Radon gas survey: Seasonal variation in Canadian homes

In September 2023, the National Radon Program started a study to assess the seasonal variation of indoor radon levels in Canadian homes over a 12-month period.

On this page

• Summary of early findings
• Background and survey information
• Details of the results so far

Summary of early findings

Health Canada recommends that 3-month radon tests should only be done during the heating season, which is limiting. We are studying if radon testing outside of the heating season can provide a reliable estimate of the annual average radon level in a home.

The results so far support that radon levels tend to be higher in colder months and confirm that our current recommendations best identify homes that exceed the Canadian guideline level of 200 Bq/m³. In all regions of Canada, radon tests during the summer have underestimated annual radon levels. This means that the annual average for the home could be above the guideline level even when summer test results are below.

These results represent only the averages of the various regions. Individual results can vary significantly. These early values should not be used to modify or correct any radon measurements or be used to inform when to test for radon. Further data is needed for the National Radon Program to update our recommendations on when to test for radon.

These early results also show promise for expanding our recommended testing window and introducing separate guidance for testing during the summer. At the conclusion of this study, Canadians will benefit from a more accessible and extended testing window.

Participants continue to be recruited to this study on a limited basis. If you want to participate in this study, you can express your interest by completing our online form:

Participate in the study

Background and survey information

Radon is a naturally occurring radioactive gas and the second leading cause of lung cancer after smoking. Health Canada recommends Canadians take action to reduce their radon levels if they measure above the guideline level of 200 Bq/m³. The guideline should be compared with the annual average radon level in the home, obtained from either a 12-month test or a 3-month test performed during the heating season, providing a conservative estimate.

The recommendation to test during the heating season is based on the assumption that radon levels are highest in winter due to reduced ventilation inside the home. However, limited data exist to confirm how radon levels vary by season in Canadian homes. International studies suggest that seasonal changes can significantly affect radon concentrations. This study helps determine to what extent other seasons can be used reliably for testing, making it easier for more Canadians to test their homes.

About the survey

This national study, led by Health Canada's Radiation Protection Bureau, continues to recruit participants from across Canada. Each participant conducts four consecutive 3-month radon tests as well as one simultaneous 12-month measurement for comparison. Each 3-month test is taken during a different season of the year. Participants also complete a household questionnaire and an end-of-project questionnaire.

Details of the results so far

Data analysis is ongoing as more participants collect radon measurements from their homes. As of May 2026, 397 participants have completed a full 12-month testing period between September 2023 and March 2026.

Results are divided into four seasons across five different regions of Canada. These same five regions were used as part of a 2024 Cross-Canada Survey of Radon Exposure in Residential Buildings .

For each participant, the results from their 3-month tests are individually compared to the result of their 12-month test by calculating the relative percent difference (RPD):

RPD (%) = 100 × (R_3m − R_12m) / R_12m

R_3m - Result of 3-month radon test
R_12m - Result of 12-month radon test

A positive RPD (+%) means the result of a 3-month test was higher than the 12-month test result. A negative RPD (-%) means it was lower. This measure tells us exactly how well a 3-month measurement estimated the annual average in that specific home.

The RPD is used for participants with 12-month test results greater than 25 Bq/m³ (377 participants in total). Below this, at very low radon levels, the relative uncertainty of a radon measurement can limit the ability to observe the impact of any seasonal variation. For this reason, participants with very low radon levels are excluded from this analysis.

A snapshot of the results from data collected to date is shown in table 1, where average RPD values are reported. These early results are based on data collected up to May 2026 for participants with 12-month test results greater than 25 Bq/m³ (377 participants in total).