike pathogens historically of concern to drinking water suppliers—that is, enteric pathogens—OPPPs are microorganisms that are naturally present in soils and aquatic environments. They have adapted to grow and persist in built environments, such as DWDSs and premise plumbing. In premise plumbing in particular, OPPPs establish themselves, grow and become part of the native drinking water microbiome, where they are considered “normal inhabitants”. Their survival and persistence are due to certain common features, including their ability to: adhere to surfaces and form biofilms; grow in free-living amoeba (FLA); resist disinfectants; tolerate a wide range of temperatures; and grow at low oxygen and organic carbon concentrations.

Although a number of OPPPs have been identified, members of the bacterial groups Legionella, Mycobacterium and Pseudomonas are of greatest public health significance in North America. Some studies have linked human illness from OPPPs to exposure via premise plumbing. In particular, L. pneumophila, Mycobacterium avium complex, and P. aeruginosa are most commonly associated with human illness. Acanthamoeba spp. and Naegleria fowleri are also increasingly recognized for their contribution to illness related to premise plumbing exposures, not only as direct pathogens, but also as hosts for other OPPPs.

OPPPs are primarily found in biofilms in areas within building plumbing systems where water has poor flow or where temperatures within hot and cold water systems fall within their optimal growth range, that is, 25 to 45 °C. Plumbing fittings are ideal locations for biofilm formation, and OPPPs have been detected in a variety of residential plumbing fittings. Showerheads and their hoses, in particular, have been shown to harbour biofilms that enrich OPPPs. Also, unlike enteric pathogens, OPPPs are largely transmitted by inhalation or aspiration of aerosols or through direct contact with the skin, eyes or nasal passages, rather than through ingestion. Inhalation of contaminated aerosols is thought to be the most common way that people get infected.

Water is the most significant pathway of OPPP exposure. As OPPPs are normal inhabitants of natural waters, DWDSs and premise plumbing, humans are regularly exposed to OPPPs in the community. Residential plumbing systems are suspected to be a substantial source of OPPP exposure. Legionella spp., nontuberculous mycobacteria (NTM), P. aeruginosa and Acanthamoeba spp. have all been detected in the premise plumbing of residences located in temperate climate zones in North America. Moreover, L. pneumophila and M. avium have been detected by molecular and culture methods in residences supplied by municipal drinking supplies and by molecular methods in homes supplied by private wells. Reported rates of detection and concentrations have been low in general and lower for homes on private wells. Nevertheless, certain characteristics of residential plumbing—such as growth-permissive water temperatures, stagnant water, or low disinfectant residuals—can lead to increased detection and concentrations.

Despite their presence in DWDSs and premise plumbing, OPPPs typically do not cause illness in healthy individuals. They are called “opportunistic” pathogens since they predominantly cause disease in sensitive populations possessing one or more specific risk factors—for example, older age, vulnerable immune systems, underlying disease—that make them more susceptible to infection. As populations age, the number of people that can be considered at risk to OPPPs is expected to increase.

Certain events must occur, and specific conditions must be present, in order for OPPPs to cause illness. This series of events and conditions is referred to as the chain of transmission. It includes: the presence of OPPPs in premise plumbing, and their persistence and growth due to conditions therein (see List 1); the capacity of premise plumbing components to act as a source and dispersal mechanism for OPPPs; a pathway for entry into the human body; the ability of OPPPs to reach target areas in the body and initiate infection; and the individual susceptible to infection.

The health effects caused by OPPPs are diverse and range from respiratory illness to infections of the eye, skin, central nervous system or gastrointestinal tract. In severe cases, illnesses caused by OPPPs can be fatal. Molecular matches have been demonstrated between isolates originating from plumbing systems in single-family homes and apartments, and clinical isolates from cases of disease caused by L. pneumophila, NTM and Acanthamoeba spp. It is difficult to quantify the public heath risk associated with OPPPs in residential plumbing systems. OPPP-related illnesses are sporadic with numerous possible origins and not captured in national surveillance programs, with the exception of those caused by Legionella spp. Existing data from surveillance and literature show that the incidence of illnesses caused by OPPPs is increasing in Canada.

A.4 Chemical contaminants

Consumers can be exposed to inorganic and organic chemical contaminants in drinking water and one of the potential sources is leaching from the DWDS, including premise plumbing. Lead is one of the main chemical contaminants of concern in drinking water since no level of exposure is considered to be safe. A range of typical concentrations to which people living in Canada are exposed to through drinking water is hard to obtain for specific contaminants as the concentrations are highly variable due to several factors.

A.5 Impacts of climate change

Changes in precipitation and temperature are some of the expected impacts of climate change. An increase in the occurrence of extreme events, such as storms, floods, droughts and wildfires, is also expected. These impacts may create conditions that favour OPPPs’ survival, growth and transport in natural and built environments. In turn, this may result in more human exposure to OPPPs through premise plumbing. Changes to key physico-chemical characteristics of drinking water are also expected, which may lead to increased concentrations of inorganic contaminants in premise plumbing. The elements expected to have the most significant impact are climate warming, which will make it more difficult to cool buildings and keep water temperatures below 25 °C, and the increased adoption of water- and energy-efficient behaviours in buildings, which can have unintended consequences on water quality.

A.6 Management strategies

As noted earlier, OPPPs have adapted to persist in built environments, including DWDSs, even those that are well-maintained. In the distribution system, they are typically at concentrations that are not considered a concern for public health. However, water from the DWDS can (sporadically) seed premise plumbing with OPPPs. Once in premise plumbing, OPPPs can amplify to a level that can cause significant health risks if conditions are suitable for their growth. Because drinking water risks are affected by conditions in premise plumbing, where drinking water suppliers have little control, managing risks from OPPPs and lead requires the combined efforts of drinking water suppliers, building owners/managers and homeowners/residents. For their part, drinking water suppliers should strive to maintain the distribution system in a way that reduces the occurrence and prevalence of OPPPs entering premise plumbing. At the same time, homeowners and property managers must take steps to avoid creating conditions that support the growth or buildup of OPPPs in biofilms, which could lead to their spread to residents.

Drinking water suppliers can minimize the opportunity for OPPPs to be retained and grow in the distribution system by employing best practices (see List 2), including optimizing treatment

and producing biologically-stable water (Health Canada 2022b).

Building owners or managers, homeowners and tenants can limit exposures through various maintenance and cleaning actions (see List 2). These actions are intended to keep water temperatures outside the growth range for OPPPs, limit stagnation, and minimize biofilm growth at fittings (especially showers), which are important sources of transmission.

A.7 Monitoring

Drinking water suppliers should have comprehensive site-specific monitoring programs in place that include operational and verification monitoring. These are essential in ensuring that water quality goals are consistently being met for microbial parameters. Monitoring plans should be based on an assessment of hazards that are specific to each system and include parameters, frequencies and locations where monitoring should be conducted.

As the management of biological activity and biofilms in the distribution system is an important focus of a drinking water supplier’s approach to OPPP control, it is important that routine monitoring programs include measures for assessing drinking water biostability (Health Canada 2022a). There are no guidelines or regulations that establish concentration or monitoring requirements for Legionella organisms in treated drinking water. Research indicates that the bacteria is seldom detected in well-managed DWDSs at concentrations estimated to pose a meaningful public health risk. There are also resource demands that arise with testing. With no clear benchmark against which to assess monitoring results and limited available resources for water providers, it is not clear that routine monitoring of finished drinking water for L. pneumophila is an effective option under an overall approach to providing safe drinking water and protecting public health. Routine testing for L. pneumophila is not recommended for drinking water suppliers. Rather than investing in frequent testing, water suppliers are encouraged to concentrate on managing their water systems effectively to reduce potential risks. Verification monitoring can be conducted by analyzing operational and treatment information with supporting water quality data. A drinking water supplier-specific Legionella hazardous conditions inventory and risk assessment can help define monitoring activities and inspection points for assessing whether controls are working as intended. Temperature and disinfectant residual are important parameters for identifying where conditions exist in municipal systems that can favour amplification and increase the risk of Legionella and other OPPPs. In general, bacterial growth is more likely in DWDSs when water temperatures increase above 15 °C; thus, drinking water suppliers may consider this temperature as a trigger to increase sampling frequency for biostability parameters.

Monitoring for lead should occur at the tap since it is representative of exposure and captures the sources of exposure, such as the lead service line and plumbing system. Guidance on lead monitoring, assessing corrosion, and triggers for implementing corrosion control measures for distribution systems in residential settings are available from Health Canada (Health Canada 2019e; 2025b). For monitoring arsenic, copper, iron and manganese and other inorganic contaminants, monitoring information is provided in Health Canada’s Guidelines for Canadian Drinking Water Quality: Guideline Technical Documents for the respective parameters (Health Canada 2006; 2019b; 2019e; 2019f; 2020b; 2024a; 2025b). For single-family homes and multi-dwelling residential buildings up to six units, monitoring for OPPPs in these premises is not practical and is not recommended. Where it is suspected that the service line for a residence contains lead, the water should be tested. It is recommended that homes on private wells test their general water quality every two years as well as the effectiveness of treatment devices if they have been installed. The local drinking water or public health authorities can be contacted to find out what chemicals to test for.

For multi-dwelling residential buildings with more than six residences and a centralized hot water heater, water management programs (WMPs) are recommended. These outline the requirements for monitoring, based on an assessment of the building water system. The requirements will be unique to each building and influenced by the complexity of the building water system design, the operating characteristics of the system and the parameters being analyzed. Typically, building WMP resources recommend monitoring for temperature to verify that the recommended premise plumbing temperatures are being achieved for microbiological control and prevention of scalding risk. These resources also recommend monitoring disinfectant residual in the cold water supply to confirm that levels reaching points of use (POUs) are within target levels and ideally approach levels in water provided by the drinking water supplier. The need for testing of microbiological hazards is determined by an assessment of the building water system conducted by the WMP team. When required, testing is most commonly conducted for L. pneumophila, using an accredited laboratory. Building owners are responsible for assessing their own plumbing systems for lead and the municipality or local drinking water authority can be contacted for advice and assistance with testing.

Part B. Supporting information

B.1 Premise plumbing

B.1.1 Definitions

The majority of people in Canada receive their drinking water from municipal drinking water supplies. This means that source water, such as a river, is pumped into a water treatment facility, where it undergoes a rigorous treatment process. Treated water then enters an extensive network of pipes (also known as watermains), valves, hydrants, and storage facilities, known as the DWDS (Figure 1). The DWDS is connected to buildings via a service line (Figure 2). Once water moves through the service line, it enters plumbing within the building, referred to as premise plumbing. In this document, premise plumbing is considered to be the portion of the DWDS beyond the property line and in buildings (NRC 2006; Pruden et al. 2013). Buildings include schools, hospitals, residences (that is, dwellings), offices and other premises.

Figure 1: Text description

The flow of water from its source through various stages until it reaches buildings:

• the “source” is a water body. This is where the water supply begins.
• the “treatment”
• the “distribution system” to deliver treated water to different buildings. Characteristics associated with the distribution system include:
  • single system
  • single owner or operator
  • stability in operation
  • limited environmental changes
  • routine monitoring
  • guidelines and regulations
• the “building plumbing system”. These structures are connected to the distribution pipes, with water entering individual properties through the premise plumbing. Characteristics associated with building plumbing systems include:
  • multiple unique systems
  • multiple owners or operators
  • varying operational conditions
  • fluctuating environmental conditions
  • no or rare monitoring
  • legally grey areas

Figure 2: Text description

The diagram illustrates the division of responsibility between a water distributor and a building owner as water moves from the drinking water distribution system into a building.

The water main is part of the drinking water distribution system. A service line connects the water main to the building and supplies water to the building's plumbing. The property line divides the portions of the water system for which the water distributor and building owner are responsible. The water distributor's responsibility extends from the water main to the property line, while the building owner's responsibility extends from the property line into the building.

Inside the building, the service line connects to the premise plumbing, which distributes water to various fixtures and systems. The premise plumbing includes a water heater, shower, two kitchen sinks and an exterior irrigation system. The water heater supplies hot water to the shower and kitchen sinks, while the irrigation system is supplied through the building's plumbing.

For those relying on a private well for their drinking water, the source water is typically groundwater, which may undergo treatment. Well water enters the building via a water supply line and flows into premise plumbing.

Typically, premise plumbing consists of the pipes, tanks, fittings, fixtures, and other devices that convey water within a building. Fittings are devices designed to control and guide the flow of water, such as faucets, shower heads, water heaters, shutoff valves, shower valves, and tub spouts. Fixtures are devices for receiving water and/or waste, such as toilets, sinks, bathtubs and shower bases.

Premise plumbing comprises both the supply side (drinking water) and the waste side (wastewater). The supply side refers to the pipe systems that carry potable water to places where it can be used by consumers, as well as those that convey water for other purposes (such as heating and cooling systems, fire protection systems) (Proctor et al. 2022). The waste side consists of pipe systems that direct water and/or waste into a sanitary drainage system.

B.1.2 Roles and responsibilities

In most cases, the drinking water supplier (usually the municipality) is responsible for the drinking water distribution system up to the property line (Figure 2). This is generally delineated by the curb stop, that is, the municipal water service shutoff valve. The building owner or homeowner is responsible for materials and water quality beyond the property line, including their portion of the service line and all of the premise plumbing.

For private wells, the well owner is responsible for their premise plumbing, including the water/supply line from the well into the building, as well as for maintaining and monitoring the quality of their well water.

B.1.3 Scope

The focus of this document is on water quality and its management on the supply side in premise plumbing in dwellings. For the purposes of this guidance, dwellings refer to residential buildings, such as detached, semi-detached, town and row houses, duplexes, triplexes, fourplexes, along with apartment and condominium complexes. The terms “single-family home” and “multi-dwelling residential buildings” are used in this document, to represent the many types of dwellings found in Canada. In addition, multi-dwelling residential buildings are described as having up to six units or more than six units. The rationale for this distinction is detailed below (see B.1.4 Dwelling types).

This document does not address guidance for premise plumbing in hospitals, long-term care homes, retirement homes or office buildings. For guidance relating to commercial, institutional and industrial buildings, please refer to the following resources: American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE 2023a; 2023b; CCIAQ 2023; CDC 2021a; PWGSC 2016). This document, likewise, does not address systems that collect rainwater or recycled water for potable or non-potable use or commissioning requirements for buildings and homes.

Aspects of water quality addressed here include the presence of OPPPs and inorganic contaminants. The OPPPs discussed in this document include Legionella spp. (with emphasis on Legionella pneumophila), the NTM group within the genus Mycobacterium spp. (with emphasis on Mycobacterium avium), Pseudomonas aeruginosa, Acanthamoeba spp. and Naegleria fowleri (see B.3 Opportunistic premise plumbing pathogens).

This document focuses on fittings within premise plumbing, as they represent a potentially important source of OPPPs (see B.2.10.1 Design). It does not address cooling towers, water features (for example, swimming pools, hot tubs, decorative fountains) or other water-using devices (for example, humidifiers). Guidance relating to these can be found elsewhere (AIHA 2022; NSF International 2017; PWGSC 2016; Régie du bâtiment du Québec 2014).

B.1.4 Dwelling types

Given the wide variety of dwellings found in Canada, it is not possible to discuss considerations related to each. Instead, monitoring and management guidance is focused on two categories of dwellings: 1) single-family homes and multi-dwelling residential buildings up to six units; and 2) large multi-dwelling residential buildings, that is, those with over six units. While there are no standardized categories of small and large multi-dwelling residential buildings based on their size and scale of plumbing complexity, provincial and municipal methodologies for property assessments classify large residential buildings as those having five to seven or more residential units (City of Edmonton 2024; MPAC 2016; SAMA 2025; Ville de Montréal 2025). Buildings having more than five to seven residential units are considered in a separate category because, among other criteria, they require different management approaches. In the Guideline Technical Document for Lead and the Guidance on Sampling and Mitigation Measures for Controlling Lead Corrosion (Health Canada 2019e; 2025b), building types are categorized into residential dwellings (up to six residences) and multi-dwelling buildings having more than six residences. To harmonize with these resources, the six-unit distinction is used in this document.

B.2 Causes of water quality deterioration

Drinking water suppliers aim to provide consumers with safe and high-quality water by protecting source waters, using the most appropriate water treatment processes and conducting routine water quality testing. However, once water leaves a treatment plant, it enters the municipal DWDS, where it is exposed to conditions that can lead to deterioration of water quality (Health Canada 2022a). Similarly, when distributed water or water leaving a private well enters premise plumbing, it is subjected to an environment that can result in further water quality degradation.

Many of the causes of worsening water quality in the DWDS (Health Canada 2022a) also apply to premise plumbing. Certain characteristics of premise plumbing (see Figure 3 and List 3) can lead to further water quality deterioration. The following characteristics are discussed:

• presence of microorganisms, biofilm, nutrients and inorganic contaminants
• use of various plumbing materials
• high surface-to-volume ratio of piping
• increased water age
• fluctuating water temperatures
• low or no disinfectant residual
• variable flow rates
• design and age

Figure 3: Text description

The diagram shows the flow of water from a drinking water distribution system to premise plumbing and highlights the conditions that can affect this water, including temperature.

Drinking water flows from the water main into the plumbing within a property. The premise plumbing includes a water heater, showerhead, kitchen faucet and toilet. The water heater supplies hot water to the showerhead and kitchen faucet, while the toilet receives cold water directly from the water supply. The plumbing also includes areas where hot and cold water can mix.

Water conditions within premise plumbing can differ from those in the drinking water distribution system. These conditions include:

• Fluctuating temperatures
• Variable flow rates
• Variable and long periods of stagnation
• Many different materials and fittings
• Low disinfectant residual

The diagram identifies three water-temperature zones within the plumbing: cold water, hot water and mixing zones, where water temperature can vary between hot and cold.

B.2.1 Presence of microorganisms and biofilm

Microorganisms are present in all municipal DWDSs (Health Canada 2022a) and wells, and can therefore be introduced into premise plumbing. Once introduced, they are likely to join pre-existing populations of microorganisms that are part of biofilms. Microorganisms may also enter premise plumbing via intrusion. Intrusion occurs when there is an integrity breach, such as a pipe break or leak, or via a cross-connection. Microorganisms in premise plumbing, like those in municipal DWDSs (Health Canada 2022a), are largely found attached to internal pipe surfaces as part of biofilms and/or loose deposits, rather than free-floating (see Figure 4). Biofilm-associated microorganisms (BAMs) account for an estimated 95% of the microbial population (Flemming et al. 2002). They, along with the microorganisms in water, are collectively referred to as premise plumbing microbiome or resident microbiome. The premise plumbing microbiome can vary within different parts of a building and between buildings, as well as with time and changing water quality conditions (Huang et al. 2021).

Biofilms are crucial to the survival and proliferation of microorganisms, including OPPPs, in premise plumbing, as they confer increased resistance to a variety of stressors (see B.3.1.1 Surface adherence and biofilm formation). Premise plumbing has several characteristics that may facilitate biofilm formation. The small diameter piping present in premise plumbing results in high surface-to-volume ratios. This allows increased contact between microorganisms and pipe surfaces and can lead to biofilm formation (see B.2.5 High surface-to-volume ratio of piping).

Because of their unique characteristics (see B.3.1 Characteristics of opportunistic premise plumbing pathogens), OPPPs have adapted to grow and persist in biofilms, and are considered “normal”, typically harmless inhabitants of premise plumbing (Falkinham 2015b; Falkinham et al. 2015b). However, under certain conditions (see B.3.3 Transmission), they can infect susceptible consumers (see B.3.4 Health considerations).

In addition to housing various microorganisms, biofilms contain organic and inorganic matter. Inorganic contaminants, such as corrosion by-products, may readily attach to biofilms which can impact water quality (see B.2.3 Presence of inorganic contaminants).

Biofilms can contribute to a host of water quality issues in premise plumbing, such as loss of disinfectant residual (see B.2.8 Low or no disinfectant residual), and they are difficult to remove. Biofilm detachment can also lead to deteriorating water quality.

Figure 4: Text description

The image shows the inside of a water pipe and highlights the presence of biofilm.

Within the pipe, there is:

• water
• the pipe wall
• free-floating microorganisms (inside the water)
• biofilm (along the bottom and sides of the pipe wall)
• biofilm-associated microorganisms (embedded in the biofilm

B.2.2 Presence of nutrients

A number of nutrients, such as carbon, copper, iron, zinc and ammonia, may be present in premise plumbing and can promote microbial growth, either by serving as fuel for microorganisms or by consuming disinfectant residual (NRC 2006). This can lead to challenges in maintaining biological stability (Health Canada 2022a). Nutrients can be introduced into premise plumbing via the distribution system (Health Canada 2022a) or through well water. They may also originate within premise plumbing through leaching of compounds from pipes and other materials. Premise plumbing materials can leach a significant amount of nutrients (Cullom et al. 2020; Neu and Hammes 2020). Shower hoses, for example, consist of plastics and plasticizers (substances added to a material to promote flexibility) and are particularly prone to leaching (Neu and Hammes 2020). Leached compounds can act as growth-supporting nutrients for microorganisms. A detailed discussion of premise plumbing materials is provided in B.2.4 Variety of plumbing materials. Biofilms in premise plumbing constitute a large source of organic nutrients, which can fuel microbial (re)growth and consume disinfectant residual. Similarly, sediment that collects at the bottom of hot water heaters may also serve as a source of nutrients (Cullom et al. 2020). Filters, such as those used in POU devices in premise plumbing, can accumulate nutrients, serving to promote microbial growth.

OPPPs have been shown to react differently to varying nutrient concentrations. Mycobacteria, Pseudomonas spp. and Acanthamoeba spp. are able to grow under low nutrient conditions, whereas Legionella spp. seem to adapt to fluctuating nutrient conditions by differentiating into cell types that vary in their infectivity and resistance to disinfection (NASEM 2020; Robertson et al. 2014).

B.2.3 Presence of inorganic contaminants

Inorganic contaminants can be introduced into drinking water from source water (for example, arsenic, iron and manganese), chemicals used during drinking water treatment (for example, aluminum or iron from coagulants) or from the municipal DWDS and premise plumbing (for example, lead or copper) (Friedman et al. 2010). They can be released through hydraulic, chemical changes (to water quality) or microbially-induced processes and thereby introduced into the distribution system and premise plumbing. Metal ions can form scale deposits that can be qualified as precipitates (Friedman et al. 2010; Health Canada 2025b). Scales can range from being highly soluble and friable to adherent and protective to the pipe wall. Scale deposits (and corrosion products) present in distribution systems and premise plumbing, particularly those containing aluminum, iron and manganese, can also act as accumulation sinks for other contaminants such as arsenic, chromium, copper or lead (Cantor 2017; Schock et al. 2008; Tully et al. 2019). They can be released into the distributed water and travel into premise plumbing.

Metal ions can also fuel microbial (re)growth. Iron, for example, is an essential nutrient for Legionella (Percival and Williams 2014; Portier et al. 2016), and its presence may contribute to Legionella growth and persistence in premise plumbing. Iron also accelerates disinfectant residual decay that can lead to microbial (re)growth (Zhang and Edwards 2009).

Given the variety of materials that may be present in the municipal DWDS, and differing system conditions (Health Canada 2025b), it is difficult to predict the type and concentrations of inorganic contaminants that may make their way into premise plumbing. Lead, copper, iron (including galvanized iron) and cement are the materials most likely to produce contaminants that exceed Canadian drinking water guidelines, pose health risks to the public or provoke consumer complaints (Health Canada 2025b). The primary contaminants that can leach from corrosion of materials in DWDSs include lead, manganese, copper, antimony, cadmium, iron and arsenic (Cantor 2017; Health Canada 2025b; Schock et al. 2008; Tully et al. 2019). Inorganic contaminants can also leach from materials present in premise plumbing and service lines (see B.2.4 Variety of plumbing materials).

B.2.4 Variety of plumbing materials

As noted earlier, premise plumbing consists of a variety of components (see B.1.1 Definitions) that are comprised of a wide range of materials (see Table 1). Materials influence water chemistry and impact microbial (re)growth, biofilm formation and the diversity of the resident microbiome (Buse et al. 2014a; 2014b; Cullom et al. 2020; Neu and Hammes 2020; Proctor et al. 2017). Conversely, water quality affects these materials and, consequently, their performance (NRC 2006).

Some materials and components used in municipal DWDSs have been discontinued or are no longer allowed (Health Canada 2025b). Knowing the age of a dwelling and when plumbing components were replaced can aid in identifying which inorganic contaminants may be present. Adopting best practices and current standards for building construction, renovation and maintenance is important to ensure acceptable materials are used.

B.2.4.1 Direct effects on water quality

Service lines and pipes

As noted in B.1.1 Definitions, a dwelling is connected to the municipal drinking water distribution system via a service line. Service lines can consist of various materials and have direct effects on water quality.

Lead service lines

Lead released from lead service lines and their connecting components (such as pigtails and goosenecks) can be deposited and accumulate on materials within premise plumbing (Health Canada 2019e). After extended stagnation times, lead service lines can contribute from 50% to 86% of the total lead concentrations measured at the tap, representing the largest source of lead in premise plumbing (Health Canada 2025b). After full lead service line replacement, lead levels were significantly lower (Deshommes et al. 2018; Lytle et al. 2019; Trueman et al. 2016). However, partial lead service line replacement (replacing only the utility or consumer’s portion) does not result in a proportional decrease in lead levels when compared with full lead service line replacement. Studies have also shown that iron can be released after full or partial lead service line replacements, and a correlation was observed between particulate lead at the tap and other metals such as iron, zinc, tin and copper (Camara et al. 2013; Deshommes et al. 2010; McFadden et al. 2011). In these cases, iron deposits, which contain lead, formed in the lead service line and premise plumbing, were disturbed and released into the water. More information on lead service lines can be found elsewhere (Health Canada 2019e; 2025b).

Galvanized pipes

Galvanized pipes and lead-lined galvanized iron pipes can be significant contributors to lead in drinking water (Health Canada 2025b). Cadmium can also be present as impurities in the pipe coating and be a source of cadmium in drinking water (Health Canada 2025b). The presence of cadmium can indicate that a galvanized pipe is the source of lead (Clark et al. 2015). Lead from upstream sources, such as service lines or solders, can sorb onto the accumulated materials on galvanized steel and become a source of lead in premise plumbing (McFadden et al. 2011; Pieper et al. 2017; Wasserstrom et al. 2017). More information on lead and cadmium release from galvanized pipes can be found elsewhere (Health Canada 2019e; 2020b).

Copper pipes

Copper and its alloys are common in premise plumbing (Cullom et al. 2020). However, copper pipe has limitations for use in certain water qualities under the certification requirements of NSF/ANSI/CAN Standard 61 (NSF International 2023). Generally, copper pipes can leach copper, with concentrations highest for newly installed pipes or in homes supplied with water having low or unstable pH and decreasing over time (Health Canada 2019b; Singh and Mavinic 1991).

Copper corrosion is influenced by several factors, including the age of the piping and water qualities or situations that are potentially corrosive (for example, low pH, low or high alkalinity, dead ends) (Health Canada 2019b). Under specific water qualities, copper pitting or pinhole leaks can also occur in premise plumbing and lead to plumbing failure and intrusion of contaminants (AWWA 2017b; Health Canada 2019b; Lytle and Nadagouda 2010).

Copper leached from pipes can reach concentrations capable of limiting microbial (re)growth, although copper has also been shown to have growth-promoting effects on microorganisms, including OPPPs (Cullom et al. 2020). The apparent contradiction in the effects of copper may be related to pipe age and water chemistry (such as pH, presence of corrosion inhibitors, oxidants); strain-specific differences in copper resistance (Bédard et al. 2019; 2021) and the role of copper as an essential micronutrient used in various bacterial processes (Cullom et al. 2020; Galai et al. 2012; Samanovic et al. 2012). Bédard et al. (2021) reported that the expression of the copper resistance gene was significantly higher for premise plumbing isolates of L. pneumophila than clinical isolates, suggesting an in-situ adaptation brought about through contact with copper pipes. Evidence of a copper-induced viable but nonculturable state has also been reported, including for P. aeruginosa and M. avium (Bédard et al. 2014; Rowland and Niederweis 2012).

Plastic pipes

Plastic pipes, such as polyvinyl chloride (PVC) and cross-linked polyethylene (PEX), are becoming more prevalent, as they are often used in new construction and in renovations (Ahmed et al. 2010; Health Canada 2025b; Kelley et al. 2014; Lee et al. 2013; Salehi et al. 2017). PEX, PVC and chlorinated polyvinyl chloride (CPVC) pipes and components (for example, tubing and fittings), used for drinking water must comply with the leaching requirements of NSF/ANSI/CAN Standard 61.

Plastic water supply connectors used for fixtures and fittings, such as faucets and refrigerator water dispensers, are made of various plastics with the greatest release of organic carbon seen from new PVC. Connector plastics have been shown to sorb over 90% of hydrocarbon contaminants during exposure, which remain in the plastics after decontamination, and release contaminants into the water during decontamination (Isaacson et al. 2024). This highlights the need for replacing connectors after wildfire or hydrocarbon contamination events. Plastic pipes can leach organic carbon compounds that act as nutrients for microorganisms, including OPPPs. Several studies have reported an association between some PEX pipes and increased biofilm and OPPP growth in comparison to copper and iron (Dai et al. 2018; Proctor et al. 2017; Van Der Kooij et al. 2005). Like PEX, PVC has been shown to promote biofilm and OPPP growth (Cullom et al., 2020). In general, plastic pipe material is associated with higher OPPP growth than copper and other metal pipe materials (Cullom et al. 2020; Logan-Jackson et al. 2023).

B.2.4.2 Fittings and other components

Fittings, along with other premise plumbing components, can consist of a host of materials whose composition depends on when they were manufactured and the requirements in the plumbing code at that time (Lytle and Schock 1996; NSF International 2023).

Brasses

The allowable lead content of brass fittings has decreased over time; thus, legacy materials are of more concern than newer materials. Corrosion of brass fittings used to connect plastic piping can also be a major source of metals released in plastic plumbing systems (Lytle and Schock 1996; Pieper et al. 2016; Salehi et al. 2018).

Removal of zinc from brasses, through a process referred to as dezincification, can lead to physical damage to plumbing, resulting in potential leaks (Health Canada 2019b; Lytle and Schock 1996). Some water qualities make brasses more susceptible to dezincification. Fittings with high zinc content (yellow brass) used in PEX plumbing systems have been associated with widespread plumbing failures caused by dezincification (Health Canada 2019b). Like copper, zinc is an essential nutrient for microbial growth (Cuajungco et al. 2021; Hood and Skaar 2012), and its presence has been associated with increases in OPPP presence and growth (Kirschner Jr et al. 1992; Reeves et al. 1981; States et al. 1985). Zinc can also be toxic to microorganisms (Choudhury and Srivastava 2001; Cuajungco et al. 2021).

Solders

Solders may also be a source of contaminants as solder particles can detach from plumbing and enter the distributed water (Bosscher et al. 2019; Cartier et al. 2011; Deshommes and Prévost 2012; Health Canada 2019e; Triantafyllidou et al. 2007). Lead solders were commonly used in premise plumbing in Canada until they were prohibited by the National Plumbing Code (NPC) in 1990 (NRCC 2015). To minimize lead leaching, they were replaced by tin-antimony, tin-copper, and tin-silver solders (CSA 2018; NSF International 2023).

Accumulation of materials behind aerator screens

Lead-containing material, including lead solder particles, pipe deposit solids (for example, iron corrosion particles), or other metals to which lead can be attached, can act as a carrier and can accumulate behind screens or aerators in water outlets (Bosscher et al. 2019; Cartier et al. 2011; Deshommes and Prévost 2012; Health Canada 2019e; Triantafyllidou et al. 2007). Metals identified in the particles are typically found in premise plumbing and include lead, copper, tin and zinc.

Other premise plumbing materials

Aluminum and magnesium are released from anodes in water heaters. While magnesium is an essential nutrient for Legionella (Reeves et al. 1981), it is unclear what role aluminum plays in the (re)growth of opportunistic pathogens in premise plumbing. Little is known regarding the potential effects of other premise plumbing materials (for example, rubber, stainless steel, silicone, polyamide) on microbial water quality, including OPPPs (Cullom et al. 2020).

B.2.4.3 Indirect effects on water quality

Premise plumbing materials can also impact water quality by reacting with the disinfectant residual. Plastic pipes, for example, are generally considered to be non-reactive to disinfectants, whereas copper pipes are more reactive, and iron pipes are highly reactive (Cullom et al. 2020; Health Canada 2024a; McCuin et al. 2022). The more reactive a material, the more the disinfectant degrades, and the less there is available to control microbial (re)growth. Nitrification can decrease pH and indirectly increase lead concentrations. When chloramine is used as a secondary disinfectant, nitrification can occur in the distribution system and decrease pH, especially in low alkalinity water. This was clearly demonstrated when nitrification was shown to have caused the pH to decrease in a dead-end main, resulting in lead release from lead service lines (Douglas et al. 2007). Warmer temperatures have also been shown to impact nitrification.

Metallic premise plumbing materials can promote the growth of autotrophic bacteria that use chemical energy to fix carbon, which aids OPPPs in overcoming carbon limitations (Brazeau and Edwards 2013; Morton et al. 2005). Scaling on metal pipes, for example, caused by corrosion or higher pHs, can lead to increased pipe roughness, which facilitates adherence of microorganisms and biofilm formation (LeChevallier et al. 1993; Niquette et al. 2000), creating a favourable environment for OPPPs.

B.2.4.4 Other considerations

Given the variety of materials present in premise plumbing, it is possible that there may be synergistic or antagonistic interactions between them, and that these interactions could influence the growth of OPPPs. Copper, for example, has been shown to catalyze degradation of plastic pipes, thereby enhancing organic carbon release (Huang et al. 2019; Tanemura et al. 2014). A galvanic connection between metal plumbing materials (for example, copper and lead pipes connected with a brass fitting) can increase corrosion, leading to changes in water quality (Nguyen et al. 2010; Tada et al. 2004). Other material interactions may create microenvironments that are favourable for OPPP growth (Cullom et al. 2020).

B.2.5 High surface-to-volume ratio of piping

In comparison to the municipal water distribution system, premise plumbing uses small diameter piping (such as 1 to 2 cm versus 10 to 200 cm) (Logan-Jackson et al. 2023). This allows for a significantly (approximately 10- to 16-fold) higher surface-to-volume ratio, leading to increased contact between microorganisms and pipe surfaces, which is favourable for adherence and biofilm formation. At the same time, smaller pipe diameters directly increase flow velocity and thus shear stress, which can affect biofilm structure and lead to detachment (Douterelo et al. 2013; Fish et al. 2017; Kim et al. 2013; Ling et al. 2018). Smaller pipe diameters can also result in lower disinfectant residual concentrations as more of the pipe’s surface is available for interaction with disinfectants, leading to their decay (Ling et al. 2018; NRC 2006; Prévost et al. 1998; Rossman et al. 1994; Tsvetanova and Hoekstra 2010). The higher surface-to-volume ratio in premise plumbing also results in more pronounced pipe material impacts through increased leaching and the permeation of contaminants (Connell et al. 2016; Deshommes et al. 2010; Elfland et al. 2010; Gagnon and Doubrough 2011; Neu and Hammes 2020). A higher surface-to-volume ratio also results in heat being dissipated more quickly. As a result, water heated to 60 °C, such as in a water heater, to control microbial (re)growth (see B.6.4.2 Managing risks) will quickly cool once in dwelling pipes, at which point it may no longer be effective in doing so.

B.2.6 Increased water age

B.2.6.1 Water age

Water age, also called residence time, refers to the time spent in pipes from the treatment facility to the water meter of a dwelling as well as from the water meter to the POU (such as a tap) (AWWA 2017c). Compared to the distribution system, the residence time in premise plumbing is relatively long (NRC 2006).

Water age is one of the most important factors affecting water quality in premise plumbing and is intrinsically linked to water demand (usage) as well as premise plumbing design and condition. As demand decreases, the time that water resides in premise plumbing increases. This increased residence time allows for microorganisms, including OPPPs, to adhere to surfaces and form biofilms. It also allows for more interaction between disinfectants and premise plumbing materials along with organic material in the distributed water and on pipe walls. These interactions can lead to disinfectant decay and result in the formation of disinfection by-products (DBPs). Disinfectant decay, in turn, may result in limited suppression of microbial (re)growth. Increased water age can also lead to temperatures suitable for microbial (re)growth, including OPPPs. Long residence times in premise plumbing have been linked to significantly higher concentrations of microbial populations and shifts in microbial community composition (Bédard et al. 2018; Lautenschlager et al. 2010; Lipphaus et al. 2014; Manuel et al. 2010; Pepper et al. 2004).

B.2.6.2 Stagnation

Stagnation refers to water not flowing or moving for extended periods in a pipe and is typically the result of intermittent water use (Leslie et al. 2021). Water can sit stagnant in dwellings for extended periods if they are not fully occupied (for example, an empty unit) or if residents are away (for example, during vacations). Even under full-time occupancy, some sections of the premise plumbing may not be commonly used. Flow patterns can vary considerably depending on the water use patterns of the occupants and lead to stagnation (NRC 2006).

Stagnation is one of the most impactful conditions on water quality in premise plumbing and has been linked to corrosion and OPPP proliferation (Bédard et al. 2016b; Hayes-Phillips et al. 2019; Ji et al. 2015; Lautenschlager et al. 2010; Ling et al. 2018; Lipphaus et al. 2014; Tolofari et al. 2022). For example, the concentrations of lead from brass fittings and copper from copper pipe have been shown to increase significantly following stagnation periods of between 4 and 20 hours (Health Canada 2025b). It can be difficult to distinguish the effects of stagnation versus water age as they are interconnected; stagnation contributes to overall water age.

B.2.7 Temperature fluctuations

Water temperature affects all physical, chemical, microbiological, and biochemical processes (Health Canada 2021a), including those that occur in premise plumbing. These, in turn, affect disinfectant efficacy and water quality. Typically, as water temperatures increase, disinfectant efficacy increases; however, so does the rate at which disinfectant degrades (Baribeau et al. 2005). This can lead to an overall decrease in the biocidal effect of disinfectant residual, resulting in microbial (re)growth (Baribeau et al. 2005; Hua et al. 1999; Monteiro et al. 2015). Increased temperatures lead to accelerated proliferation of microorganisms, depending on their optimal growth ranges (see Table 3). There is no simple relationship between temperature and corrosion processes. Hot water is often observed to be more corrosive than cold water (Schock and Lytle 2011), resulting in a more direct impact on the solubility of lead and iron at elevated temperatures. It is important to note that consuming hot water from the tap is not recommended due to the potential for increased metals, including lead (Health Canada 2016). In contrast, the solubility of several corrosion by-products decreases with increasing temperature (Edwards et al. 1996; McNeill and Edwards 2001; 2002; Schock 1990). Water temperature increases can also lead to increased formation of DBPs (Liu and Reckhow 2013).

Since premise plumbing consists of both cold and hot water supply systems, temperatures in premise plumbing can be much higher than those in the distribution system (Rushing and Edwards 2004). In temperate regions, such as Canada, temperatures in the distribution system of surface water supplies typically reach 20 °C or more in summer months (Li et al. 2014). In comparison, hot water in premise plumbing can exceed 60 °C, particularly in the water heater. Even the cold water line in premise plumbing can be 10 to 15 °C warmer than the water in the municipal distribution system (NRC 2006). Cold water temperatures are expected to increase due to climate change (see B.5 Impact of climate change). Water in pipes and cisterns (where present) will also acclimatize to the ambient temperature conditions. This means that hot water temperatures can fall and cold water temperatures can rise, resulting in temperatures within the growth range for OPPPs, that is, 25 to 45 °C (Figure 5; Table 3). As a result, a fundamental control strategy for OPPPs in premise plumbing is to keep cold water systems ideally under 20 °C, water heaters above 60 °C, and hot water in pipes between 50 °C and 55 °C (see B.6 Management strategies).

Figure 5: Text description

Water temperature (°C)	Building water systems	Legionella activity
10-20	Unheated or stagnant	Dormant (< 20 °C)
20-30	Unheated or stagnant	Active
30-40	Household use	Active
40-50	Household use	Active
50-60	Water heating	Viable but non-culturable or cell death
60-70	Water heating	Viable but non-culturable or cell death
70-80	n/a	Viable but non-culturable or cell death Disinfection (specifically, 66°C)
80-90	n/a	Viable but non-culturable or cell death Disinfection (specifically, 66°C)
90-100	n/a	Viable but non-culturable or cell death Disinfection (specifically, 66°C)

Other notes:

• Temperatures below 50°C in building water systems are for scalding prevention and energy conservation
• Temperatures above 60°C in building water systems are for Legionella prevention

Legionella activity can last for:

• 5 to 6 hours at 55°C
• 30 minutes at 60°C
• less than 5 minutes at 66°C

B.2.7.1 Cold water

As noted above, cold water in premise plumbing can be warmer than the water in the municipal distribution system (NRC 2006). It is not uncommon for cold water temperatures in premise plumbing to exceed 20 °C, particularly if cold water pipes are not sufficiently insulated, located close to hot water pipes, or run through heated rooms (Leslie et al. 2021). Cisterns or cold water storage tanks (particularly those not properly insulated or situated) can also be susceptible to increases in temperature above 20 °C when the weather or ambient temperature is warmer (ESGLI 2017; O'Keeffe 2024). At these “cold” water temperatures, OPPPs can persist and, in some cases, proliferate.

B.2.7.2 Hot water

In premise plumbing, water for specific uses, such as showering, is produced by a water heater. The choice of water heater and its temperature setting determine the temperature and quality of water entering the hot water supply system of premise plumbing (see B.2.10.1 Design). The temperature set at the water heater is not equivalent to the temperature at the tap, as the temperature of hot water is difficult to maintain once it leaves the heater. The high surface-to-volume ratio of premise plumbing piping causes heat to quickly dissipate (see B.2.5 High surface-to-volume ratio of piping), with water temperatures falling to within the optimal growth ranges for OPPPs (see Table 3), especially at taps at distal ends of premise plumbing. Similarly, low flow rates in premise plumbing can lead to decreases in hot water temperatures, as there is insufficient hot water being drawn to maintain the temperature, therefore facilitating microbial (re)growth (see B.2.9 Variable flow rates and water use).

B.2.8 Low or no disinfectant residual

To maintain water quality during distribution, drinking water suppliers typically add disinfectant to water leaving the treatment plant to provide a disinfectant residual in the distribution system. Disinfectant residual may also be added at locations throughout the distribution system via booster stations. Disinfectant residual is intended to ensure microbial safety and limit (re)growth. Most drinking water suppliers in Canada use free chlorine as a residual disinfectant, while the rest use chloramines (Health Canada 2009a). These disinfectants have different disinfectant abilities, reactivity with organic and inorganic material, and biofilm penetration. Disinfectant residual declines in the municipal distribution system due to various reasons (Health Canada 2022a). By the time water reaches dwellings, particularly those furthest away from the point of disinfectant addition, there can be little remaining disinfectant residual. Canadian data provided in 2005 by six provinces and territories indicated that typical levels of free chlorine in drinking water systems ranged from 0.4 to 2.0 mg/L when leaving the treatment plant, from 0.4 to 1.2 mg/L at intermediate points in the distribution system, and from 0.04 to 0.8 mg/L at the far ends of the distribution system. A study of water quality in a green, that is, net-zero energy, residential building in Indiana (United States [U.S.]) found no detectable total chlorine residual in over 10% of samples obtained from the service line (Salehi et al. 2020). As water enters premise plumbing, it is exposed to an environment that can lead to further depletion of disinfectant residual (Huang et al. 2020; Leslie et al. 2021; Logan-Jackson et al. 2023; NRC 2006). Reactive premise plumbing materials (see B.2.4 Variety of plumbing materials) and increased water temperatures (see B.2.7 Temperature fluctuations) can result in significant disinfectant residual decay, which is exacerbated by stagnation (see B.2.6 Increased water age).

The lack of an adequate and persistent disinfectant residual throughout the premise plumbing results in a compromised ability to control microbial (re)growth, including that of some OPPPs that are inherently disinfectant-resistant (Falkinham 2015b; Falkinham et al. 2015b; Pruden et al. 2013). Low disinfectant residual in premise plumbing can also lead to biofilm formation. As it is difficult for some disinfectants, such as free chlorine, to penetrate biofilm, OPPPs that take up residence in them are protected (see B.2.1 Presence of microorganisms and biofilm) and are also less likely to be washed away (Falkinham 2015b; Falkinham et al. 2015b; Pruden et al. 2013).

Historically, it has been recommended that a “detectable” level of disinfectant residual be maintained throughout the distribution system. There is increasing recognition that higher minimum disinfectant residual concentrations of 0.2 mg/L free chlorine (Health Canada 2022a), in combination with low water temperatures (ideally, less than 18 °C), are required to control microbial (re)growth in biofilms, including that of OPPPs, throughout DWDSs (Gagnon et al. 2008; Gillespie et al. 2014; LeChevallier et al. 2015b; Rand et al. 2014). There is also evidence to support a minimum chlorine residual of 0.1 mg/L to reduce Legionella colonization of dwelling premise plumbing (Gleason et al. 2023; Straus et al. 1996). Inhibiting biofilm formation entirely requires very high chorine residual concentrations, on the order of 1.0 mg/L free chlorine (for systems that chlorinate) and 1.8 mg/L total chlorine (for systems that chloraminate) (Gagnon et al. 2008; Gillespie et al. 2014; LeChevallier et al. 2015a; 2015b; Rand et al. 2014).

B.2.9 Variable flow rates and water use

Water flow rates influence the transport of microorganisms, particles, nutrients, and disinfectants, as well as temperature and the rate of chemical reactions in distributed water (Leslie et al. 2021; Prest et al. 2016). Flow rates also affect biofilm formation and integrity, and, consequently, the occurrence of OPPPs. They also impact internal pipe velocities, the duration and timing of stagnation events, and the presence of stagnation zones (Pruden et al. 2013). As such, changes in flow rate can lead to changes in water age (see B.2.6 Increased water age) that therefore influence OPPP growth. Low flow rates typically lead to water quality degradation. Biofilm (and sediment) accumulation is more likely to occur under low flow conditions, leading to consumption of disinfectant residual (see B.2.8 Low or no disinfectant residual). Biofilms in low flow pipes may remain more intact that those in high flow pipes, permitting OPPP colonization and proliferation within them (Liu and Tay 2002; Moreira et al. 2013). Low flow rates can also cause temperatures in hot water pipes to drop into the optimal growth range of OPPPs because the rate of heat dissipation exceeds the rate of hot water replenishment (see B.2.7 Temperature fluctuations). While high flow rates are helpful in flushing out OPPPs, they also expedite the delivery of nutrients that can encourage OPPP growth (Liu et al. 2006; Ohl et al. 2004; Rhoads et al. 2015) and mobilize particles in premise plumbing (such as lead solder particles) (Health Canada 2019e; 2022b).

Flow rates in dwellings can be highly variable due to consumer water use patterns (NRC 2006), and depend on residence type. Additionally, due to the rapidly changing flow rates experienced in premise plumbing when opening and closing a faucet, dislodgement of biofilms and other inorganic deposits (for example, iron or lead) can occur (Health Canada 2019e; 2025b). Flows of 10 m/sec (reported as 33 ft/sec) have been reported (NRC 2006), making premise plumbing more susceptible to shearing of biofilms than the municipal distribution system (Lehtola et al. 2006; Liu et al. 2020). Detached materials can lead to deterioration of water quality.

Changes in the plumbing components and design intended to promote sustainability and reduce water use/consumption can also contribute to low flow conditions (Huang et al. 2020). The use of water-efficient fittings can significantly limit water flow (see B.2.10.1 Design). Water-efficient shower heads, for example, typically use between 6 to 8 L/min as compared to 15 to 25 L/min for regular shower heads (Leslie et al. 2021). Most hot water usage in dwellings tends to be of short duration (for example, handwashing) with the exception of showering and bathing. This means that the volume of hot water needed is usually less than the volume of water in the pipes between a water heater and faucet – that is, ‘slugs’ of heated water are being pulled without ever actually reaching the POU (Leslie et al. 2021). This may also result in water arriving at a blended lower temperature (Omaghomi and Buchberger), leading to water quality degradation (see B.2.7 Temperature fluctuations). Water-efficient buildings, also referred to as green buildings, have design elements that limit water flow and have been associated with diminished microbial water quality and elevated OPPP concentrations (Ley et al. 2020; Rhoads et al. 2016). These findings highlight the possible unintended consequences associated with water conservation (Ley et al. 2020).

B.2.10 Design and age

B.2.10.1 Design

For the purposes of this document, the term “design” is intended to encompass the type (composition, diameter and length) and configuration of piping as well as the type and location of fittings, fixtures and equipment in premise plumbing.

Premise plumbing systems can differ considerably in their design (Proctor et al. 2022). In Canada, the NPC establishes provisions for the design and installation of new plumbing systems, along with alterations to existing plumbing systems (NRCC 2020b). Provinces and territories choose whether to adopt the NPC, with or without modifications, into their plumbing codes. While plumbing codes aim to safeguard human health and safety and protect buildings from the environment as well as water and sewage damage, they do not address OPPP risk and its management.

Certain premise plumbing designs can create conditions that are favourable for OPPP growth. Long pipe lengths between a water heater and faucet(s), for example, can lead to heat loss, resulting in temperatures within the growth range for OPPPs (see B.2.7 Temperature fluctuations). The presence of dead ends can lead to stagnation and create habitats for microbial growth and increase lead and copper concentrations (Health Canada 2019e; 2025b). The presence of fire suppression systems in premise plumbing can also create conditions ideal for OPPP growth as these systems contain water that can become stagnant and contaminated. If a pressure drop occurs, contaminated water could be drawn into the plumbing and/or drinking water distribution system. It is essential that backflow prevention devices be part of the premise plumbing design. The use of water efficient fittings can lead to reduced flow that can increase water age, resulting in biofilm formation (see B.2.9 Variable flow rates and water use), and a higher risk of OPPP contamination. These and other fittings are discussed below.

Few studies have explored the impact of residential premise plumbing configuration on water quality (Logan-Jackson et al. 2023). Yao et al. (2023) determined that plumbing configuration played a key role on the aesthetic, chemical, and microbiological quality of water. Similarly, Schück et al. (2023) concluded that premise plumbing layout affected water quality and that deterioration of water quality could be minimized by certain design best practices.

Fittings and fixtures

The choice of fittings, fixtures and equipment in a dwelling and their location can contribute to water quality degradation. Like pipes (see B.2.4 Variety of plumbing materials), fittings consist of various materials that influence water chemistry and microbial growth differently. In addition, fittings and fixtures suffer from recurring stagnation events and associated disinfectant residual decay as well as temperature fluctuations because they can be located at distal parts of the premise plumbing, contain internal structures, have large surface-to-volume ratios and are typically not in use for much of the day. This makes them ideal locations for microbial attachment and biofilm formation, along with OPPP growth (Logan-Jackson et al. 2023; NASEM 2020). OPPPs have been detected in a variety of residential fittings and fixtures (see Table 2) (NASEM 2020).

Showerheads and their hoses, in particular, have been shown to harbour biofilms that serve to enrich OPPPs, particularly Legionella and Mycobacterium spp. (Collins et al. 2017; Feazel et al. 2009; Gebert et al. 2018; Hayes-Phillips et al. 2019; Proctor et al. 2016; 2018).

Water efficient and scald-prevention faucets

To reduce water and energy consumption, a variety of water-efficient fittings and related technologies have been developed. Although these fittings vary in their design, they typically restrict water flow (for example, the flow from a water efficient faucet is 5.7 L/min [1.5 US gal/min] vs a conventional faucet at 8.3 L/min [2.2 US gal/min]). This can result in increased water age and stagnation (see B.2.6 Increased water age), which can subsequently result in water quality deterioration. Water-efficient faucets tend to have a higher surface area than traditional faucets, which facilitates microbial adherence and biofilm development (NASEM 2020). The soft rubberized materials that are included to control water flow serve as ideal surfaces for microbial colonization (Leslie et al. 2021). Water-efficient faucets have been associated with a higher risk of biofilm formation and OPPP contamination (Bédard et al. 2016b; Halabi et al. 2001; Liu et al. 2006; Merrer et al. 2005; Sydnor et al. 2012; Yapicioglu et al. 2012).

Like water-efficient faucets, scald-prevention faucets also contain several surfaces for microbial attachment and consist of various materials that may impact water quality (NASEM 2020). As they are designed to prevent scalding, they blend hot and cold water through the use of devices that limit the temperature of the water before it reaches consumers (for example, automatic compensating valves, such as thermostatic mixing valves, and temperature limiting devices). The resulting warm water can be at temperatures favourable for OPPP growth (see B.2.7 Temperature fluctuations) (Singh et al. 2020). These devices can fail, leading to contamination between cold and hot water lines, resulting in water quality deterioration (Boppe et al. 2016; Rhoads et al. 2014; Singh et al. 2020).

Water heaters

Water heaters can serve as ideal habitats for OPPPs, particularly if temperatures are not high enough. The type of water heater can affect the potential for OPPP colonization. In North America, electric and gas-fueled storage water heaters are the most common, accounting for over 90% of residential hot water heaters (Maquire et al. 2013). Hot water heaters are typically categorized into two groups: storage and tankless (also known as on-demand or instantaneous water heaters). Electric storage water heaters heat water through electrical elements located on the side of the tank, leading to vertical thermal stratification. This results in lower temperatures at the bottom of the tank and the accumulation of sediment, providing favourable conditions for the growth of Legionella and other OPPPs (see B.2.7 Temperature fluctuations) (Alary and Joly 1991; Brazeau and Edwards 2011; Falkinham 2020).

Gas and oil-fired storage water heaters heat via a burner located at the bottom of the tank, which avoids vertical thermal stratification (Brazeau and Edwards 2011). However, sediment accumulation is still possible and, in areas with hard water, scale build-up at the bottom of the tank may reduce some heat transfer (Brazeau and Edwards 2011). Due to their design, electric storage water heaters are more likely to be contaminated with Legionella than gas or oil-fired water heaters (Alary and Joly 1991; NASEM 2020). In studies in the Québec City area, L. pneumophila was detected by culture in 37% (66/178) and 41% (84/205) of samples from electric water heaters, with temperatures of 30 to 40 °C at the bottom of the heater being strongly associated with positivity (Alary and Joly 1991; Dewailly and Joly 1991). In contrast, none of the samples from oil and gas water heaters were positive for L. pneumophila (Alary and Joly 1991; Dewailly and Joly 1991). Tankless water heaters work by passing water through a heat exchanger where a gas burner or electric element heats it to a pre-set temperature, only when it is needed (Brazeau and Edwards 2011). These water heaters have a low risk for OPPP contamination, since there is no storage tank for microbiological growth (Brazeau and Edwards 2011). Water temperature can be more variable when using a tankless system, with increased water temperatures at taps closest to the water heater (Brazeau and Edwards 2011; NRCC 2020b).

B.2.10.2 Age

For municipal distribution systems (Health Canada 2022a), aging premise plumbing systems become more susceptible to biofilm development (Zheng 2013). In addition to higher biofilm densities, older premise plumbing systems contain a higher density of other deposits, such as organic matter, minerals, and corrosion products (Chawla et al. 2012; Health Canada 2025b). These deposits can limit the flow of water, consume disinfectants (see B.2.8 Low or no disinfectant residual) and stimulate OPPP growth, leading to further deterioration in water quality (DiGiano and Zhang 2005; Gauthier et al. 2001; Kowalska et al. 2006; Lehtola et al. 2004; Reynolds et al. 2008; Wingender and Flemming 2004). Lead concentrations originating from lead-containing solders and brass fittings decline with age (Health Canada, 2025b). Similarly, the age of the plumbing materials is particularly important for copper and brasses. The iron concentration and the rate of corrosion increase with time when a pipe is first exposed to water, but both are then gradually reduced as the scale builds up. However, concentrations of lead, copper, iron and other metals in drinking water can increase significantly following a period of water stagnation of a few hours, regardless of age or other factors such as the water quality (for example, pH) and the composition, diameter and length of the materials (Health Canada 2025b). Older residences may also contain legacy materials that further contribute to water quality degradation (see B.2.4 Variety of plumbing materials).

B.2.11 Other water quality considerations

B.2.11.1 Disinfection by-products

DBPs are formed by the reaction of natural organic matter (NOM) present in raw water and disinfection (for example, chlorination or oxidation) during drinking water treatment (Health Canada 2025a; Reckhow and Singer 2011). Trihalomethanes (THMs) and haloacetic acids (HAAs) are the most commonly detected DBPs and include numerous substances (Rodriguez et al. 2004). THMs are considered stable once they have been formed and continue to form in the distribution system when chlorine is used as the residual disinfectant. However, HAAs can degrade after their formation (Reckhow and Singer 2011). Exposure to THMs occurs through ingestion of drinking water as well as through inhalation and dermal absorption as they are highly volatile and permeable through the skin. Exposure from bathing and showering are included in the calculation of the guideline value for THMs (Health Canada 2025a).

THM concentrations vary temporally and spatially within a distribution system, which impacts the concentrations entering residential premise plumbing. Factors such as pH, temperature, free chlorine residual and reaction time affect the fate of THMs in residential hot water heating. Thus, general conclusions about impacts to THMs are not possible (Legay et al. 2019; Liu and Reckhow 2013; 2015). One study demonstrated that THM concentrations increase considerably when cold water stagnates in premise plumbing but more significantly in hot water tanks (Dion-Fortier et al. 2009). It was also observed that HAA levels increased but less than THM levels in water stagnating in premise plumbing pipes and hot water tanks (Dion-Fortier et al. 2009). Residential water heating increased the THM concentrations, but the increase varied between residences and seasons (Legay et al. 2019). The authors concluded that exposure to chlorinated DBPs is higher from hot tap water than cold tap water (Legay et al. 2019). These studies also highlighted that, in premise plumbing including hot water tanks, the different DBP species do not behave similarly.

More detailed information on the formation of THMs in distribution systems is presented in the Guideline Technical Documents for THMs and HAAs (Health Canada 2008; 2025a).

B.2.11.2 Taste and odour

Taste and odour problems can be associated with raw water, the treatment method, the distribution system or a combination of the three (Health Canada 2005). An overview of different potential causes for taste and odour problems in drinking water is presented elsewhere (Health Canada 1979; 2005; 2020a). NOM can contribute to undesirable tastes and odours in drinking water caused by volatile compounds that are produced by the microbial biomass (for example, actinomycetes, cyanobacteria, fungi) (Health Canada 2020a). The taste of water can be affected by its mineral content and temperature, with dissolved anions and cations contributing to the mineral content (Dietrich and Devesa 2019). DWDSs and premise plumbing can be the source of compounds causing taste and odour events. Some metallic elements can flavour drinking water, such as iron, copper, zinc and manganese.

Iron, in its aqueous form [Fe(II)] has a metallic odour and an astringent taste or mouthfeel (Health Canada 2024a). However, depending on the form of iron, there are different taste thresholds and it can produce a weak bitter or metallic and salty taste (Dietrich 2015; Sain and Dietrich 2015). At concentrations of 0.3 mg/L of iron, it is possible for consumers to notice the taste of ferrous iron (Dietrich and Burlingame 2015). However, different population taste thresholds are reported in the literature and range from 0.003 to more than 5 mg/L (Omur-Ozbek and Dietrich 2011).

Aside from potentially discolouring the water, copper can also be detected by taste, with reported taste thresholds ranging between 0.0035 to more than 5 mg/L (Health Canada 2019b). The solubility of copper affects its aesthetic properties, with soluble copper being more readily tasted than particulate copper (Cuppett et al. 2006). The different copper ions [Cu(I) and Cu(II)] produce a weak bitter or metallic and salty taste and an astringent mouth feel (Dietrich et al. 2004; Omur-Ozbek and Dietrich 2011).

Dissolved manganese [Mn(II)] has a population best-estimate taste threshold of 101 mg/L for Mn(II) from manganese sulphate. However, this was likely influenced by both the sulphate and Mn(II) taste contributions. The 50% population taste threshold for Mn(II) was 75.4 mg/L. No taste threshold has been established for particulate manganese. Manganese can also cause a bitter taste in drinking water, with a taste threshold of 75 to 100 mg/L for Mn(II) (Sain and Dietrich 2015). Zinc can have an undesirable astringent taste at concentrations above 5.0 mg/L (Health Canada 1987). Hydrogen sulphide has an offensive rotten egg odour and has a low odour threshold. Hydrogen sulphide (H₂S) is produced from the breakdown of organic matter in the absence of oxygen and may also be formed by sulphate-reducing bacteria. Its concentrations in groundwater are typically higher than in surface water (Health Canada 2025c).

B.2.11.3 Discoloration

Discoloured water should not be considered safe to consume or treated as only an aesthetic issue, since contaminants such as arsenic, lead, manganese, cadmium and radionuclides can be adsorbed onto the iron or manganese particles being released in water in the distribution system or premise plumbing (Health Canada 2019f; 2024a). Different colours observed in water provide an indication of the potential source of discoloration (for example, red, orange or yellow water may be linked to iron, blue or black to copper, and grey or black to manganese) (Health Canada 2025b). The accumulation and release phenomenon can also result in water discoloration. The occurrence of discoloured water should trigger sampling for metals and potentially additional distribution system maintenance (Health Canada 2019e; 2019f; 2025b).

Discoloured water occurs more frequently in stagnant areas of a municipal DWDS that is supplied with iron pipes. It can also occur when soluble iron enters the distribution system and is then oxidized to particulate iron (Health Canada 2024a; Sarin et al. 2004). The range of colours that can occur depend on the iron concentration and iron oxidation state, such as reddish-brown and yellow-orange (Health Canada 2024a).

Blue or green water events can be caused by the dispersion of fine copper corrosion products, which can also appear black depending on its composition (Health Canada 2019b). It can potentially stain bathroom fixtures (Health Canada 2019b). Depending on the distance of the observer from the coloured water, different concentrations can be visible with particulate copper being more readily visible in a bathtub than in a glass of water (Edwards et al. 2000).

Release of accumulated manganese in distribution systems can result in discoloured water – for example, when physical disturbances cause the release of scales containing manganese (Health Canada 2019f). Even at low concentrations, manganese oxides are visible, whereas dissolved manganese is not. In a study, particulate Mn(IV) was visible at a concentration of 0.005 mg/L (Sain et al. 2014). Manganese oxides can appear as a dark brown, grey or black solid and its colour varies depending on its oxidation state (Health Canada 2019f). Discoloured water can also occur due to hydraulic disturbances such as watermain breaks, hydrant flushing, changes in flow velocity and flow reversals. It can increase the turbidity of the distributed water (Health Canada 2024a; 2025b; 2025c).

B.3 Opportunistic premise plumbing pathogens

B.3.1 Characteristics of opportunistic premise plumbing pathogens

OPPPs are microorganisms that are naturally present and ubiquitous in soils and aquatic environments, including drinking water sources (Pruden et al. 2013). These microorganisms are also found in built environments, such as drinking water treatment plants and distribution systems, and premise plumbing. In premise plumbing, in particular, they establish themselves due to its features (see B.2 Causes of water quality deterioration) and, unlike enteric pathogens, are able to grow and become part of the native drinking water microbiome (Hayward et al. 2022; LeChevallier et al. 2024; Wang et al. 2017). As such, they are considered “normal inhabitants” (Cavallaro et al. 2023; Falkinham 2015b; 2020; LeChevallier et al. 2024; Mapili et al. 2022) and are primarily found in biofilms, along with other BAMs (see B.2.1 Presence of microorganisms and biofilm). Despite their presence in premise plumbing, they typically do not cause illness in healthy individuals. Infections occur predominantly in susceptible individuals, such as the elderly, those with underlying diseases or in the immunocompromised (see B.3.4 Health considerations). For this reason, they are referred to as “opportunistic” pathogens. Unlike enteric pathogens, OPPPs are not typically transmitted via ingestion. Transmission occurs largely via inhalation or aspiration of aerosols generated through the use of fittings, such as showers, but can also occur by ingestion and direct contact with skin or mucous membranes of the eye or nasal passages. Inhalation of aerosols is thought to be the most important way people become infected (Hamilton et al. 2019; Joshi et al. 2025). The health effects caused by OPPPs are diverse and range from respiratory illness to infections of the eye, skin, central nervous system or the gastrointestinal tract (see Table 4). In severe cases, illnesses caused by OPPPs can be fatal (Health Canada 2022b).

Although a number of OPPPs have been identified, members of the bacterial groups Legionella spp., Mycobacterium spp. and Pseudomonas spp. are of greatest public health significance in North America. These OPPPs have been repeatedly linked to human exposure via premise plumbing (Falkinham 2015a; 2015b; Pruden et al. 2013). The genus Legionella includes several well-known pathogenic species, of which Legionella pneumophila is the most common and virulent pathogen, responsible for more than 70% to 90% of all cases of Legionnaires’ disease (LD) in North America and Europe (Barksey et al. 2022; ECDC 2023; Peci et al. 2016). The most common serogroup causing disease is L. pneumophila serogroup 1 (Barksey et al. 2022; ECDC 2023; Peci et al. 2016). The contribution of other serogroups and species to the burden of illness is not well understood because the most widely used diagnostic method, that is, the urine antigen test (UAT), is unable to detect them (McMullen et al. 2024). NTM is a broad term that is applied to a group of approximately 186 currently-recognized environmental species of Mycobacterium spp. (Donohue 2018; Falkinham 2016). Of the more than 90 clinically relevant NTM species, members belonging to the Mycobacterium avium complex have a higher percentage of the health burden (Donohue 2018). Several species of Pseudomonas are known to cause disease in humans, with P. aeruginosa receiving the most attention because of the frequency with which it is involved in human disease (Chakravarty and Anderson 2015). Acanthamoeba spp. and Naegleria fowleri are also increasingly recognized for their contribution to illness related to premise plumbing exposures (Falkinham 2015b; Lu et al. 2016; Wang et al. 2013).

This document provides information on the occurrence, health risks and management of the primary OPPP groups with emphasis on the established pathogens of most concern: Legionella pneumophila, Mycobacterium avium, Acanthamoeba spp., Naegleria fowleri and Pseudomonas aeruginosa. Table 3 summarizes some of their key attributes. Additional information can be found in (Health Canada 2022b).

Other OPPPs, such as Acinetobacter baumannii, Stenotrophomonas maltophilia, Burkholderia cepacia and Sphingomonas paucimobilis, are less common and their public health significance is unclear. Information about these OPPPs and others can be found elsewhere (Falkinham 2015b; LeChevallier et al. 2024). It is important to note that, in most cases, efforts to manage the established OPPPs will also be effective for managing these microorganisms (LeChevallier et al. 2024).

The established OPPPs share common features (Falkinham 2015b; 2023a; Leslie et al. 2021) that contribute to their survival and proliferation in DWDSs and premise plumbing, including:

• surface adherence and biofilm formation
• growth in amoebae
• disinfectant resistance
• temperature tolerance
• growth at low oxygen and organic carbon concentrations

B.3.1.1 Surface adherence and biofilm formation

A key attribute shared by OPPPs is their relatively high cell surface hydrophobicity that drives their preference for surface attachment over water suspension (Buse et al. 2018; Falkinham 2021; Lytle et al. 2004), leading to the formation of biofilms. Biofilms are complex structures consisting of microorganisms that cling to each other and/or to surfaces, and are embedded in a gelatinous matrix of compounds, referred to as extracellular polymeric substances (EPS), which they excrete (see B.2.1 Presence of microorganisms and biofilm). The EPS structure provides protection against predators and disinfectants (Simões et al. 2009), and aids in access to nutrients and in their uptake and utilization (Flemming and Wingender 2010; LeChevallier et al. 1988; Prest et al. 2016). OPPPs in biofilms display increased resistance to a range of stressors in comparison to their free-floating counterparts (Costerton et al. 1995).

Surface adherence and biofilm formation can be enhanced by conditions in premise plumbing (see B.2 Causes of water quality deterioration).

B.3.1.2 Growth in amoebae

Another common feature of OPPPs is their ability to reside and proliferate within FLA (Falkinham 2015b; Falkinham et al. 2015a; 2015b; Pruden et al. 2013). FLA, such as Acanthamoeba and Naegleria, graze on other microorganisms, including other OPPPs, found in biofilms in DWDSs and premise plumbing (Smirnov et al. 2005; Thomas and Ashbolt 2011). When consumed by FLA, these OPPPs typically resist digestion and survive (Thomas and Ashbolt 2011). Legionella, mycobacteria and Pseudomonas are among the OPPPs shown to reside within amoeba. In the case of L. pneumophila, its primary growth is within Acanthamoeba (Bouyer et al. 2007).

These amoebae provide OPPPs with a source of nutrients and a means of transport (Buse et al. 2012; NASEM 2020; Percival and Williams 2014), therefore ensuring survival. Microorganisms are also shielded from harsh conditions, such as the presence of disinfectants and elevated water temperatures. Moreover, damaged microorganisms, such as those in a viable but nonculturable state, are able to heal within amoeba hosts (Li et al. 2014; Oliver 2010). Growth of microorganisms in FLA has also been shown to endow these microorganisms with novel or modified characteristics, such as increased virulence (Brieland et al. 1997; Cirillo et al. 1997). The overall result is that OPPPs within FLA are protected and may grow to concentrations that are considered to be a potentially infectious dose (Ashbolt 2023; Shaheen and Ashbolt 2018) (see B.6 Management strategies).

While OPPPs are capable of persisting in biofilms in the absence of a host FLA, their replication occurs predominantly in amoebae (Ashbolt 2023; NASEM 2020). Gene expression studies suggest that they have evolved in order to control their amoeba hosts, ensuring their growth and transmission (Burstein et al. 2016; Chauhan et al. 2023; Li et al. 2020; Quan et al. 2020). Further proof of their reliance on amoeba for dissemination comes from detection studies which frequently report that they occur together (Lau and Ashbolt 2009; Logan-Jackson and Rose 2021; Thomas et al. 2006; 2008).

Given the crucial role that FLA such as Acanthamoeba and Naegleria play as a host, reservoir and vehicle for other OPPPs, they represent a much broader public health concern than the direct role they play as pathogens (Pruden et al. 2013; Thomas and Ashbolt 2011).

B.3.1.3 Disinfectant resistance

Most OPPPs are inherently more resistant than fecal indicators to disinfectants used in water treatment (Falkinham 2015b; Falkinham et al. 2015b; Pruden et al. 2013). Resistance varies between genera and among species and strains. Mycobacterium spp., for example, are highly resistant. M. avium, in particular, is approximately 1 000 times more resistant than fecal indicators to chlorine and other (chemical) disinfectants (Taylor et al. 2000). Acanthamoeba and N. fowleri, particularly their cysts, are also considerably resistant to disinfectants (Bartrand et al. 2014; Health Canada 2019c; Hijnen et al. 2011; Loret et al. 2008). P. aeruginosa and Legionella are typically inactivated by free chlorine at concentrations commonly applied during water treatment. However, Legionella can adapt to fluctuating conditions by differentiating into cell types that are more resistant to disinfection (NASEM 2020; Robertson et al. 2014). As noted above, OPPP resistance to disinfectants is enhanced in biofilms and in FLA.

As a result of disinfectant resistance amongst OPPPs, they may survive water treatment and enter the DWDS, where they grow and are transported to residences.

B.3.1.4 Temperature tolerance

OPPPs are able to survive over a wide range of temperatures, well outside their optimal growth ranges (see Table 3) (Pruden et al. 2013). Acanthamoeba cysts, for example, have been shown to withstand temperatures ranging from -20 to 56 °C (Chalmers 2014). N. fowleri cysts can tolerate temperatures up to 60 °C (Health Canada 2022b). In the case of Legionella, survival was observed after several hours at 50 °C (Bartram et al. 2007; Dennis et al. 1984; Konishi et al. 2006), several minutes at 60 °C (Leoni et al. 2005), and even after a heat shock treatment of 30 minutes at 70 °C (Allegra et al. 2011). Survival of Legionella has also been observed at low temperatures, between 20 and 25 °C, where they were still able to grow, although slowly (Kusnetsov et al. 2003; van der Kooij 2014). Mycobacteria and P. aeruginosa can grow at temperatures as low as 15 °C (George et al. 1980) and 4 °C (Chakravarty and Anderson 2015), respectively.

B.3.1.5 Growth at low oxygen and organic carbon concentrations

Dissolved oxygen concentrations decrease in water in premise plumbing during periods of low flow or stagnation (see B.2.9 Variable flow rates and water use) (Wang et al. 2012a). However, some OPPPs can adapt and grow at low oxygen levels (Falkinham 2015b; Falkinham et al. 2015a; 2015b). P. aeruginosa and M. avium, in particular, can grow well in microaerobic environments (Falkinham 2015b; Leslie et al. 2021; Lewis and Falkinham 2015).

In an effort to minimize degradation of microbial water quality during distribution, drinking water suppliers try to limit the level of nutrients, including assimilable organic carbon (AOC), in treated water (Camper et al. 2003; LeChevallier et al. 1991; van der Kooij 2003). While this strategy has proven effective in minimizing total bacterial growth, it does not appear to limit OPPP growth (Falkinham 2023b; Proctor et al. 2017). Mycobacteria and P. aeruginosa, in particular, seem to prefer a lower range of AOC (Proctor et al. 2017). L. pneumophila and amoeba growth is not limited by low levels of AOC, as these organisms turn to alternative nutrient sources (Temmerman et al. 2006; Thomas and Ashbolt 2011).

B.3.2 Exposure

Water exposures are the most significant pathways of concern for OPPPs. As discussed in B.3.1 Characteristics of opportunistic premise plumbing pathogens, these microorganisms are normal inhabitants of natural waters, DWDSs and premise plumbing. Therefore, humans are regularly exposed to OPPPs in the community. Depending on the conditions in the plumbing system (see B.2 Causes of water quality deterioration), OPPPs can amplify to levels likely to impact persons that are at greater risk for infection (see B.3.3 Transmission). Data from studies of the occurrence of OPPPs in residential plumbing systems were analyzed to determine the frequency of detection in these locations (see Part D). Studies were limited to those conducted within the northern U.S. and Canada to assess findings from locations that have plumbing systems and climate conditions relevant to Canada. Given the various study approaches and analytical methods applied, comparisons of occurrence and concentration across studies are challenging. Figure 6 shows the percentage of residential dwellings in the northern U.S. and Canada that tested positive for OPPPs in drinking water using culture and polymerase chain reaction (PCR) methods. The following scale was used to evaluate the frequencies of OPPP detections reported in individual studies: Never: 0%, Rarely: 1% to 15%, Occasionally: 16% to 49%, Often: 50% to 75%, Usually: 76% to 99%, Always: 100%.

Figure 6: Text description

Water source type	Microorganism	qPCR	Culture
Municipal	Acanthamoeba spp	14%	15%
Municipal	Legionella spp.	53 to 100%	8 to 14%
Private well	Legionella spp.	5 to 71%	0 to 18%
Municipal	L. pneumophila	0 to 63%	0 to 40%
Private well	L. pneumophila	15%	2%
Municipal	NTM or Nontuberculous Mycobacteria	100%	42 to 58%
Private well	NTM or Nontuberculous Mycobacteria	0 to 41%	0%
Municipal	M. avium	24 to 75%	23 to 38%
Private well	M. avium	10 to 100%	0%
Municipal	P. aeruginosa	3 to 100%	n/a

elec. – electric

^* Small buildings (single story homes and businesses); ^† private and community wells; ^‡ data reported as all households, but no detections were noted for private wells.

Data from the studies presented in Part D and Figure 6 showed that:

• Legionella spp. and NTM were detected with wide variation in frequency by culture and quantitative PCR (qPCR) in water from plumbing systems of residential dwellings on municipal drinking water supplies and residential homes on private wells. In general, detections were more frequent in municipally-supplied homes than in private well-supplied homes.
• Both L. pneumophila and M. avium could be detected at hot (or mixed hot and cold) and cold water taps, by culture and qPCR.
• In studies of municipally-supplied homes where the majority of water heaters were gas hot water tanks, L. pneumophila detection varied from never to often (0% to 63%) by qPCR and was rarely detected (less than 15%) by culture.
• Culturable L. pneumophila in water was occasionally detected in homes possessing electric hot water tanks (39% to 40%) but never in homes with oil or gas hot water tanks (0%) in Québec City area studies.
• M. avium was occasionally detected in water (23% to 38%) in municipally-supplied homes by culture and occasionally to often detected (24% to 75%) by qPCR.
• P. aeruginosa and Acanthamoeba spp. were variably detected in water in municipally supplied homes by qPCR or culture.
• In homes supplied by private wells, one study reported detection of intact L. pneumophila using viability qPCR, and no studies reported detection by culture. Culturable M. avium was never detected in water samples but was detected in a biofilm sample in one study.
• No studies were found concerning the detection of Naegleria fowleri in residential premise plumbing in the northern U.S. and Canada.

Table D. 1 and Table D. 2 in Part D provide details on the OPPP concentrations encountered in the studies. Data from municipally-supplied homes showed that, for the broader OPPP genera, Legionella spp. and Mycobacterium spp. (NTM), qPCR concentrations in positive samples spanned a large range from below quantifiable limits to approximately 6 log and 7 log gene copies/L, respectively (Haig et al. 2020; Rhoads et al. 2020; Schwake et al. 2016; Wang et al. 2012a). NTM have been found to be, on average, the most abundant group of bacteria found in showerhead biofilm samples (Feazel et al. 2009; Gebert et al. 2018). For L. pneumophila qPCR concentrations in municipal homes, one study reported that the 90^th percentile of positive samples were below quantifiable limits (Rhoads et al. 2020), whereas others reported values ranging from approximately 2 to 4 log gene copies/L (Donohue et al. 2019a; Wang et al. 2012a). Culture-based data for L. pneumophila in municipally-supplied homes showed that positive samples often had concentrations that were higher than the 10 colony forming units (CFU)/mL (10 000 CFU/L) value for uncontrolled growth in potable water proposed by the CDC (CDC 2021c). Values of 4 to 5 log CFU/L were reported for some hot water tank and hot water tap samples (Arnow et al. 1985; Dewailly and Joly 1991; Gleason et al. 2023; Stout et al. 1992b). For M. avium qPCR values in municipal homes, concentrations in positive samples ranged from 1.5 to 4.4 log gene targets/L (Donohue et al. 2019a; Pfaller et al. 2022; Wang et al. 2012a) and no culture-based count data were reported.

Among the data for homes served by private wells, Clark et al. (2025) showed that L. pneumophila qPCR concentrations were above the level of quantitation at only 1 of 23 sites where it was detected. Pfaller et al (2022) reported qPCR concentrations for M. avium ranging from 1.7 to 2.7 genomic units/L and for M. intracellulare ranging from 1.4 to 7.1 genomic units/L in private well homes. No studies reported culture-based detection of L. pneumophila or M. avium.

Very few studies reported data on the detection of OPPPs in drinking water in multi-dwelling residential buildings (Arnow et al. 1985; Donohue et al. 2019a; Feazel et al. 2009; Marrie et al. 1994). Arnow et al. (1985) reported similar rates of detection of culturable L. pneumophila among homes (25%) and multi-dwelling residential buildings (32%). Higher rates of OPPP detection among residential apartments compared to single-family homes were reported by Marrie et al. (1994) for Legionella spp. (25% versus 8% respectively) and Donohue et al. (2019a) for M. avium (50% versus 28% respectively). Feazel et al. (2009) reported higher detection rates of M. avium gene sequences in showerhead biofilms from condominiums and apartments (22%) compared to single-family homes (0%). However, follow-up testing indicated that the detection rates in both types of dwellings may be underestimates (Feazel et al. 2009).

Additional insights are provided from studies modelling residential water activities using a simulated home plumbing system. Gomez-Alvarez et al. (2023) observed OPPP growth within the electric hot water heater and other parts of the hot water system (average temperature: 25.5 to 31.3 °C), with L. pneumophila (5.5 to 5.7 log gene copies/L) being the most prevalent pathogen, followed by M. intracellulare (3.4 log gene copies/L) and P. aeruginosa (3.3 log gene copies/L). M. avium and M. abscessus were not detected. In a follow-up study, Gomez-Alvarez et al. (2024) showed that temperature stratification in the electric hot water heater set to 49 °C produced conditions (temperatures of 35 to 42 °C, lower chlorine residual, prolonged stagnation) resulting in higher Mycobacterium gene copy numbers and culturable L. pneumophila counts at the bottom of the tank.

Correlations between OPPP detections in homes and plumbing system characteristics have been observed. The temperature of hot water is the most important determinant for multiplication of Legionella and other OPPPs in residential premise plumbing. Authors reported more frequent Legionella spp. and L. pneumophila detection associated with hot water heater temperatures less than 40 °C in the bottom sediments of electric heaters and less than 51 °C among homes that (for the most part) had a gas heater (Dewailly and Joly 1991; Gleason et al. 2023; Rhoads et al. 2020; Stout et al. 1992b). Temperatures of less than 54 to 56°C at distal fittings were also associated with more frequent Legionella spp. and L. pneumophila detection (Alary and Joly 1991; Arnow et al. 1985; Dewailly and Joly 1991; Gleason et al. 2023; Stout et al. 1992b). In an Australian survey, Legionella qPCR detection was also associated with instantaneous hot water heaters where the mean maximum hot water heater temperature recorded in the study was below 50 °C (Hayes-Phillips et al. 2019). The type of hot water heater is also an important contributor to the detection of Legionella in residential homes.

Canadian studies have demonstrated a higher prevalence of L. pneumophila in homes with electric water heater tanks (Alary and Joly 1991; Dewailly and Joly 1991). Rhoads et al. (2020) reported very low detection of L. pneumophila in Flint, Michigan homes during a major disease outbreak at a local hospital and despite hot water heater temperatures often being below the recommended levels for inhibiting Legionella growth. The high proportion of gas water heaters in the study (60%) was proposed as a factor contributing to the low prevalence (Rhoads et al. 2020). Rhoads et al. (2020) observed higher Legionella spp. (but not L. pneumophila) qPCR concentrations in homes with influent free chlorine of less than 0.4 mg/L compared to homes that had influent free chlorine of greater than 0.4 mg/L. Gleason et al. (2023) observed that samples with chlorine of less than 0.1 mg/L were more likely to have culture detection of Legionella spp. compared with samples with a chlorine residual of greater than or equal to 0.1 mg/L. Haig et al. (2020) found that NTM had a higher frequency of detection among homes where chloramines were used as a residual disinfectant. Studies have shown that chloramine residual disinfectant can provide better control of Legionella in plumbing systems but can also lead to a higher frequency of the detection of some Mycobacterium spp. (Donohue et al. 2019b; Pfaller et al. 2022). Chloramine penetration into biofilms can shift host amoebae populations to the cyst stage, thereby reducing Legionella growth potential because it is only capable of growing in trophozoites (NASEM 2020). In contrast, mycobacteria are very chlorine- and chloramine-resistant and can compete better in biofilms when other less-resistant microorganisms have been killed (Bal Krishna et al. 2021; Waak et al. 2019). In general, the findings indicate that maintenance of an effective disinfectant residual, regardless of type, can help limit the occurrence of Legionella spp. and mycobacteria in drinking water biofilms (Bartrand et al. 2024; Buse et al. 2019; Donohue et al. 2019b; Norton et al. 2004).

The detection of OPPPs in residential premise plumbing, in some cases at considerable frequencies and concentrations, is evidence that OPPPs are normal inhabitants of premise plumbing. Further, characteristics of the residential plumbing environment (for example, where there are growth-permissive water temperatures or the water stagnates) can result in conditions that are favourable for OPPP growth. OPPP species (such as L. pneumophila and M. avium) that are important causes of infectious disease have been detected by molecular and culture methods in municipally-supplied residences and by molecular methods in private well-supplied homes. PCR methods detect OPPPs at higher frequencies than culture-based methods, and it is difficult to interpret what DNA detection means (Whiley and Taylor 2016). Detection of gene markers does not provide information on viability and infectivity but indicates that the organisms were alive at some point in the system (Dai et al. 2019; Schwake et al. 2016), and high levels can provide evidence of multiplication. Overall, the evidence suggests that detection of the species most significantly associated with human illness, namely L. pneumophila and M. avium, is more frequent among large multi-unit residential buildings than among single-family homes (Arnow et al. 1985; Feazel et al. 2009; Marrie et al. 1994). Additionally, reported rates of detection of culturable L. pneumophila and M. avium in plumbing systems for single-family homes were low in general and the frequency of detection for plumbing systems of homes supplied by private wells appears lower than that observed for municipally-supplied homes (Byrne et al. 2018; Clark et al. 2025; Pfaller et al. 2022; Rhoads et al. 2020; Richards et al. 2018; Tzou et al. 2020).

It is evident that a wide variety of factors influence pathogen fate, transport in drinking water systems and the capability for growth in residential plumbing systems (see B.2 Causes of water quality deterioration), ultimately influencing detection patterns in individual residences. Having good water quality management practices in place helps to limit OPPP growth in residences (see B.6 Management strategies).

B.3.3 Transmission

Despite the presence of OPPPs in premise plumbing, their ability to cause illness depends on the occurrence of specific events and favourable conditions. This series of events and conditions is referred to as the chain of transmission, and includes: 1) the presence of OPPPs in premise plumbing, and their persistence and growth due to conditions therein (see B.2 Causes of water quality deterioration); 2) the capacity of premise plumbing components, such as fittings, to act as a source and dispersal mechanism for OPPPs (for example, to generate aerosols containing OPPPs); 3) having individuals present in the dwelling and a pathway of entry into their body (for example, by the inhalation of aerosols); 4) the ability of OPPPs to reach target areas in the body (for example, to be deposited into the lungs) and be capable of initiating infection; and 5) the individual being susceptible to infection (ASHRAE 2023a; CCIAQ 2023; NASEM 2020). Figure 7 illustrates the primary chain of transmission for Legionella.

Figure 7: Text description

• Conditions that favor Legionella growth in building water systems:
  • high-rise with cooling towers on the roof
  • bacteria in biofilm and water in water pipes
  • water of different temperatures
• Aerosolization of contaminated water
  • cooling towers spraying mist
  • showerheads spraying water
  • decorative fountains with water jets
• Inhalation or aspiration of aerosols containing Legionella species
  • inhaling water from, for example, showerheads, containing bacteria
• Ability of Legionella species to initiate infection and susceptibility of the human host
  • Legionnaires’ disease
  • Pontiac fever
  • No illness/asymptomatic

As noted in B.3 Opportunistic premise plumbing pathogens, OPPPs are not typically transmitted via ingestion. Instead, infections are primarily associated with the inhalation or aspiration of aerosols (Health Canada 2022b; Pruden et al. 2013). Thus, the generation of aerosols, also referred to as aerosolization, is a critical step in the chain of transmission. OPPPs are particularly prone to aerosolization due to their relatively hydrophobic cell surfaces (Falkinham 2021; Lytle et al. 2004). Aerosols can be generated by various components of premise plumbing. They are created when water is splashed, dripped, sprayed, misted, bubbled or jetted (CCIAQ 2023). While both fittings and fixtures can generate aerosols, fittings, especially showers, represent an important source of Legionella and mycobacteria (see B.2.10.1 Design), which can lead to infection (Falkinham 2011; 2015a; 2021; Falkinham et al. 2008; Feazel et al. 2009; Hamilton et al. 2019; NASEM 2020; Pruden et al. 2013).

Once aerosols are generated, they can remain suspended in the air and travel (long distances), depending on air currents (NASEM 2020). Legionella in aerosols has been shown to survive for several hours, depending on the humidity (Berendt 1980; Dennis and Lee 1988; Hambleton et al. 1983). In order for aerosolized OPPPs to be inhaled and (potentially) cause illness, they must remain airborne. A critical factor determining how long aerosols remain in the air is their size. The larger the aerosol, the greater the settling velocity, and the faster it will fall to the ground (Hinds 2012). Aerosol size varies depending on a number of factors, including the source of the aerosol and the water temperature. In the case of showering with hot water (43 to 44 °C at showerhead, 35 to 37 °C at mannequin), for example, Zhou et al. (2007) reported aerosols with a mean diameter between 6.3 to 7.5 μm inside the shower stall. Aerosols were significantly smaller (50% smaller, 1 to 5 μm) when showering with cold water (24 to 25 °C at showerhead and mannequin). Aerosols generated using cold water were less concentrated inside the shower stall than those generated using hot water and were not detected outside the stall. Aerosol size was independent of the water flow rate.

In order for infection to occur, OPPP-containing aerosols must be inhaled by susceptible individuals (see B.3.4.1 Health effects) and find their way into the lungs. Typically, aerosols of less than 10 μm will be deposited into the lungs (NASEM 2020); there, OPPPs contained in these aerosols can replicate, initiate infection, and cause illness in susceptible persons.

B.3.4 Health considerations

B.3.4.1 Health effects

As noted in B.3.1 Characteristics of opportunistic premise plumbing pathogens, infections caused by OPPPs can be in the lungs, skin, bloodstream, eye or nasal passages depending on the point of entry (POE) of the infecting microorganism. Also, OPPPs are microorganisms that, in general, pose a higher risk for individuals possessing one or more specific risk factors that make them more susceptible to infection. The characteristics of susceptible individuals varies with OPPP (see Table 4).

Overall, immunocompromised individuals are considered to be at high risk for OPPP infections and for experiencing more severe illnesses if infected (CDC 2025a; 2025c). Several diseases and conditions can weaken the immune system such as cancer, diabetes, kidney failure, liver failure, HIV/AIDS and certain medications like chemotherapy or those taken after a transplant operation. As lung infections are the most common form of illness from exposure to OPPPs in premise plumbing, persons with lung diseases such as Chronic Obstructive Pulmonary Disease, emphysema or cystic fibrosis are also notably at increased risk for infection and for becoming sick if infected (CDC 2025a; 2025c).

The number of people considered to be at risk for OPPP infections in general may be greater than what has traditionally been implied by the term “opportunistic” (Pruden et al. 2013). OPPP infections have occurred in healthy individuals with no known risk factors and, for some OPPPs, simply being of older age puts a person at increased risk (Pruden et al. 2013). As a result, a sizable portion of the population could be considered “at risk” (Pruden et al. 2013). Between 2022 and 2050, it is projected that the global number of persons aged 65 and older will more than double (UN 2022). Collectively, Europe and North America may expect to see a 44% increase in the size of this age group over this period (UN 2022). As the number of older individuals and those with immunodeficiencies and/or underlying diseases increases, so will the impact of OPPPs.

B.3.4.2 Waterborne illness

The degree to which exposure to OPPPs in residential premise plumbing systems contributes to human illness is difficult to quantify because not all cases of disease are recognized, investigated and reported. In Canada, there is no national surveillance system specific to waterborne illness (Pons et al. 2015). Provinces and territories report notifiable disease data to the federal government (Public Health Agency of Canada 2024) based on the disease (for example, campylobacteriosis) rather than the route of transmission (for example, waterborne). Thus, there is limited information regarding the magnitude and sources of illness attributed to drinking water. Additionally, sources of OPPP infections are difficult to confirm for many reasons, including: 1) the majority of cases are sporadic and not associated with outbreaks; 2) there are a multitude of different water sources a person could be exposed to; 3) patient isolates may not be available; 4) the time delay between when a case is reported to public health and when environmental sampling is conducted; and 5) the limitations of detection methods (Kirschner 2016; Orkis et al. 2018). Sources of NTM infection can be particularly difficult to identify due to the lengthy time lag (such as months to years) that can occur between infection and when diagnosis is made (Falkinham 2015a; Field et al. 2004).

Legionellosis is the only OPPP-related disease that is a nationally notifiable disease in Canada. From 2015 to 2019, an average of 438 LD cases per year were reported in Canada (Public Health Agency of Canada 2023). However, when accounting for underdiagnosis and underreporting, the true number of waterborne LD over that period is an estimated average of 1 113 cases per year (3.06 cases/100 000 population), roughly 2.6 times higher than what is reported (McMullen et al. 2024). NTM is thought to be as important as Legionella as a cause of illnesses, hospitalizations and deaths (Collier et al. 2021; Greco et al. 2020); however, national data for NTM are not available in Canada because it is not a nationally notifiable disease. Marras et al. (2013) reported that the rates of NTM pulmonary disease in Ontario from 2006 to 2010 ranged from 9.7 to 10.7 cases/100 000 population per year. Estimates of the burden of waterborne illness in the U.S. and Ontario suggest that Legionella spp., NTM and Pseudomonas aeruginosa account for most of that burden, contributing to over 70% of estimated hospitalizations and more than 90% of estimated deaths (Collier et al. 2021; Greco et al. 2020). The incidence of disease caused by Legionella spp., NTM and Acanthamoeba spp. is increasing in Canada (Karsenti et al. 2022; Marras et al. 2013; Public Health Agency of Canada 2023) and is similar to trends reported in the U.S. (Pruden et al. 2013). For legionellosis, the rate of reported cases has increased more than eightfold since 2000 (Public Health Agency of Canada 2023).

In the U.S., Legionella is the leading cause of outbreaks associated with drinking water, accounting for nearly half of all reported outbreaks (Holsinger et al. 2022; NASEM 2020). However, according to CDC data, less than 5% of all U.S. LD cases are attributed to outbreaks (CDC 2011). Most outbreaks of OPPP disease have been linked to large building water systems in healthcare and hotel settings, with a smaller number of outbreaks having been associated with residential buildings and residential homes (Falkinham et al. 2015a; Hlavsa et al. 2018; Holsinger et al. 2022). Of the 184 Legionella drinking water outbreaks reported to the CDC’s surveillance system, between 2001 and 2007, 13 (7%) were associated with apartment buildings and one (0.5%) was associated with a private residence.

For most OPPPs (Legionella spp., NTM, Acanthamoeba spp., N. fowleri), instances of illness are sporadic community-acquired cases of disease (CDC 2022b; Griffith et al. 2007; Kaufman and Tu 2022; Yoder et al. 2010). Residential premise plumbing systems are suspected to be substantial sources of sporadic cases of OPPP-related illness (Hayward et al. 2022; Logan-Jackson et al. 2023; Orkis et al. 2018). Orkis et al. (2018) reviewed and ranked sources of sporadic LD based on the strength of the epidemiological evidence. Potable water systems in single-family homes and apartment buildings were identified as definite sources of LD (Orkis et al. 2018). Although the data were not sufficient to show the relative importance of different sources, apartment buildings (and other large buildings) were highlighted as sources likely to pose a significant risk (Orkis et al. 2018). Studies in Canada and the U.S. have confirmed molecular matches between isolates from plumbing systems in single-family homes and apartments and clinical isolates from cases of sporadic disease for multiple OPPPs, including L. pneumophila (Laverdière et al. 2001; Schumacher et al. 2020; Silk et al. 2013; Stout et al. 1992a), NTM (Falkinham 2011; Falkinham et al. 2008; Marras et al. 2005; Tichenor et al. 2012) and Acanthamoeba spp. (Mubareka et al. 2006).

In other North American studies, water from residential premise plumbing was identified as the likely source of exposure for various OPPP infections based on the epidemiological and environmental investigations, even in the absence of a confirmed molecular linkage (Barton et al. 2017; Cope et al. 2015; Granseth et al. 2017; Phillips et al. 1987; Rose et al. 1983; Yoder et al. 2012). For P. aeruginosa, most infections occur within health care settings (sporadically and in outbreaks), while community-acquired infections are primarily limited to recreational water exposures involving the use of contaminated swimming pools or hot tubs at hotels and motels (Bédard et al. 2016b; Pruden et al. 2013). In very rare cases, infections with N. fowleri have been linked to premise plumbing in southern U.S. states and attributed to activities such as nasal cleansing or recreational water uses (Bartrand et al. 2014; Yoder et al. 2010; 2012).

Enhanced surveillance, where intensive environmental sampling is conducted, has been used in some European countries to better understand the sources of community-acquired LD (Buchholz et al. 2020; Den Boer et al. 2015). Studies in the Netherlands (Den Boer et al. 2015) and in Germany (Buchholz et al. 2020) have estimated that 20% to 25% of LD cases reported in these countries could be attributed to residential drinking water.

B.3.5 Risk of illness (and Quantitative microbial risk assessment)

As mentioned earlier (see B.3.4.2 Waterborne illness), it is challenging to determine the extent to which exposure to OPPPs in residential premise plumbing contributes to human illness. Quantitative microbial risk assessment (QMRA) is a tool that can be used to estimate the health risks associated with exposure to OPPPs (Haas et al. 2014; Health Canada 2019a) and inform risk management decisions and strategies, such as the establishment of water treatment requirements or critical concentrations. QMRA follows a common process that includes hazard identification, exposure assessment, dose-response assessment and risk characterization (Health Canada 2019a). QMRA estimates are typically expressed in relation to a target risk level, also referred to as a level of acceptable risk. In the U.S., it is common to use an annual risk level of 10⁻⁴ infections/year (Regli et al. 1991), whereas the World Health Organization and the Guidelines for Canadian Drinking Water Quality use 10⁻⁶ disability-adjusted life years (DALY)/person-year (Havelaar and Melse 2003; Health Canada 2019a; NASEM 2020; WHO 2022a). It is important to note that both acceptable risk levels were developed based on the risks associated with enteric pathogens, which typically include mild to moderate health consequences. Cryptosporidiosis, for example, is associated with a 0.3 ratio of DALYs per 100 infections. In the case of Legionella, the severity of LD corresponds to more than a 300-fold higher ratio of DALYs per infection (NASEM 2020).

Several QMRA models have been developed to characterize the legionellosis risk from exposure to waters containing L. pneumophila under various scenarios (Ahmed et al. 2010; Armstrong and Haas 2007; 2008; Azuma et al. 2013; Bouwknegt et al. 2013; de Man et al. 2014; Sales-Ortells and Medema 2014; 2015; Schoen and Ashbolt 2011; Storey et al. 2004). Models vary in terms of their assumptions and mathematical approaches, as no standardized Legionella QMRA approach exists. In an effort to harmonize methodologies, Hamilton and Haas (2016) proposed a framework that can be applied when conducting a Legionella QMRA (see Figure 8). This framework outlines a number of key parameters and steps in the chain of transmission, including the risk from inhaled aerosols, using dose-response modelling.

A QMRA requires a substantial number of input parameters, each of which may have uncertainty. However, in the absence of precise data, risk can be calculated by estimating the uncertainties for each input and carrying them through the calculations (NASEM 2020). A sensitivity analysis can also be conducted to identify the parameters with the greatest impact on the uncertainty surrounding the risk estimate.

Figure 8: Text description

1. recovery-corrected concentration of Legionella in water and biofilms
2. Legionella from biofilms enter distributed water
3. aerosol generation rate (scenario specific)
4. bacterial enrichment for systems in which bubbles or condensed mists are generated
5. size distribution of generated aerosol
6. size-resolved bacterial partitioning in aerosol
7. aerosol transport, dispersion, evaporation and deposition
8. bacterial survival, virulence, viability during transport as a function of environmental conditions
9. inhalation
10. size-resolved deposition of aerosol at the alveoli
11. dose-response model
12. risk characterization and sensitivity analysis

Using the above framework, Hamilton et al. (2019) estimated critical concentrations of L. pneumophila in aerosols from indoor residential water uses from showers, faucets and toilets. Showering risks were highest and risks from toilets were the lowest. When using a target risk level of 10^-4 infections/person/year, the median concentration of L. pneumophila was 1.41×10³ CFU/L. This is in line with most guidance document recommendations of less than 1 CFU/mL (ASHRAE 2023b; CDC 2021c; ESGLI 2017; HSE 2024). In contrast, when using a target risk of 10^-6 DALY/person/year, the median concentration of L. pneumophila was 1.44×10¹ CFU/L, which is at or below the limit of detection of culture monitoring methods (ASHRAE 2023b; Bukhari et al. 2023). Clark et al. (2025) modelled the risk of L. pneumophila infection and illness in Illinois rural homes supplied by private wells and assessed that it was lower compared to that estimated for homes supplied by municipal systems. However, the L. pneumophila illness risk for private well-supplied homes was not negligible and was greater than the 10⁻⁴ target metric (or 1 in 10 000) for the annual risk for adults when showering, particularly for the elderly age group (Clark et al. 2025). In general, burden of illness estimates suggest that populations served by private wells are at greater risk of contracting acute gastrointestinal illness from home water supplies than OPPP-related illness (Hynds et al. 2014; McMullen et al. 2024; Murphy et al. 2016).

Risk estimates should be interpreted with caution, as they are dependent on several assumptions, including those related to the spatial and temporal distribution of L. pneumophila within premise plumbing. They also depend on the quality of data used to generate them and how those data were acquired -- for example, detection using culture versus molecular methods (Hamilton and Haas 2016). Bentham and Whiley (2018) discuss the uncertainty that is associated with factors used to develop a QMRA. A limitation of most Legionella QMRA models is the assumption that hosts are equally susceptible to infection. To date, only one QMRA model has incorporated varying host susceptibilities when estimating legionellosis risk within premise plumbing (Weir et al. 2020). Despite their limitations, these models fill key gaps and help answer questions about how OPPP risks can be best mitigated and/or managed.

QMRAs for other OPPPs, such as Pseudomonas aeruginosa and NTM, have not been performed due to the lack of accepted dose-response models for these microorganisms (Ashbolt 2015; Hamilton et al. 2017). Hamilton et al. (2017) conducted a critical review of the infectivity data for M. avium and identified dose-response models that can be applied for lung infections, such as was done in developing Legionella QMRA estimates. The authors have also proposed a framework for incorporating the dose-response models into a QMRA.

B.4 Chemical contaminants

B.4.1 Sources of exposure

Consumers can be exposed to inorganic and organic chemical contaminants in drinking water. These contaminants can come from different sources such as surface water or groundwater, from treatment processes (such as aluminum- or iron-based coagulants) or leaching/release from the distribution system, and/or premise plumbing system materials (for example, lead service lines, brass fittings, copper pipes, galvanized pipes and components). Inorganic contaminants (such as lead, arsenic, cadmium, copper, iron and manganese) present in drinking water can be deposited in the distribution system where they accumulate (Bradham et al. 2023; Friedman et al. 2010; Health Canada 2025b; Hill et al. 2010; Lytle et al. 2014; Peng et al. 2012; Schock et al. 2008; Triantafyllidou et al. 2019). The primary mechanisms by which inorganic contaminants accumulate in the distribution system are adsorption and co-precipitation onto solids, particularly iron particles and corrosion scales (for example, hydrous iron oxides), aluminum solids and manganese solids (for example, hydrous manganese oxides) (Health Canada 2024a).

If physical (hydraulic), chemical, and microbially-influenced changes occur, these inorganics can be released into the water, potentially resulting in increased concentrations at the tap. The accumulation and release phenomenon can also result in discoloration (for example, red, yellow, orange, grey or black water) events. These events are an indicator of potential exposure to chemical contaminants of health concern, such as lead, manganese and arsenic. When iron, manganese and aluminum are present in the distribution and plumbing systems, they act as sinks, adsorbing metals and transporting them to the tap (Health Canada 2019e; 2019f; 2021b; 2024a; 2025b).

Organic contaminants such as DBPs can also be formed within DWDSs as a result of disinfection processes. DBPs are formed within the DWDS and premise plumbing, and consumers can be exposed at the POU.

B.4.2 Exposure

Exposure to these contaminants can occur at home, school or the workplace where different contaminants may be present. It is not possible to provide a typical range of concentrations for specific contaminants to which people in Canada can be exposed through drinking water, as they can be highly variable due to several factors. Data on individual contaminants are listed by jurisdiction in each specific Guideline Technical Document under the Guidelines for Canadian Drinking Water Quality.

Wide variations in concentrations of lead have also been reported in the literature across a distribution system or within an individual site based on a number of factors such as source of lead (such as lead service line), stagnation time, age of the plumbing system, water quality (for example, pH and alkalinity) and sampling protocols for a variety of objectives (such as compliance, exposure or corrosion control). Due to this high variability, it is challenging to assess the exact exposure to lead from drinking water (Health Canada 2019e). Lead service lines have been shown to be a consistently high source of lead and contribute 50% to 86% of the total lead at the tap after extended stagnation times (Lytle et al. 2019; Sandvig et al. 2008).

Copper concentrations are less variable than lead and generally well below the guideline value for health. Exposure from drinking water is generally considered low although some high concentrations have been reported in the compliance data submitted in the copper guideline document (Health Canada 2019b).

B.4.3 Health effects

Key health information associated with exposure to these contaminants is presented in each specific Guideline Technical Document under the Guidelines for Canadian Drinking Water Quality. For all chemical contaminants, potential health effects depend on the exposure conditions, concentrations and duration of exposure. A summary of health effects of the contaminants can also be found in Table 2. Chemical Parameters of the Guidelines for Canadian Drinking Water Quality Summary Table.

Corrosion tends to increase the concentrations of many metals at the consumer’s tap. Corrosion deposits in pipes also provide a major reservoir for a broad variety of contaminants, some of which are a health concern, particularly lead. The toxicity of lead has been extensively documented in humans through blood lead levels (BLLs), and it is the main contaminant of concern for health with respect to corrosion. Health effects of lead that have been studied include increased blood pressure and renal dysfunction in adults, as well as adverse cognitive and behavioural effects in children. The strongest association observed to date is between increased BLLs in children and reduction in intelligence quotient (IQ) scores, which is the key health endpoint of concern. No safe level of exposure to lead in children has been identified (Health Canada 2019e). Some studies have determined that lead in drinking water can significantly contribute to the total lead exposure of children (increased BLLs) even at very low concentrations (Deshommes et al. 2013; Levallois et al. 2018; Ngueta et al. 2014). Consequently, every effort should be made to maintain lead levels in drinking water as low as reasonably achievable (ALARA) (Health Canada 2019e).

B.5 Impact of climate change

Climate change impacts, principally changes in precipitation and temperature, are expected to bring about more frequent and extreme events, such as storms, floods, droughts and wildfires. Climate change is also expected to alter key physico-chemical characteristics of drinking water sources, thus having impacts on more than only OPPPs in premise plumbing. These climate and source water changes will affect the presence, proliferation and likelihood of exposure to OPPPs in premise plumbing.

Table 5 provides an overview of the key climate drivers and expected impacts on drinking water quality in premise plumbing (Blanc et al. 2021b; Bruaset and Sægrov 2018; Dao et al. 2023; Dupke et al. 2023; Hohner et al. 2016; LeChevallier et al. 2024; Lyle et al. 2023; O’Keeffe 2022).

The possible outcomes of these impacts on drinking water systems are:

• more difficulty for drinking water suppliers to treat and maintain water quality during distribution and associated changes in the properties of distributed water (for example, increased NOM, change in DBP formation);
• change of chemical reactions such as disinfectant decay;
• impacts on corrosion in DWDSs and premise plumbing; 
• increased nutrients and/or warmer temperatures favouring propagation and spread of OPPPs in source waters, DWDSs, storage facilities and premise plumbing that are more frequent and persist for longer periods during the year;
• expansion of the geographical range of some OPPPs, for example, N. fowleri;
• change in concentration and type of inorganic contaminants present in source water; and
• increased exposure to chemical contaminants in drinking water resulting from wildfires. 

The climate scenarios that could impact drinking water suppliers and building water systems and the extent of climate-related impacts will vary by region. The elements expected to have the most significant impact are climate warming, making it more difficult to cool buildings and keep water temperatures below 25 °C, and the increased adoption of water and energy-efficient behaviours in buildings, which can have unintended consequences on water quality. The responsible authority can be consulted to discuss relevant forecast scenarios, if available. Climate change impacts will heighten the importance of every aspect of drinking water management, from source to tap. Thus, drinking water suppliers and building owners/managers should integrate the risks associated with climate change into their water management plans to maximize the reliability, robustness and resilience of their water systems. Resources are available to assist drinking water suppliers and building owners/managers in assessing climate-related risks for their systems and in developing risk adaptation plans (Conservation Ontario 2020; European Commission 2023; Institute for Catastrophic Loss Reduction 2024; WHO 2017).

B.6 Management strategies

B.6.1 Unique challenges posed by opportunistic premise plumbing pathogens

Historically, drinking water risk management has primarily focused on enteric pathogens, such as (pathogenic) E. coli, Cryptosporidium and enteroviruses (Health Canada 2019c; 2019d; 2020d). OPPPs are different from enteric pathogens in many ways (see B.3 Opportunistic premise plumbing pathogens). Their ability to take up residence in biofilms and form symbiotic relationships with FLA, for example, has enabled them to grow and persist in DWDSs and premise plumbing. Particularly in premise plumbing, OPPPs are considered “normal inhabitants” rather than transient contaminants. On the whole, OPPPs are contributing to the changing picture of waterborne disease and are a risk management challenge.

While there are some fundamental principles of enteric pathogen control that are transferable to the control of OPPPs, these pathogens present some unique challenges to drinking water suppliers (see Table 6).

B.6.2 Shared responsibility concept for drinking water management

OPPPs found at low levels in drinking water can become established in building water systems; when conditions favour their growth in premise plumbing, they can amplify to levels that pose significant public health risk (NASEM 2020). For chemical substances, lead is also an example of a contamination where premise plumbing is the largest source of exposure, but the conditions in the drinking water supply have a profound effect on exposure at the tap.

Managing risks from OPPPs and lead in drinking water requires barriers and controls in place throughout the DWDS and in building water systems to be maintained with high reliability to prevent conditions favourable for entry, growth, accumulation and exposure (Bartrand et al. 2024). As discussed in B.1 Premise plumbing, drinking water suppliers are responsible for water distributed to the property line. From the property line, the control of the quality of the water travelling through the premise plumbing is the responsibility of the dwelling owner. Thus, drinking water suppliers, building owners and managers and homeowners all have roles and responsibilities in managing OPPPs and inorganic contaminants in drinking water supplies. This “shared responsibility” represents a fundamental shift in the current drinking water risk management paradigm.

For managing OPPP risks, most of the available information is for the management of L. pneumophila. On the whole, OPPPs share many common characteristics (see B.3 Opportunistic premise plumbing pathogens) and control strategies for Legionella are generally effective for reducing the growth and transmission of other OPPPs (Falkinham 2023a; Ford et al. 2004). Still, prevention measures can affect specific OPPPs in different ways (Proctor et al. 2022). Management resources can be improved by considering all OPPPs together (Proctor et al. 2022). OPPP risk management is most important for those with increased susceptibility to OPPP-related disease such as persons with weakened immune systems and the elderly. People considered at high risk of contracting OPPP infections (see B.3.4 Health considerations) should consult with their health care provider regarding actions they can take as individuals to protect themselves from exposure.

Strategies for controlling inorganic contaminants are mainly focused on lead as it is the main contaminant of concern for health. The most sensitive population to lead exposure is young children (Health Canada 2019e). Management strategies for lead should prioritize minimizing exposure in dwellings supplied by a lead service line in schools and daycares. Strategies for controlling lead can also aid in reducing exposure to other metals.

B.6.3 At the drinking water treatment plant and within the distribution system

OPPPs can break through or bypass treatment barriers and be introduced into water and biofilms in DWDSs (LeChevallier 2019a; Wang et al. 2012b). Well-managed drinking water supplies have generally had infrequent detections of L. pneumophila with concentrations in positive samples below levels of direct public health concern (Bartrand et al. 2024; Burlingame and Bartrand 2024; 2019a; LeChevallier 2019b; Wang et al. 2012a). Bartrand et al. (2024) conducted the most comprehensive survey of L. pneumophila occurrence and concentration in U.S. DWDSs to date. Samples from 57 utilities were collected and analyzed to determine their relationship to system characteristics (disinfectant residual, temperature, other water quality parameters, sample location). In this study, 68% (39/57) of water suppliers had no detections, L. pneumophila was detected in only 1.2% (109/9181) of the samples and, when detected, 73% of the positive samples were less than 10 MPN/100 mL (Bartrand et al. 2024). These concentrations are several orders of magnitude lower than the 1 × 10⁻⁴ risk of infection (413 CFU/L) and illness (3.53 x 10⁵ CFU/L) for a single exposure from a conventional shower estimated by a QMRA model (Hamilton et al. 2019).

Still, drinking water distribution supplies, even in well-managed systems, can seed building water systems with low numbers of L. pneumophila and other OPPPs (Bartrand et al. 2024). Although drinking water suppliers cannot fully eliminate the risk of OPPPs in building water systems, it is critical that they engage in best practices for managing water supplies and distribution to minimize the frequency and concentrations of OPPPs entering building water systems. This, in turn, helps building water managers maintain control.

The water safety plan approach is recognized as the most effective way for drinking water suppliers to manage potential risks (microbiological, chemical, radiological) from poor water quality (WHO 2009; 2011). Detailed information on management strategies for microbiological, inorganic and organic contaminants in DWDSs is available in other Guidelines for Canadian Drinking Water Quality Guideline Technical Documents (Health Canada 2019c; 2019d; 2019e; 2020a; 2021a; 2022a; 2022b; 2025b). Specific activities important for controlling the occurrence and abundance of OPPPs in drinking water under a Water safety plan include (Health Canada 2020a; 2021a; LeChevallier et al. 2024; NASEM 2020):

• maintenance of an effective disinfectant residual paired with water testing to ensure it is maintained throughout the system;
• implementing a routine unidirectional flushing program for distribution system mains and periodic inspection and cleaning of storage facilities;
• reducing water age through activities such as managing turnover in storage facilities and flushing of stagnant zones;
• maintaining the physical and hydraulic integrity of the distribution system through the use of best practices for pressure management and sanitary procedures during main construction and repairs;
• optimizing treatment to minimize nutrient concentrations in treated water and the amount of scaling and/or corrosion in the distribution system, and to prevent nitrification in the distribution system; and
• implementing strategies (monitoring, process adjustments and procedures) to understand and manage the effects of temperature on water treatment, distribution and storage, particularly during the warmest months.

Many of the listed activities will also be of benefit for controlling and minimizing the accumulation and release of contaminants in the distribution system. Additional specific activities include (Health Canada 2019b; 2019e; 2019f; 2020b; 2022a; 2024a; 2025b):

• minimizing the concentration of inorganic contaminants (such as aluminum, arsenic, iron and manganese) that enter the distribution system from treatment processes; and
• implementing strategies to limit pH variability within ±0.2 units throughout the distribution system.

Most or all of these activities may already exist in drinking water suppliers’ water quality management procedures.

Advanced management of L. pneumophila in drinking water is possible through recognizing specific conditions that can lead to intrusion or amplification of the pathogen in the distribution system; identifying the locations where the conditions may develop; determining controls that prevent conditions from developing or having an impact; and incorporating the information into the water supplier’s risk management plan (Burlingame and Bartrand 2024). The authors identified an inventory of specific conditions (source water, water treatment process, distribution and storage) that could promote L. pneumophila presence and growth in drinking water systems. For example, conditions that promote retention and growth in the distribution system include sections of a distribution system where water is stagnant, disinfectant residual is low, water is seasonally warm (for example, more than 18 °C or during the warmest months), biofilm grows or is not well controlled, sediment collects or there is increased iron corrosion. The inventory is a suggested starting point for utilities to develop a system-specific hazards list. Once hazardous conditions are inventoried and the likelihood of occurrence is assessed based on technical knowledge provided by the water system’s operators, appropriate preventive and mitigative controls can be put in place.

In drinking water supplies in Canada, requirements for disinfectant residual concentrations are determined by the responsible authority and may vary amongst the provinces and territories. Most Canadian drinking water supplies target free chlorine residuals between a measurable level (generally 0.1 to 0.2 mg/L) and 2.0 mg/L (Health Canada 2009a). Where chloramines are used, the residual level target is typically from 0.25 mg/L to a maximum of 3.0 mg/L (Health Canada 2020c). These goals are consistent with recommendations of the voluntary Partnership for Safe Water program (AWWA Partnership for Safe Water 2023), which requires that all member systems use secondary disinfection and that optimized systems meet residual disinfectant goals throughout the distribution system of greater than or equal to 0.20 mg/L and 0.50 mg/L for free chlorine and total chlorine (chloramines), respectively, and less than or equal to 4.0 mg/L for both.

For Legionella management, LeChevallier (2019b) found that L. pneumophila was detected infrequently in distribution systems and that all concentrations of L. pneumophila greater than 10 MPN/100 mL occurred when free chlorine residuals were less than 0.1 mg/L. In the Bartrand et al. (2024) study, occurrence of L. pneumophila was limited by disinfectant residuals of 0.2 mg/L for both chloramine and free chlorine systems. Increases to 0.4 mg/L produced a smaller reduction, and further increases produced very small reductions (Bartrand et al. 2024). Based on the findings of this study, the authors recommended that, for L. pneumophila management, utilities should strive to maintain a minimum disinfectant residual of 0.2 mg/L for both free chlorine and chloramine (as total chlorine) in 100% of samples (Bartrand et al. 2024). It is noted that the minimum residual concentration for chloramine is often higher than that for free chlorine and utilities may wish to use the more stringent Partnership for Safe Water goal. Maintaining a disinfectant residual throughout the distribution system at all times with high reliability is central to the management of L. pneumophila (Bartrand et al. 2024; LeChevallier et al. 2024).

The appropriate amount of disinfectant needed to maintain water quality in the distribution system will depend on (among other factors) the species of bacteria, the presence of biofilms and sediments, the distribution system conditions (water temperature, residence time, hydraulic conditions, pipe material and diameter, pH, corrosion rate), the amount of biodegradable material and the nutrients (C:N:P) present in the treated water (Health Canada 2009a; 2020a; 2020c; 2021a). Some utilities may require residual concentrations that are much higher than the minimum recommendations to address their specific distribution system water quality. For water utilities that experience seasonal changes in water temperature, higher minimum disinfectant concentrations will be required to meet operational goals during periods of elevated water temperature (Bartrand et al. 2024; Health Canada 2021a). Utilities are advised to conduct an assessment of their system to determine system-specific goals.

Having minimum goals and adjusting disinfectant residual concentration goals upward, if needed, provides flexibility for utilities to simultaneously meet utility-specific goals and promote customer satisfaction with water quality (Bartrand et al. 2024). Higher minimum disinfectant residuals (for example, 0.5 mg/L) have been recommended in emergency situations in response to drinking water outbreaks of OPPP-related disease occurring in the community – for example, for the control of N. fowleri (Louisiana Department of Health and Hospitals 2013). Studies have also shown that L. pneumophila can still be detected in low concentrations in water with high residual disinfectant concentrations (Bartrand et al. 2024; 2019a; LeChevallier 2019b), demonstrating that additional barriers beyond disinfectant residual must be in place within a control program for Legionella and other OPPPs (Bartrand et al. 2024).

There is no universal recommendation for what disinfectant residual level should be maintained in building water systems (ASHRAE 2023a). Generally, maintenance of a free chlorine residual of greater than 0.2 mg/L in potable water systems is effective for limiting the growth of Legionella (Grimard-Conea et al. 2024; LeChevallier et al. 2024; NASEM 2020). In cold water, if there is an adequate disinfectant residual present in the water supplied to the building, it will typically persist throughout the cold building water system as long as there is sufficient water use to avoid excessive water age (ASHRAE 2023a). However, as distributed water travels through premise plumbing, the residual can be significantly reduced due to various factors (see B.2.8 Low or no disinfectant residual). Thus, even when water at the POE to a dwelling has a high residual disinfectant, the residual may not persist throughout the premise plumbing system. Hot water lines often contain little residual disinfectant due to decay at elevated temperatures (Lehtola et al. 2005; Martin et al. 2020), and larger, more complex water systems in buildings are more likely to experience reduced residuals (Grimard-Conea et al. 2024; Waak et al. 2024). Given these challenges, building managers should not assume that there is sufficient chlorine residual present in the premise plumbing to prevent OPPP growth. Other management actions (see B.6.4.2 Managing risks and B.6.4.3 Plumbing materials), such as proper temperature control, balancing hydraulics, and preventive flushing, have proven to be more effective for OPPP control and can be found in Bédard et al. (2016a); Boppe et al. (2016); Darelid et al. (2002); Waak et al. (2024).

Guideline documents address on-site disinfection as a control strategy for building water systems where there is a high potential for growth of OPPPs and are facilities that house occupants that are at higher risk of OPPP infection and illness. Supplemental disinfection can present significant risks and limitations associated with its implementation such as system damage and health hazards from the disinfectant and its by-products. It is not recommended as a control strategy for multi-unit residential buildings.

B.6.3.1 Homes served by private wells

Residential buildings served by private wells are influenced by different factors than buildings served by municipal systems. Wells can become contaminated due to vulnerabilities in well construction or through contamination of the aquifer itself (Lee and Murphy 2020). Enteric pathogens are a greater risk than OPPPs in private well water supplies (Hynds et al. 2014; Murphy et al. 2016). Thus, residents in dwellings served by a private well water supply should focus on assessing the vulnerability of their wells to fecal contamination by monitoring and maintaining the quality of their well water. Local public health units and provincial or territorial drinking water authorities can help with finding licensed well contractors and accredited laboratories, understanding what to test for and interpreting results. When test results confirm bacterial contamination after intrusion events due to well vulnerabilities or from weather events like flooding, treatment is generally advised, typically by shock disinfection (Health Canada 2024b). OPPPs can also be introduced into wells via flooding or intrusion of debris and soil (Dai et al. 2019; Mapili et al. 2022). Research suggests that shock chlorination is not an effective strategy for long-term control of OPPPs if they are naturally occurring in the groundwater supply or were pre-existing in biofilms in the plumbing system (Mapili et al. 2022). For private well systems, managing of potential risks from OPPPs is most important for individuals identified as high-risk. Actions to reduce exposure to OPPPs in dwelling premise plumbing are summarized in Table 7.

Inorganic contaminants can also be present in homes served by a private well, either as naturally-occurring contaminants in the groundwater or from leaching of well component and plumbing system materials. Some contaminants are aesthetic in nature (such as hardness and hydrogen sulphide) but can impact confidence in the water source, while other contaminants may have health impacts (for example, nitrate, lead, manganese).

When groundwater is contaminated by surface activities like farming, contaminants such as nitrates can be introduced. Naturally-occurring metals from the soil and rock can be leached into surface water and groundwater and contaminate it. Arsenic, manganese and fluoride are introduced into water through the erosion and weathering of soil, minerals, and ores. Lead and copper are present in tap water principally as a result of leaching (or corrosion) from components of distribution, household plumbing systems and well components (Pieper et al. 2018). Management strategies for homes served by private wells can be better tailored after the water is sampled to determine which contaminants are present and their potential impact on water quality. Mitigation strategies such as the installation of a POE or POU device can be considered based on the contaminants that need to be removed (see B.6.4.4 Treatment devices).

B.6.4 Residential buildings

B.6.4.1 Design

Understanding the premise plumbing system design factors that influence exposure to OPPPs, inorganic and organic contaminants is important for effective management (see B.2.10 Design and age). Plumbing systems should be designed and constructed to comply with jurisdictional plumbing and building codes. The NPC of Canada (NRCC 2020b) and the National Building Code of Canada (NRCC 2020a) are objective-based model codes that are generally adopted by provincial and territorial governments. In general, features of a good plumbing system design include: having good flow to all parts of the system (for example, minimizing stagnation, avoiding dead ends); avoiding materials that encourage growth of microorganisms or that can be sources of lead; minimizing heat transfer to and from hot and cold water supplies; and ensuring access for cleaning and maintenance of components (ESGLI 2017; NASEM 2020). Any measures to improve energy efficiency or reduce water usage should be assessed at the design stage to ensure that the control of OPPPs and inorganic contaminants is not compromised (ESGLI 2017).

For large multi-dwelling residential building water systems, ANSI/ASHRAE Standard 514 and ASHRAE Guideline 12 (ASHRAE 2023a; 2023b) provide guidance on system design, engineering and commissioning specific to the control of physical, chemical and microbial hazards.

B.6.4.2 Managing risks

Managing plumbing systems in dwellings is an important aspect of protecting occupant health. Building owners and managers and homeowners have responsibility for these systems; thus, they have a critical role in preventing the growth and spread of OPPPs and accumulation of inorganic contaminants. No single control measure ensures the control of OPPPs in premise plumbing systems (CDC 2021c; NASEM 2020). Complete eradication of OPPPs is impossible, particularly in complex building plumbing systems where recolonization often occurs. However, steps can be taken to limit growth of OPPPs in plumbing systems to reduce the risks of human exposure. It is therefore important that building owners and managers and homeowners understand the conditions and locations for OPPP growth and their spread, inorganic contaminant accumulation, as well as actions to ensure that water quality in their plumbing systems is maintained to reduce the risks of exposure to these hazards. Tenants also have roles in maintaining good water quality in dwellings.

For large multi-dwelling residential buildings, there is strong consensus that the development and implementation of a building WMP, also called a water safety plan for buildings, is a best practice to help building owners and managers oversee water quality in their buildings (NASEM 2020; WHO 2011). The WMP approach applies Hazard Analysis and Critical Control Point (HACCP) principles through a series of steps that enable a preventive and building-specific approach to preventing issues related to water quality management.

ANSI/ASHRAE Standard 514 provides minimum practices and extensive guidance for managing the overall risk from microbiological, physical and chemical hazards associated with water systems in buildings including multiunit residential buildings (ASHRAE 2023a). ANSI/ASHRAE Standard 514 works in companion with ANSI/ASHRAE Standard 188 (ASHRAE 2021) and ASHRAE Guideline 12 (ASHRAE 2023b), which address a single microbiological hazard, Legionella. Together, these materials specify the requirements for preparing and implementing a WMP for new and existing buildings, provide general guidance on physical, chemical and microbiological hazards including test parameters and methods, and outline recommendations for hazard control measures.

Key steps in developing a WMP for buildings involve establishing a team responsible for developing and implementing the program, describing the building water system with diagrams, identifying where hazardous conditions exist, assessing and determining where control measures are to be applied.  This is followed by establishing procedures for monitoring, reviewing the program’s effectiveness, and documenting and communicating results. The main benefits of WMPs are increased understanding of the building water system and improved overall water quality management (Schmidt et al. 2019). Important challenges with implementing WMPs are the demands for time and financial resources (Schmidt et al. 2019).

Under ANSI/ASHRAE Standard 514, buildings having any of the following features should develop and implement a WMP (ASHRAE 2023a):

• having multiple housing units with one or more centralized hot water systems serving two or more housing units
• more than six stories tall
• larger than 50 000 ft² (5 000 m²) and contains one or more potable water booster pumps
• having the purpose of housing occupants under the age of two or over the age of 65
• having pools, spas or equipment that releases water aerosols (including but not limited to centrally-installed humidifiers, cooling towers)

It is recommended that multi-dwelling residential buildings having more than six units and a centralized hot water heater develop and implement WMPs.

Building owners and managers should follow the recommendations outlined in ANSI/ASHRAE Standard 514 for the preparation and development of a WMP. Other references can be consulted for technical guidance on managing risks in building water systems (ASHRAE 2023b; CCIAQ 2023; CDC 2021c; ESGLI 2017; HSE 2024). Building owners and managers should consult with the appropriate authorities having jurisdiction to determine any requirements for building water quality. Consulting with building water management professionals can help building owners and managers when developing their programs. Single-family homes and multi-dwelling residential buildings with up to six units have simpler plumbing configurations and fewer routes of exposures than in multi-dwelling buildings with more than six residences. Nevertheless, the management strategies for controlling OPPP growth and transmission are similar. A WMP is overly complex and unnecessary for homeowners and managers of these buildings. However, there are actions they can take to prevent conditions that allow OPPPs to grow and spread within home plumbing systems.

In general, the overall strategy for controlling OPPPs in building water systems involves actions to minimize the presence of biofilm and sediment and to provide conditions that are more inhospitable for OPPP growth in these environments. Important strategies for maintaining water quality in all residential buildings include temperature management, flushing of infrequently used showerheads and faucets, and cleaning and maintenance of fittings and other water system components that are prone to biofilm growth or sediment buildup, in particular fittings that can generate aerosols (ASHRAE 2023b; Falkinham 2018; HSE 2024). Management strategies for controlling lead concentrations involve the removal of lead-containing plumbing materials and flushing to reduce lead released during stagnant conditions (Health Canada 2019e). Temporary mitigation measures also include the use of POU devices certified to the appropriate NSF/ANSI standard (see B.6.4.4 Treatment devices).

Table 7 summarizes key management actions that can be applied by homeowners (residents) and building owners and managers. Details on the components of management strategies are described in the parts that follow.

Temperature management

As shown in B.2.7 Temperature fluctuations, temperature is a critical factor contributing to OPPP proliferation and control in all building plumbing systems and also influences metal release and corrosionB.4.6. Maintaining hot and cold water temperatures outside of the optimal growth range for Legionella and other OPPPs is the most effective method for controlling their survival and growth in premise plumbing systems (ASHRAE 2021; NASEM 2020).

Cold water

Consumers will not have any control over the temperature of the incoming water. In temperate climates, cold water systems are generally not a problem for OPPP growth, since the water is typically supplied below 20 °C (NASEM 2020; Schwake et al. 2021). Water at these temperatures slows the growth of OPPPs but does not inactivate them. During periods where cold water is not in use, the water temperature will approach the temperature of the area in which the pipe is located (ASHRAE 2023a). The lower end of the Legionella growth range is 20 to 25 °C. In the summer months or where conditions where the pipe is located in the building may increase temperatures, there is the possibility that the temperature of cold water lines can reach the growth range for Legionella (NASEM 2020). In addition to increasing the potential for Legionella growth, these temperatures can result in increased leaching of lead from plumbing components. Regular use of outlets (for example, at least once per week – see B.6.4.2 Managing risks) prevents stagnation, thereby limiting temperature increases.

For homes and buildings that use cisterns to store drinking water (for example, in rural, remote or northern communities), maintaining water temperatures below 25 °C during storage is similarly useful in maintaining control of microbiological growth (ASHRAE 2023a; ESGLI 2017; NASEM 2020).

Hot water

As temperature increases above the optimal growth range (25 to 45 °C), OPPP growth slows, and the microorganisms begin to die. The higher the temperature, the more quickly they will be killed. The decimal reduction time (time required to reduce the concentration of microorganisms by 90%) for L. pneumophila decreases as temperatures increase, from around 100-1 000 minutes at 50 °C, to 10-15 minutes at 55 °C, to less than 5 minutes at 60 °C, to around 2 minutes at 70 °C (KWR 2021; NASEM 2020; van der Kooij 2014). Reduction times reported for M. avium (1 000 minutes at 50 °C and 60 minutes at 55 °C) and the cyst form of host protozoa Acanthamoeba spp. and Vermamoeba vermiformis (5-10 minutes at 60 °C) are longer than for L. pneumophila (Cervero‐Aragó et al. 2014; Schulze-Röbbecke and Buchholtz 1992). These values were developed for culturable cells under laboratory conditions and do not account for the effects of scale or biofilm, which can insulate and protect microorganisms. OPPPs can survive for longer periods of time due to changes in metabolic state and protection from disinfection within biofilms, sediments, scales or amoeba host organisms (Cervero-Aragó et al. 2019; Storey et al. 2004). However, in general, L. pneumophila can be properly controlled with temperatures of 60 °C or higher from hot water heaters and with temperatures at or above 50 to 55 °C reaching all parts of the system; at lower temperatures, hot water systems are more frequently positive for L. pneumophila and other Legionella spp. (Kistemann et al. 2024; KWR 2021; NASEM 2020; Quon et al. 2024; Rogers et al. 1994). These temperatures also reduce the numbers of M. avium and of the FLA hosts Acanthamoeba spp. and Vermamoeba vermiformis, which can act as natural replicators of OPPP bacteria (Cazals et al. 2022; Cervero-Aragó et al. 2015; Falkinham 2011; Schulze-Röbbecke and Buchholtz 1992).

At the temperatures required for control of Legionella and other OPPPs, the risk of scalding is a serious concern, particularly for more sensitive groups such as children, older adults and people with disabilities (ASHRAE 2023a; NASEM 2020). When using these temperatures, measures should be in place and/or steps should be taken to reduce scalding risk. Information on scalding risks is provided in this document.

For multi-dwelling residential buildings having more than six units, it is recommended that building owners and managers set centralized storage water heaters and hot water storage tanks to a minimum of 60 °C and maintain temperatures consistently above 49 °C throughout the water system, including to points of delivery and in return lines (ASHRAE 2023b; AWWA 2020; CDC 2021c). Several guidance documents suggest that maintaining hot water above 55 °C across the whole hot water system in large buildings provides the greatest risk management for Legionella (AWWA 2020; ESGLI 2017; NASEM 2020). However, producing hot water at this temperature may require greater intervention to prevent scalding (AWWA 2020).

Hot water systems with return loops require balancing so that hot water is evenly distributed throughout the building and can become unbalanced over their lifetime (AWWA 2020; Boppe et al. 2016). When a system is unbalanced, it does not provide the same hot water temperature to all recirculating portions of the system. This can result in portions of the system remaining at ideal growth temperatures for opportunistic pathogens for extended periods of time (AWWA 2020; Boppe et al. 2016). ANSI/ASHRAE Standard 514 (2023a) provides guidance on balancing requirements for recirculated building water systems. Boppe et al. (2016) identify a stepwise approach to evaluate and correct hydraulics in large buildings based on detailed temperature monitoring.

For single-family homes and multi-dwelling residences of up to six units, it is recommended that homeowners and building owners and managers set the temperature of their hot water heater (all types) to 60 °C. Under the NPC, electric storage-type service water heaters are to be shipped with the thermostat set to 60 °C to address the concern regarding growth of Legionella(NRCC 2020b).

Various energy-conserving strategies have also been introduced for hot water systems to reduce energy consumption due to water heating, such as scheduled lowering and raising of hot water heater temperature, use of hot water recirculation lines, drain water heat recovery systems and the pre-heating of cold water (KWR 2021; NASEM 2020). Features that encourage water system operation in the temperature range of 25 to 49 °C can inadvertently create conditions conducive to Legionella growth (KWR 2021; NASEM 2020). Notably, utility programs have been proposed to incentivize customers to lower electric water heater temperatures in order to reduce water energy demand during peak periods (also referred to as load-shifting or shedding). Strategies involve alternating cycles of lower temperatures (less than 60 °C) during peak demand periods and high temperature (above 60 °C) cycles intended to address the risks of Legionella growth. Under these scenarios, Legionella can be exposed to lower temperatures where the potential for growth is greater, followed by exposure to high temperatures, which is similar to applying a heat shock. Studies simulating conditions in hot water plumbing systems have demonstrated that heat shock treatment can result in a temporary reduction in the number of Legionella; however, complete elimination is often not achieved, and their numbers can return to previous levels (Cazals et al. 2022; Favere et al. 2025; Ji et al. 2018).

There are also concerns that repeated heat shock operations might select for more heat-resistant L. pneumophila (Ji et al. 2018; KWR 2021). Research indicates that management strategies involving changes in hot water heater temperature require measures to compensate for the periods of increased vulnerability during reduced temperature, including improved design to minimize thermal stratification and loose deposit accumulation and technological solutions for optimizing water heater temperatures during operation (Cazals et al. 2022). It is vital to ensure that Legionella control measures are not compromised by energy efficiency initiatives. Building owners and homeowners should assess the potential impacts of any energy efficiency measures to avoid unintended consequences on water quality and safety.

Scalding risks

The higher hot water temperatures recommended to control OPPP growth (60 °C or higher) in single-family homes and multi-dwelling buildings are associated with an increased risk of scalding, particularly for children, older adults and persons with disabilities (NASEM 2020; NRCC 2020b). Temperature management strategies must operate in accordance with regulations regarding maximum allowable temperatures at the tap. To reduce the risk of scalding, the NPC specifies that the temperature of water discharging from a showerhead or into a bathtub shall not exceed 49 °C; in seniors’ residences, it shall not exceed 43 °C (NRCC 2020b). The NPC further specifies that, to maintain temperatures at or below this limit, water supplied to showerheads or bathtubs shall be controlled by automatic compensating valves (for example, pressure-balanced or thermostatic mixing valves) or temperature-limiting devices (NRCC 2020b). Fittings must conform to the appropriate American Society of Mechanical Engineers (ASME), American Society of Sanitary Engineering (ASSE) or Canadian Standards Association (CSA) standards outlined in the requirements. The hot water temperature for other fixtures such as sinks and laundry trays is not addressed, but the risk of scalding may exist at these fixtures(NRCC 2020b).

To maintain hot water temperatures that inhibit the growth of OPPPs but at the same time reduce the risk of scalding to users, building owners and managers can consider installing mixing valves that limit water temperatures at showers and faucets to 49 °C or lower if their building does not have them (ASHRAE 2023a). Similarly, residents setting their water heater temperatures to 60 °C can take precautions against scalding by installing mixing valves at showers and faucets if their home lacks them. B.2.10.1 Design discusses how mixing valves can create conditions more favourable to the growth of OPPPs and that they can also be susceptible to breakage. Ideally, mixing valves should be installed at the POU to minimize the volume of water held after the mixing point at water temperatures within the range where Legionella can grow (ASHRAE 2023a). A licensed plumber can be consulted for information on the installation of mixing valves and their maintenance requirements.

Flushing and maintenance

Flushing

Good water flow to all areas of the plumbing system is important to prevent stagnation and the establishment of conditions that favour OPPP growth and the release of metals such as lead (see B.2.6 Increased water age and B.2.9 Variable flow rates and water use). The risks of stagnation increase where faucets and showers are not in regular use – for example, in vacant units, unused portions of the home, during vacations or in unoccupied rental or vacation properties. Flushing in building water systems serves a dual purpose, by removing microorganisms that have accumulated in stagnant water and by providing water with improved temperature and chemical composition – for example, lowering lead concentrations and increasing disinfectant residuals – to the ends of the system (AWWA 2020; HSE 2024; NASEM 2020; Waak et al. 2024). Two common flushing strategies employed in building water systems are remedial flushing (an intensive and high effort flushing of all outlets and replenishment of water in the plumbing system in response to prolonged periods of stagnation) and routine flushing (a preventive flushing often focused on unoccupied or unused outlets in stagnant areas of the building) (AWWA 2020).

For multi-dwelling residential buildings with more than six residences, procedures required for flushing and mixing water in stagnant and low-flow areas should be outlined in WMP documents (ASHRAE 2021; 2023a). ASHRAE Standards and guidance outline the requirements and general considerations for flushing procedures as part of a WMP (ASHRAE 2023a; 2023b). Building owners and managers can also consult technical guidance (AWWA 2020; Waak et al. 2024) and knowledgeable professionals (contractors, authorities having jurisdiction) for help with developing flushing plans for their buildings. Developing flushing procedures under a flushing plan requires various considerations, including understanding water system design, operation and baseline water quality of the cold and hot water supplies, determining the scope of flushing strategies, taking precautions not to harm water systems and managing the potential health and safety risks to personnel and occupants (AWWA 2020; Waak et al. 2024). Where buildings have typical occupancy and enough water use to regularly replenish building water, routine flushing is the control measure most often used to help reduce water age (ASHRAE 2023b; AWWA 2020; Waak et al. 2024).

There is no standard flushing frequency or duration that applies to all large multi-dwelling buildings (AWWA 2020). Guidance documents specify weekly flushing at outlets not used at least once in a seven-day period as a place to start when developing flushing plans (AWWA 2020; ESGLI 2017; Waak et al. 2024). Flushing practices can be scaled up or down based on review and validation of the WMP or flushing plan performance (AWWA 2020). A general approach to flushing is to start at the outlet nearest to the building service line and systematically progress to locations farther away in order to prevent unintentionally spreading microorganisms from one location to another (AWWA 2020; Waak et al. 2024). The recommended procedure for flushing showers and faucets is to first flush the cold water until the water is representative of water entering the building (based on disinfectant residual, temperature or volume flushed), then flush the hot water until the temperature stabilizes. Lastly, run the cold water again for 10 seconds to introduce water with a residual back to the POU (AWWA 2020; Waak et al. 2024). Outlets with mixing valves should be flushed at least as long as is required at other outlets to achieve stable cold and hot temperatures (AWWA 2020). Flushing can occur with aerators and showerheads installed, though these should be removed and cleaned on a regular basis (AWWA 2020; Waak et al. 2024).

For lead, flushing cold water taps is a temporary mitigation measure that does not consistently or sufficiently reduce lead concentrations in drinking water in schools, multi-dwelling residences and large buildings (Doré et al. 2018; Health Canada 2019e; 2025b; Murphy 1993). Periodic flushing throughout the day is necessary to adequately reduce lead concentrations since lead levels can increase when water stagnates for as little as 30 to 60 minutes (Health Canada 2025b). However, flushing may not be practical, especially in larger buildings. Identifying and replacing the lead-containing plumbing components with non-leaded ones is the most effective mitigation strategy in buildings as well as residences.

For single-family homes and multi-dwelling residences of up to six units where daily water usage is common and water turnover in the system is high, the risk of prolonged stagnation is low (HSE 2024; NASEM 2020). To control OPPPs, residents (that is, homeowners or tenants) and building owners/managers should flush faucets and showers that have not been used for a week or longer (CDC 2023; HSE 2024; NASEM 2020). Some guidance materials suggest that residents can consider flushing to refresh the water in their plumbing when it has not been in use for three days or more (AWWA 2012; Purdue University 2019). After any extended period of stagnation (such as several hours), water should be flushed to lower lead concentrations prior to consumption. Where properties have not been in use for very long periods, (for example, weeks to months), homeowners and building owners/managers should follow best practices for returning the dwellings to service (AWWA 2023).

There is currently no standard protocol regarding flushing procedures for residential dwellings, except following lead service line replacement. Guidance documents recommend letting the water run for at least 2 to 5 minutes at each faucet or shower, for cold and then hot water lines (AWWA 2012; CDC 2023; CSTE 2022; Purdue University 2019). Lead flushing duration is specific to each dwelling since it depends on the configuration of its internal plumbing and other factors, such as the length of the lead service line.

To reduce exposure to lead, flushing of taps following household plumbing construction or repair is also recommended (AWWA 2012). After service line replacement, extensive flushing should be conducted. The initial flushing should be extensive and other temporary mitigation measures should be used shortly after the replacement, such as a POU filtration device (AWWA 2017a).

Residential building owners/managers should note that there is an increased potential for exposure to airborne OPPPs when flushing faucets and showers. Building managers must adhere to national and provincial/territorial occupational health and safety regulations (ASHRAE 2023b; AWWA 2020). In general, measures should be taken during flushing to minimize exposure to aerosolized water, such as running bathroom fans and opening windows to increase ventilation, leaving the room while the water is running or using appropriate personal protective equipment when necessary (AWWA 2020; CSTE 2022; Falkinham 2013; NASEM 2020).

Maintenance

There are other things residents and owners/managers of buildings of all sizes can do to help minimize OPPP growth and reduce the potential for exposure as well as minimize exposure to lead. For building owners/managers that provide rental residential accommodations, the extent of the responsibilities for activities to help minimize OPPP growth, and reduce the potential for exposure to OPPPs and lead, will depend on the details of the tenancy agreement. Owners/managers, as the person in control of the premises, are responsible for the water systems in their premises. Building managers can advise their tenants to regularly clean their faucets, showers and aerators. Tenants should consult with their building owner/manager about responsibility for activities within their premises.

Water heaters, cold water storage and hot and cold water pipes

For all residential systems, water heater tanks should be regularly drained and flushed by following manufacturer’s instructions or by contacting a licensed plumber to do the work (CDC 2023; Purdue University Center for Plumbing Safety 2020a). Various plumbing contractors recommend that, if a water heater has been installed for more than three years without any maintenance, home/building owners should consult with a licensed plumber before flushing. Flushing a water heater tank after years without previous maintenance could inadvertently cause damage to the unit (DeMark Home Ontario 2025; Hydro Solution 2025).

Cisterns should be inspected, cleaned and disinfected at regular intervals, with most guidance documents recommending at least once per year (City of Iqaluit 2021; ESGLI 2017; Government of Northwest Territories n.d.; Manitoba Conservation and Water Stewardship 2014). Home and building owners should check with their municipal or local authority for specific instructions. Cleaning or inspection that requires entry into a storage tank should be done by a trained professional. CSA Standard B126 Series on water cisterns (CSA 2023) outlines best management practices for cistern operations and management.

Building owners or managers of residential buildings having more than six units can also consider insulating cold and hot water pipes if they are located in areas where the possibility of gaining or losing heat can result in water temperatures falling within the range that supports Legionella growth (25 to 45 °C) for extended periods (ASHRAE 2023a; 2023b; CDC 2021c; HSE 2024; NASEM 2020). For example: pipes located in unheated mechanical rooms or near cold areas, especially in cold weather, may decrease the temperature of hot water; pipes in areas experiencing heat from hot equipment rooms, outside walls, uninsulated hot-water pipes or exhaust from equipment may increase the temperature of cold water.

For single-family homes and multi-dwelling residential building of up to six units, Natural Resources Canada recommends insulating the first 2 to 3 m of hot water pipe leaving the water heater as this section loses heat most rapidly (Natural Resources Canada 2012). The benefit of pipe insulation in reducing hot water temperature losses is less significant in homes and small buildings compared to large buildings since hot water usage is less continuous (EPA Water Sense 2014) and distances from the water heater to POUs are shorter.

Cleaning aerators, faucets and showerheads

Showerheads, aerators and faucets should be regularly cleaned according to manufacturer’s instructions to help minimize OPPP growth, particularly whenever biofilm (slime) buildup is visible on these fittings (ASHRAE 2023b; CDC 2021c; 2023; ESGLI 2017; NASEM 2020). Showerheads warrant the most attention for cleaning and maintenance; they produce much more aerosols than faucets and are considered the greater risk for OPPP exposure (Clark et al. 2025; Hamilton et al. 2019). Periodic disinfection of showerheads and shower hoses should be considered (Falkinham 2018; Lande et al. 2018). If manufacturer’s instructions are not available, Waak et al. (2024) outlines a procedure for cleaning and disinfection of aerators, showerheads and hoses and other removable components suitable for Legionella control. In general, fittings are uninstalled and placed in a bucket in a well-ventilated area, washed with soap and water, descaled if needed by using white vinegar or descaling product for 30 minutes to one hour, rinsed, soaked in a 50 ppm bleach solution (1 tsp. household bleach in 4.5 L of water) for a minimum of 30 minutes, rinsed and reassembled. Increased bleach concentrations (1 000 ppm and greater) are proposed in guidance for health-care facilities (Allegheny County Health Department 2014) and for disinfection procedures to control M. avium, which are much more resistant to chlorination (Falkinham 2013; Lande et al. 2018). Higher chlorine levels can damage plumbing components and fitting finishes (Waak et al. 2024); compatibility with cleaning and disinfecting agents should be checked with the manufacturer. Building owners/managers and residents should follow required or best practices regarding the safe use of chemicals.

Faucet aerators should also be cleaned regularly to help reduce the risk of lead exposure. Aerators should be inspected monthly for presence of debris or particles. Aerators should be cleaned every six months, after construction work affecting the water service of a residence or after lead service line replacement (full or partial). Construction work can include a water main break repair in the area where the premise is located, a main replacement project or road repair. More information on how to clean water aerators can be found in the infographic published on Health Canada’s website (Health Canada 2023).

B.6.4.3 Plumbing materials

The choice of plumbing materials can help lower exposure to lead or help mitigate the growth of OPPPs in residential premise plumbing (see B.2.4 Variety of plumbing materials). To lower exposure to lead from drinking water, identifying and replacing the problematic plumbing system components with non-leaded ones can be an effective mitigation strategy (Health Canada 2019e). It is recommended that consumers use materials that have been certified as conforming to the applicable health-based performance and lead content standards (NSF International 2023; 2024). Consumers are responsible for ensuring that their actions do not have an adverse impact on water quality. Selection of fittings (namely, faucets and showerheads) and fixtures with the goal of minimizing how much those fittings and fixtures may contribute to water quality deterioration in premise plumbing (see B.2.10 Design and age) can help reduce OPPP exposures. Installation and maintenance of household plumbing systems should be undertaken by qualified and authorized plumbers or other persons with appropriate expertise (WHO 2023).

Lead service line replacement

Lead service lines have been found to be the largest source of lead in dwellings (Health Canada 2019e; Lytle et al. 2019). For this reason, the removal of the entire lead service line (public and private portions) is the most effective and most permanent solution for reducing exposure to lead from drinking water. The private and public sections of lead service lines should be removed at the same time to fully remove this lead source. A partial lead service line replacement occurs when only one of either the municipal or homeowner portion of the lead service line is replaced. A partial lead service line replacement does not result in a proportional decrease in lead levels when compared with full-service line replacement. Partial lead service line replacement is not recommended. In cases when there are partial lead service line replacements, galvanic corrosion can be mitigated by using plastic couplings to connect the old lead service line with the new copper pipe. Alternatively, connecting PVC piping to the lead service line can also prevent galvanic corrosion when conducting partial lead service line replacements (Health Canada 2019e). Service lines made from galvanized iron or lead-lined galvanized iron pipe should also be fully replaced as they are also a potential significant source of exposure to lead from drinking water. If present, pigtails and gooseneck should also be replaced (Health Canada 2025b).

Selecting plumbing fittings

Under the NPC, fittings used for potable water applications must comply with ASME A112.18.1/CSA B125.1, the relevant standard for plumbing fittings (NRCC 2020b). The NPC lists the acceptable plastic materials that can be used in service lines and plumbing systems along with certification (NRCC 2020b). All plastic pipes must comply with the CSA B137 series of standards for plastic pipe including meeting the requirements of NSF/ANSI/CAN Standard 61 for leaching of contaminants (NSF International 2023). Standard ASME A112.18.1/CSA B125.1 limits the content of lead in solders to 0.2% and includes requirements to comply with NSF/ANSI/CAN Standard 61.

NSF/ANSI/CAN Standard 61 addresses the leaching of lead and also includes a requirement that the wetted surfaces of products (such as plumbing components) have a weighted average lead content of no more than 0.25% when tested in accordance with NSF/ANSI/CAN Standard 372 (Drinking Water System Components—Lead Content) (NSF International 2023; 2024). Although these requirements are included in the NPC, older materials containing higher lead levels can still be present in existing buildings. These should be replaced with certified low-lead alternatives when possible.

Similarly to leaded-solder, brasses and bronzes historically were permitted to contain higher lead levels. Legacy brasses could contain up to 8% lead. Replacing them with materials complying with more recent certification requirements will reduce sources of lead in premise plumbing.

Copper pipes connected to other copper components (such as elbows) with leaded-solder can be found in residential premise plumbing installed prior to 1990. These solders continue to be a source of lead at the tap. Any replacement component with soldered joints should meet the current requirements of the NPC, including the use of solder with no more than 0.2% lead (for example, tin-antimony solder). Other plumbing fittings such as valves and flexible connectors may also contain lead that can leach into drinking water.

Plumbing fittings and fixtures

Homeowners should install only new faucets or showerheads approved for use in their jurisdiction and meeting the most current requirements of the NPC. Caution should be taken whenever purchasing fittings offered for sale over the internet. A certified fitting should have an identifying mark stamped on it or its packaging from a certification (testing) laboratory accredited by the Standards Council of Canada (SCC) (see B.6.4.4 Treatment devices). For questions, homeowners can contact a licensed plumber, certified plumbing inspector or their provincial or territorial plumbing regulatory body.

When selecting a water heater, the following factors should be considered: individual preferences (for instance, costs, comfort and reliability); individual susceptibility to potential risks (for example, immune status, risks for scalding); household water demand needs; scaling potential; and available fuel and power sources (Brazeau and Edwards 2011).

Individuals at high risk for OPPP illness (see B.3.4 Health considerations) could consider additional actions to reduce their risk of OPPP exposure linked to plumbing materials. High-risk individuals should avoid installing low-flow fittings (faucets, showers) as these can increase water age and bring about conditions more favourable for the growth of Legionella and other OPPPs (NASEM 2020; U.S. EPA 2012). Where low-flow fittings are in place, they should be used or flushed regularly to maintain water flow and prevent stagnation (ASHRAE 2023b). Other actions can include removing bathroom faucet aerators or using faucets that produce smooth (laminar) flow streams and that do not mix water and air (Falkinham 2023c; NASEM 2020); and replacing shower hoses with shower arms made of rigid pipe material (Proctor et al. 2018). Replacing showerheads that produce a fine mist with those that produce water streams (holes larger than 1 mm diameter) may be helpful (Falkinham 2013; NASEM 2020). Showerheads with bigger holes generate larger droplets that fall out of the air more quickly (Falkinham 2023c); however, they can also have greater water and energy consumption. Investigation of the potential impacts of water-saving, low-flow showers on exposures to OPPPs is ongoing (Niculita-Hirzel et al. 2021; 2023).

B.6.4.4 Treatment devices

Generally, the use of residential-scale treatment devices on municipally-treated water is not necessary. Private well owners are responsible for ensuring the safety of the drinking water from their own well. Where specific water quality concerns exist, there are different options available for the treatment of well waters. It is generally not recommended to use supplemental treatment devices to reduce OPPPs in drinking water in residential homes and buildings.

Individuals who are at high risk for OPPP illness (such as immunocompromised individuals) can consider taking additional measures such as installing POU filters at showerheads or faucets (Falkinham 2018; NASEM 2020; U.S. EPA 2025). Various filters (typically of 0.2 µm or less porosity) that are commercially available can provide a physical barrier to the transmission to OPPPs including Legionella, NTM and P. aeruginosa (ASHRAE 2023b; NASEM 2020; Norton et al. 2020). These filters have a limited lifespan and need to be maintained and replaced at regular intervals according to the manufacturer’s recommendations. Immunocompromised individuals should always discuss potential risks and remedial measures with their physicians.

A water quality professional should be consulted before installing any treatment devices. Before a treatment unit is installed, the water should be tested to determine the general water chemistry of the source water. Testing can verify the presence of specific contaminants. Periodic testing by an accredited laboratory (CALA 2024; SCC 2025) should be conducted on both the water entering the treatment unit and the treated water to verify that the treatment unit is effective. Units can lose removal capacity through use and time and need to be maintained and/or replaced. Consumers should service and replace components in the treatment unit according to the manufacturer’s recommendations. Local public health units and provincial or territorial drinking water authorities can provide advice on finding qualified treatment professionals. Additional resources are also available elsewhere (Extension Foundation 2019).

Health Canada does not recommend specific brands of drinking water treatment devices, but it strongly recommends that consumers use devices that have been certified by an accredited certification body as meeting the appropriate NSF/ANSI standards for drinking water treatment units. The purpose of these standards is to establish minimum requirements for the materials (including material safety), design and construction of drinking water treatment units that can be tested by a third party. The standards include performance requirements that specify the removal that must be achieved for specific contaminants (for example, reduction claim) that may be present in water supplies.

Certification organizations provide assurance that a product conforms to applicable standards and must be accredited by the SCC. In Canada, the following organizations have been accredited by the SCC to certify drinking water devices and materials as meeting NSF/ANSI/CAN standards (SCC 2025):

• CSA Group
• NSF International
• Water Quality Association
• UL LLC
• Bureau de normalization du Québec (available in French only)
• International Association of Plumbing & Mechanical Officials
• ALS Laboratories (formerly Truesdail)

An up-to-date list of accredited certification organizations can be obtained directly from the SCC (SCC 2025).

In residential settings, drinking water treatment devices can be installed at a tap (POU) or at the location where water enters the home (POE) to reduce contaminant concentrations. Since levels of lead can increase between the POE and the point of consumption of the drinking water, a POU treatment device that is certified for lead removal should be used. The POU device should be installed at the tap that is most commonly used for drinking, cooking and food preparation, as it is considered to be the best approach to reduce lead concentrations immediately before consumption. Inhalation and dermal absorption are not significant routes of exposure for lead (Health Canada 2019e). Following lead service line replacement and until lead concentrations stabilize, it is recommended to use POU devices certified for the removal of lead (Health Canada 2019e). A number of residential treatment devices are available that can remove lead or OPPPs from drinking water.

Hot water heater considerations

Ion exchange softeners

In areas with hard ground water, homeowners with private wells should consider softening the cold water supply to the hot water system using an ion exchange softener (ESGLI 2017; HSE 2024). This reduces the accumulation of scale at the bottom of water heater tanks, on heat exchangers or within the pipes or fittings (for example, shower mixing valves, faucets, showerheads), which may adversely affect the efficiency of the hot water system and water quality (HSE 2024).

The use of sodium-based ion exchange softeners may introduce undesirable quantities of sodium into the treated water. It is recommended that a separate supply be used for potable water consumption and culinary purposes. When a sodium-based water softener is used, it is recommended that a portion of the water most frequently consumed (such as from a kitchen tap) bypass the softener altogether to avoid excessive salt intake (Health Canada 2025c). This is particularly a concern for individuals on sodium-restricted diets. Water softeners using potassium chloride as the ion exchange medium can increase the exposure to potassium. This is not a concern for the general population, but susceptible individuals should avoid the consumption of water treated by water softeners using potassium chloride. Susceptible individuals include those with kidney dysfunction or taking medications that interfere with normal potassium-dependent functions in the body (Health Canada 2008).

Remediation measures

Remediation measures for OPPPs in plumbing systems are nearly always implemented as an emergency response directed by public health authorities for buildings needing immediate corrective action for documented pathogen issues like a disease outbreak (NASEM 2020; U.S. EPA 2016).

Treatment methods often involve a chemical shock (also called shock hyperchlorination) by distributing a chlorine disinfectant throughout the system at concentrations well above maximum levels permitted for potable water (ASHRAE 2023b; U.S. EPA 2016). Thermal shock using very high temperature water circulated throughout the hot water system is another method, but this requires considerable energy and manpower and is not recommended in most circumstances (ASHRAE 2023a; 2023b; U.S. EPA 2016).

Remediation methods have significant shortcomings: their effectiveness is highly dependent on being able to achieve target control levels at all points in the system, they can be damaging to plumbing components and they may pose risks to occupants (ASHRAE 2023a; 2023b; U.S. EPA 2016). In addition, they only provide short-term control—OPPP concentrations are reduced but not eliminated, and regrowth can occur after treatment is completed (ASHRAE 2023b; U.S. EPA 2016).

For building owners and managers of multi-dwelling residential buildings with a WMP, ANSI/ASHRAE Standard 514 (ASHRAE 2023a), ANSI/ASHRAE Standard 188 (ASHRAE 2021) and ASHRAE Guideline 12 (ASHRAE 2023b) outline the requirements for program documents and procedures for emergency remediation under contingency response plans. The resources also provide guidance on factors to be considered in assessing needs for remediation, available methods and cautions.

When properly applied, control strategies for Legionella and other OPPPs are effective in reducing concentrations in plumbing systems (Waak et al. 2024). Where the results of a WMP indicate that a water system has, for example, poor Legionella control or that control measures are ineffective, additional interventions may be necessary (ASHRAE 2023a; 2023b; CDC 2021c). Any considerations for additional control interventions should be done under the guidance of public health or water treatment professionals (ASHRAE 2023b; CDC 2025b).

For homeowners and small building owners and managers that do not have a WMP for their buildings, any remediation or control interventions required would be those directed by public health authorities in response to documented cases of illness.

B.6.5 Additional management steps for drinking water suppliers

Under a water management framework like a water safety plan, supporting programs such as education and outreach initiatives are important mechanisms for encouraging incremental improvement of the quality of drinking water supplies (WHO 2023). Publishing information on public health aspects of drinking water quality and the performance of suppliers can address consumer concerns and encourage users to follow good management and use practices (WHO 2023). Drinking water suppliers and stakeholders can work together to expand outreach efforts to educate property owners about the risks posed by water quality degradation in premise plumbing. The Water Research Foundation’s Project No. 4664, “Customer Messaging on Opportunistic Pathogens in Plumbing Systems” outlines a communications plan and example messaging for informing those who maintain premise plumbing about OPPPs with a focus on Legionella (Masters et al. 2018).

B.7 Monitoring

B.7.1 At the drinking water treatment plant and in the distribution system

Drinking water supplies that meet the Guidelines for Canadian Drinking Water Quality for microbiological parameters have a strong foundation for controlling the risk from exposure to Legionella and other OPPPs (Health Canada 2022b). Drinking water suppliers should have comprehensive monitoring programs in place that include operational and verification (sometimes called validation) monitoring to verify water quality and system performance, and demonstrate that water quality goals are consistently being met for microbial risks. Monitoring plans should be based on an assessment of hazards that are specific to each system and include parameters, frequencies and locations where monitoring should be conducted. Monitoring should be representative of exposure for the purposes of compliance. For example, since the primary sources of lead originate from the service line and plumbing system, sampling for lead compliance should be done at the tap, using a protocol representative of exposure (Health Canada 2019e).

A key pillar of managing OPPPs in drinking water systems is controlling their occurrence and growth in DWDSs (LeChevallier et al. 2024). In order to maintain water quality in the distribution system, it is essential to understand when changes occur. To achieve this goal, it is important that routine monitoring programs include measures for assessing drinking water biostability (Health Canada 2022a). Monitoring locations for various parameters should be based on historical data, distribution system characteristics and population or consumer distribution. Locations should have good spatial representation that is based on distance from the treatment facility(ies) or travel time within the distribution system, population density for serviced areas and where the system ends. For all systems, areas with low flow or dead ends should be monitored, as should storage facilities, due to the possibility of increased water age and/or poor hydraulics.

B.7.1.1 Operational monitoring

Operational monitoring for parameters used to characterize NOM should also be conducted as part of a source-specific monitoring plan. This will help to ensure that water utilities adequately remove NOM, which in turn helps to meet associated water quality goals related to microbial risks, DBPs, biological stability and corrosion control. Parameters and methods to consider are found in the relevant guideline technical document or guidance document under the Guidelines for Canadian Drinking Water Quality. Many of these parameters (such as disinfectant residual, DBPs, pH, alkalinity) are already being monitored in most treatment facilities as part of a source-to-tap approach to producing safe drinking water. System operators should employ a combination of the most appropriate methods and parameters, based on a system-specific assessment, cost and ease-of-use, to routinely monitor distribution system water quality and establish baseline conditions, monitor changes and detect potential or actual contamination events.

B.7.1.2 Legionella monitoring

There are no guidelines or regulations that establish concentrations or monitoring requirements for Legionella spp. organisms in treated drinking water. Studies show that the bacteria are rarely found in distributed water at levels high enough to present a significant health risk. There are also financial and human resource demands associated with routine testing. Instead, a priority focus on drinking water process management is the recommended strategy for water utilities to manage potential risks. Verification monitoring (activities to assess that the hazard is being controlled) can be accomplished using operations and treatment information with supporting water quality data (Bartrand et al. 2024; Burlingame and Bartrand 2024). A water supplier-specific Legionella hazardous conditions inventory and risk assessment can help define operational monitoring activities and inspection points to assess whether controls are working as intended (see B.6.3 At the drinking water treatment plant and within the distribution system). Water temperature and disinfectant residual are important parameters for identifying conditions in municipal systems that can favour amplification and increase the latent risk of OPPPs. These conditions include water temperature greater than 18 °C and low disinfectant residual in areas of the distribution and storage where water is stagnant, biofilm grows, sediment collects, enhanced corrosion is occurring or where there is significant occurrence of pressure transients or evidence of intrusion (Burlingame and Bartrand 2024; LeChevallier 2019a). Reviews of water quality data and distribution system hydraulic modelling can help identify vulnerable locations to prioritize as monitoring sites (Besner et al. 2001; Bukhari et al. 2023). Bacterial growth is generally more likely in DWDSs when water temperature increases above 15 °C (Besner et al. 2001; LeChevallier et al. 1996). Therefore, utilities may consider this temperature as a trigger to increase sampling frequency for some parameters such as biostability parameters (Health Canada 2021a).

It may be useful to monitor for L. pneumophila in special instances – for example, as part of an investigation of a waterborne disease outbreak (Payment and Pintar 2006; WHO 2022a). As well, increasing public awareness or reported increases in the incidence of waterborne illness as demonstrated by national or regional surveillance data could prompt drinking water suppliers to consider voluntary monitoring activities to better understand potential system vulnerabilities or baseline levels (Bukhari et al. 2023). More detailed information on approaches to monitoring L. pneumophila in drinking water systems can be found elsewhere (Bartrand et al. 2024; Bukhari et al. 2023). Although studies suggest that L. pneumophila occurrence in DWDSs is low, with numbers largely below levels estimated to be health risks, monitoring for Legionella is still likely to find that these microorganisms are present (Bartrand et al. 2024; Bukhari et al. 2023; Wang et al. 2012a). Therefore, water suppliers considering monitoring should have proactive communication to help contextualize monitoring data and educate customers of its significance and the risk management actions in place (Bartrand et al. 2024; Bukhari et al. 2023).

B.7.1.3 Monitoring for inorganic contaminants

For inorganic contaminants, such as arsenic, copper, lead and manganese, monitoring information is provided in Health Canada's Guidelines for Canadian Drinking Water Quality: Guideline Technical Documents for the respective parameters (Health Canada 2006; 2009a; 2009b; 2019b; 2019e; 2024a; 2025b). For lead in particular, detailed information can be found in the guideline technical document (Health Canada 2019e) and in the Guidance on Sampling and Mitigation Measures for Controlling Lead Corrosion (Health Canada 2025b). It is important to select the appropriate sampling protocol depending on the objective (Health Canada 2019e; 2025b).

Of note, water should be monitored at the tap when discoloured water events occur. Discoloration is likely to be accompanied by the release of accumulated contaminants, including arsenic, copper, lead, manganese and radiological parameters and can deplete the disinfectant residual (Health Canada 2006; 2009a; 2009b; 2019b; 2019e; 2024a; 2025b). Therefore, discoloured water events should not be considered only an aesthetic issue; they should serve as a trigger for monitoring metals (Friedman et al. 2016).

B.7.2 Single-family homes and multi-dwelling residential buildings (up to six units)

Regular monitoring for OPPPs in these premises is not practical and therefore not recommended. Testing is typically limited to research studies and during investigations of cases or outbreaks of legionellosis.

Homeowners and building owners are responsible for evaluating the types of materials they have in their plumbing system (for example, whether there is a lead service line, a galvanized iron or lead-lined galvanized service line, copper piping). Building owners and homeowners can contact their water supplier or municipality as they may have useful information including whether service lines in their area may be made of lead and whether they have had reports of discoloration events. If the homeowner suspects that their service line is made of lead, they should have their water tested. They can also ask their municipality or water supplier if a lead testing program exists or request information on where and how to test. Regular monitoring of residential dwellings is also conducted by authorities responsible for DWDSs as part of compliance monitoring. Specific information on monitoring strategies for lead (such as sampling flow charts) can also be found in the Guideline Technical Document – Lead and in Guidance on Sampling and Mitigation Measures for Controlling Lead Corrosion (Health Canada 2019e; 2025b).

Homeowners with private wells are encouraged to have their water tested for general water quality every two years. The local authorities can be contacted to find out what chemicals to test for. In addition, private well owners using residential treatment devices should regularly test the water entering and exiting the treatment device according to manufacturer’s instructions to verify that the device is effective.

B.7.3 Multi-dwelling residential buildings (greater than six residences)

B.6.4.2 notes that the development and implementation of a comprehensive WMP, or building water safety plan, is the recommended best practice for managing water quality in buildings (NASEM 2020; WHO 2011). ANSI/ASHRAE Standard 514 (2023a) and its companion resources, ANSI/ASHRAE Standard 188 (2021) and ASHRAE Guideline 12 (2023b), provide guidance and information on requirements for developing monitoring procedures for building water systems under a WMP and identify considerations for measuring and testing physical, chemical and microbiological parameters. Building owners and managers should consult ANSI/ASHRAE Standard 514 for detailed information. Guidance on some key aspects of monitoring is provided here.

Prior to developing monitoring procedures, building owners/managers must first conduct an assessment of their building water system to identify hazardous conditions and the control measures that are to be applied and subsequently monitored (ASHRAE 2023a). It is also important that building owners and managers understand how to interpret and respond to test results before beginning measuring parameters and collecting samples (ASHRAE 2023a). Some monitoring activities may be carried out by the building manager or operator, provided they have been trained, whereas others may require a service provider (for example, a qualified building water management company or consultant). Building owners and managers with no experience collecting samples can seek help from building water management professionals, laboratories and government (such as public health or environmental) authorities.

Under a WMP, monitoring includes measuring physical and chemical parameters such as temperature and disinfectant residual at representative points throughout the building water system to verify that the program is working as implemented and validate that the hazards are being controlled. Routine measurements for physical (scalding), microbial and chemical hazards can also be conducted if the WMP team determines this is also needed for validation (ASHRAE 2023a).

B.7.3.1 Monitoring parameters and methods

Building WMP resources recommend temperature and disinfectant residual as the primary parameters that should be monitored (ASHRAE 2023b; AWWA 2020; CDC 2021c).

Temperature

Water temperature is one of the most important factors influencing OPPP survival and growth in premise plumbing (see B.2.7 Temperature fluctuations). Measuring temperature can be used to verify that the recommended premise plumbing temperatures are being achieved for microbiological control and prevention of scalding risk (ASHRAE 2023a; 2023b). Temperature can be checked at POUs with or without mixing valves by running the water and recording the temperature after it has stabilized (AWWA 2020). There are no detailed standardized methods for monitoring water temperature in premise plumbing. Standard Methods for the Examination of Water and Wastewater provides basic recommendations for temperature measurement devices and their accuracy and calibration (APHA et al. 2023a). ASTM E2877 Standard Guide for Digital Thermometers (ASTM 2019) can be consulted for guidance on thermometer selection, specifications and minimum requirements for certification. Digital thermometers or temperature meters having a minimum range of 0 to 100 °C and a resolution of 0.1 °C have been used by researchers in investigative studies (Bédard et al. 2015; Rhoads et al. 2015). Instruments are available for remote online measurement of water temperature in building premise plumbing. Continuous remote water temperature monitoring can generate more comprehensive and accurate readings, though it does have additional costs in terms of initial equipment installation and maintenance.

Disinfectant residual concentration

As noted in B.2.8 Low or no disinfectant residual, a disinfectant residual is applied to limit microbiological regrowth (Health Canada 2009a; 2020c). Measuring cold water disinfectant residuals can be used to confirm that levels reaching POUs are within target levels and ideally approach levels in water provided by the drinking water supplier (AWWA 2020). Building owners or managers can contact their drinking water supplier for information on the type of disinfectant residual and level in water distributed in their area (AWWA 2020). It is necessary to measure free chlorine for buildings supplied with water with a free chlorine residual and total chlorine for buildings supplied with chloramine residual (AWWA 2020). Methods for testing chlorine or chloramine residual using portable analyzers are available (ASHRAE 2023a; Health Canada 2009a; 2020c).

Legionella spp.

When using ANSI/ASHRAE Standard 514, ANSI/ASHRAE Standard 188 and ASHRAE Guideline 12, the building owner/manager or program team determines if testing for microbial hazards is needed for validation based on the assessment of their building water system (ASHRAE 2021; 2023a; 2023b). Legionella (especially L. pneumophila) is the most commonly tested microbial hazard to confirm that the conditions for microbial growth are well controlled (ASHRAE 2023a). For Legionella spp. monitoring in residential buildings, testing should focus on L. pneumophila, the species of greatest public health significance (KWR 2021; LeChevallier et al. 2024). Routine testing for L. pneumophila can be useful in buildings where there is difficulty meeting target limits for water temperatures and disinfectant residual consistently throughout the building, and in buildings housing populations at increased risk for opportunistic infections, such as apartments for older adults (ASHRAE 2023b; CDC 2021c; ESGLI 2017). For other OPPPs, testing is not recommended due to various factors including the complexity of the methods, cost, lack of consensus protocols, variable presence of the microorganisms and knowledge gaps in the understanding of their infectivity, dose and disease risk (APHA et al. 2023c; Wang et al. 2017).

Building owners and managers should be aware of the various challenges associated with monitoring for L. pneumophila in building water systems including resources required (costs, personnel time), lab availability, developing the monitoring program design and interpreting the results. When testing, analyses should be carried out by laboratories that include detection of L. pneumophila from environmental samples within their scope of accreditation (ASHRAE 2023b; ESGLI 2017). Resources are available to help building owners and managers find laboratories qualified for L. pneumophila testing (CALA 2024; CDC 2022a; SCC 2025).

Standard culture and qPCR methods for the detection and quantification of L. pneumophila are available (AFNOR 2017; 2019; APHA et al. 2023b; ASTM 2021; CDC 2005; ISO 2017; 2019; 2024). Most accredited laboratories use culture methods; comparatively fewer laboratories offer qPCR testing within their scope of analysis. Monitoring programs should ideally include both methods for a comprehensive understanding of L. pneumophila presence in building water systems (NASEM 2020). qPCR is useful for quick screening of samples for L. pneumophila, and positive qPCR tests can be followed up with culture testing to detect the presence of viable, culturable cells (ASHRAE 2023b; CDC 2021c; Dowdell et al. 2022). Building owners or managers can consult with laboratories and water management professionals to select the proper test methods for their buildings (ASHRAE 2023a).

In general, the hot water side of the plumbing supply is an important target for Legionella monitoring. Cold water systems should not require routine monitoring for Legionella unless temperatures and stagnation times are elevated in the system (for example, greater than 25 °C, dead ends) (Wang et al. 2017). When sampling for Legionella, water samples (as opposed to biofilm or aerosol samples) are the focus for testing as they are easier to collect, various volumes can be sampled, and standard methods are available for consistent sample collection and reporting of results (ASHRAE 2023b; NASEM 2020; Wang et al. 2017). Legionella test results indicate conditions at the time of testing and do not provide an indication of health risk nor are they predictive of disease (ASHRAE 2023b; CDC 2021c). Results should be interpreted based on concentration, extent of detection and species of Legionella (ASHRAE 2023a). ASHRAE standards and guidelines (ASHRAE 2023a; 2023b) and the CDC (CDC 2021c) provide guidance for building owners/managers or the program team for responding to Legionella test results, as well as guidelines for evaluating the performance of WMPs. In general, indicators that building water system conditions support Legionella growth include Legionella concentrations greater than 1 CFU/mL (1 000 CFU/L), increases in concentrations of ten times or more and increases in the number or percentage of sample locations testing positive (ASHRAE 2023b; CDC 2021c). Test results where the growth of L. pneumophila is not well controlled indicate a need for a higher level of response under the WMP (ASHRAE 2023b; CDC 2021c). This is done by reviewing operating conditions, adjusting control measures as necessary and retesting (ASHRAE 2023b; CDC 2021c).

Heterotrophic Plate Count (HPC)

Monitoring for heterotrophic plate count (HPC) bacteria is generally not recommended as part of a building WMP (CDC 2021c; WHO 2011). HPC methods detect only a small proportion of the total bacterial concentration in drinking water (Prest et al. 2016; Van Nevel et al. 2017; WHO 2003). Given the limitations of HPC methods, it is recommended that other monitoring methods and parameters be considered for assessing general microbiological water quality (Health Canada 2022a). For example, adenosine triphosphate (ATP) is a quick, easy and cost-efficient indicator that has been used successfully in building water monitoring to assess the general quality of drinking water and the biostability and effectiveness of corrective actions, like flushing (Grimard-Conea et al. 2022; Grimard-Conea and Prévost 2023; Lautenschlager et al. 2010; Li et al. 2022; Siebel et al. 2008; Yao et al. 2023). A standard test method is available for detection of ATP content from the microorganisms present in water (ASTM 2023), and commercial kits, compliant with this method, are available. Some authors have recommended threshold values to facilitate the interpretation of ATP results; for example, ATP ≥ 10 pg/mL has been suggested as indicative of a need for corrective action (Chen et al. 2024; Grimard-Conea et al. 2022). In practice, system-specific baselines and trends should be established over a sufficient test period, similar to practices recommended for any microbiological monitoring technique (Chen et al. 2024).

Inorganic contaminants

Owners of buildings are responsible for assessing their own plumbing systems for lead. The municipality or local authority can be contacted for advice and assistance with testing. More information can be found in Health Canada’s Water Talk on lead in drinking water (Health Canada 2016). Specific information on monitoring strategies for lead can be found in the Guideline Technical Document – Lead and in Guidance on Sampling and Mitigation Measures for Controlling Lead Corrosion (Health Canada 2019e; 2025b).

Discoloration events are likely to be accompanied by the release of accumulated contaminants (see B.4.1 Sources of exposure). Therefore, discoloured water events should trigger sampling for arsenic, copper, lead, iron and manganese and other inorganic contaminants, as appropriate (Health Canada 2006; 2019b; 2019e; 2019f; 2021b; 2024a; 2025b). Metal scans, in which multiple trace metals can be analyzed simultaneously in one sample, can also be performed to determine contaminants present. Building owners need to consider customer complaints as they are a good source of information on releases from the distribution system and locational information for management actions.

B.7.3.2 Monitoring considerations

Monitoring procedures should be outlined in the building WMP (where such a program is developed). Sampling strategies will be unique to each building and influenced by the complexity of the building water system design, the operating characteristics of the system and the parameters being analyzed (ESGLI 2017; NASEM 2020). ANSI/ASHRAE Standard 514, ANSI/ASHRAE Standard 188 and ASHRAE Guideline 12 outline considerations for choosing sampling locations (ASHRAE 2021; 2023a; 2023b). Other resources are available that provide practical guidance on sampling, site selection, collection and transport (ASHRAE 2023b; CCIAQ 2023; CDC 2021c; PWGSC 2016). Building water management professionals can also assist in developing a sampling plan.

As discussed in B.2 Causes of water quality deterioration, building characteristics, operational settings and fittings may lead to increased potential for OPPP growth and exposure if not properly managed. These include dead ends, infrequently used outlets, areas with poor circulation, areas within the hot and cold water systems that are prone to temperatures falling within the range of 25 to 45 °C, storage water heaters, hot water mixing valves, showerheads and faucets (AWWA 2020; ESGLI 2017; Hamilton et al. 2017). ASHRAE Guideline 12 specifies that sampling plans for Legionella testing should include, at a minimum, central distribution points, areas identified to have low-flow conditions, and POUs close to and far from where the drinking water system distribution system connects to the building water system (ASHRAE 2023b).

Monitoring temperature of cold and hot water systems and disinfectant levels of cold water systems is a practical approach to identify areas of low usage or where operational targets are not being met (Bédard et al. 2015; NASEM 2020). Building an informative profile of temperature and disinfectant residual in a building water system requires numerous sampling points that encompass the range of scenarios that may be encountered, including periods of stagnation and different seasons or times of year (Bédard et al. 2015; Rhoads et al. 2014). Bédard et al. (2015) discusses the specifics of temperature profiling and understanding temperature distribution within a building hot water distribution system.

Sampling location

Sampling locations and frequency will be influenced by the complexity of the building water system design, the operating characteristics of the system and the parameters being analyzed. Monitoring for lead should be conducted at the tap that is used most frequently for drinking and food preparation (Health Canada 2019e). In areas of the distribution system that are subject to discoloration events (for example, buildings supplied by or downstream of cast iron pipes), sampling for metals during and after discoloration events should be done at the tap (Health Canada 2019e; 2024a; 2025b).

For lead, sampling frequency for multi-dwelling residential buildings is described in Guidance on Sampling and Mitigation Measures for Controlling Lead Corrosion (Health Canada 2025b) and sampling plans should be developed specifically for each building. Lead levels are highly variable in large buildings, meaning that the concentration of lead at a specific tap cannot predict the lead concentration for other taps in a building. Sampling plans need to ensure that every tap in the building is sampled (Health Canada 2025b).

B.8 International considerations

B.8.1 OPPPs

Some international organizations have drinking water guidance for OPPPs. However, it is limited to management and control of Legionella, with a focus on water systems in buildings.

Two international agencies have established drinking water regulations for Legionella. The European Union (EU) in its updated Drinking Water Directive (European Union 2020), requires that Legionella be addressed through a risk-based approach to water safety. Member States are required to carry out a general analysis of potential risks associated with domestic distribution systems and monitor Legionella at locations where risks are identified, with a specific focus on priority premises.^{Footnote 1} A parametric value of 1 000 CFU/L for Legionella spp. in drinking water is established in the Directive. Where risks from domestic distribution systems are identified, or where monitoring shows that the parametric value is not met, Member States are required to take steps to eliminate or reduce the risks. This can include advising consumers and owners of public and private premises about the conditions of the water and actions to eliminate or reduce the risks. The U.S. Environmental Protection Agency (EPA) has established a Maximum Contaminant Level Goal (MCLG, a non-enforceable public health goal) of zero Legionella spp. organisms in drinking water in its treatment-based Surface Water Treatment Rule (U.S. EPA 1989). The EPA states that if Giardia spp. and viruses are removed/inactivated, according to the treatment techniques in the Surface Water Treatment Rule, Legionella spp. will also be controlled.

The World Health Organization and the Australian government (Australian National Health and Medical Research Council) provide fact sheets that contain detailed information on individual waterborne pathogens, including OPPPs (NHMRC and NRMMC 2011; WHO 2022a).

In most countries, management of Legionella in building water systems occurs on an ad hoc basis through the use of a range of codes, standards, and guidance materials. Standards and guidance documents have been developed for buildings that include multi-dwelling residential buildings; however, their scope does not include single-family homes (ASHRAE 2021; 2023b; CDC 2021c). Comparisons of recommendations from international agencies for hot water temperature and action levels for Legionella applicable to residential dwellings is provided in Table E. 1 and Table E.2

B.8.2 Lead and other inorganic contaminants

Various national and international organizations have established values for lead and other inorganic contaminants in drinking water. However, few have specific information presented in one document on how to minimize exposure to these contaminants in drinking water consumed at home. For example, the website of the U.S. EPA presents general information on lead in drinking water and what people can do (U.S. EPA 2024a). Information is also provided on how to identify lead-free certification marks on plumbing components (U.S. EPA 2024c) and how to identify POU and pitcher filters certified for lead removal (U.S. EPA 2024b). The Australian government (Environmental Health Standing Committee) published a Guidance Statement on lead in drinking water from plumbing products and a document on reducing exposure to metals in drinking water (enHealth of the Australian Health Protection Principal Committee 2021a; 2021b). The World Health Organization has guidance information of minimizing lead concentrations at the tap (WHO 2022b).

However, no holistic document for minimizing exposure to lead and other inorganic contaminants potentially present in drinking water has been published by national or international organizations. The EU does not provide detailed information aside from general guidelines on acceptable levels of contaminants in drinking water (European Union 2020).

Part C. References and abbreviations

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C.2 Abbreviations

ALARA

As low as reasonably achievable

ANSI

American National Standard Institute

AOC

Assimilable organic carbon

ASHRAE

American Society of Heating, Refrigerating and Air-Conditioning Engineers

ASME

American Society of Mechanical Engineers

ASSE

American Society of Sanitary Engineering

ATP

Adenosine triphosphate

BAM

Biofilm-associated microorganisms

BLL

Blood lead level

BQL

Below quantifiable limit

CDC

Centers for Disease Control and Prevention

CFU

Colony forming units

CPVC

Chlorinated polyvinyl chloride

CSA

Canadian Standards Association

DALY

Disability-adjusted life years

DBP

Disinfection by-products

EPS

Extracellular polymeric substances

FLA

Free-living amoeba

HAA

Haloacetic acids

HACCP

Hazard Analysis and Critical Control Point

HPC

Heterotrophic plate count

HVAC

Heating, ventilation and air conditioning

IQ

Intelligence quotient

LD

Legionnaires’ disease

LOD

Limit of detection

MAC

Mycobacterium avium complex

MCLG

Maximum Contaminant Level Goal

MPN

Most probable number

N/A

Not available

ND

Not detected

NOM

Natural organic matter

NPC 

National Plumbing Code

NTM

Nontuberculous mycobacteria

OPPP

Opportunistic premise plumbing pathogen

PCR

Polymerase chain reaction

PEX

Cross-linked polyethylene

POE

Point of entry

POU

Point of use

PVC

Polyvinyl chloride

QMRA

Quantitative microbial risk assessment

qPCR

Quantitative polymerase chain reaction

THM

Trihalomethanes

UAT

Urine antigen test

U.S.

United States

UV

Ultraviolet

WMP

Water Management Program

Part D. Summaries of studies on the occurrence of opportunistic premise plumbing pathogens in residential dwellings

Part E. Summary of international recommendations for residential buildings: hot water temperature and Legionella action levels