Guidelines for Canadian drinking water quality – Malathion: Rationale

Malathion is a registered insecticide and acaricide used on a wide variety of sites including agricultural and non-agricultural sites. Despite its common use in Canada, data provided by provinces and territories that monitor for malathion in source and drinking water indicate that, when detected, levels of malathion are well below the MAC. The kidney has been identified as the most sensitive target organ for malathion toxicity. Although no human studies have investigated the effects of malathion on the kidney, animal studies conducted in rats and dogs have consistently shown nephrotoxicity following malathion exposure. Malathion also inhibits ChE but at higher doses than those causing nephrotoxicity. Its active metabolite, malaoxon, has been shown to be a more potent ChE inhibitor. Malaoxon is only a minor environmental degradate of malathion but can be produced during drinking water treatment. Nevertheless, because reported levels of malathion in source and drinking water are well below the MAC, any formation of malaoxon during water treatment is expected to be negligible. Therefore, this document does not propose an additive approach (that is, use of a TAF) for malathion and malaoxon in drinking water. An additive approach for malathion and malaoxon in drinking water would result in added monitoring requirements and costs, with no improvement in health protection.

Health Canada, in collaboration with the Federal-Provincial-Territorial Committee on Drinking Water established a MAC of 0.29 mg/L (290 µg/L) for malathion in drinking water based on the following considerations:

• an HBV of 0.29 mg/L (290 µg/L) based on an increase in severity of chronic nephropathy in female rats
• analytical methods are available to accurately measure malathion at concentrations well below the MAC
• treatment technologies are available to effectively decrease malathion at concentrations well below the MAC

The MAC is protective of potential health effects from malathion exposure. As part of its ongoing guideline review process, Health Canada will continue to monitor new research in this area, including the outcomes of PMRA's evaluations, and recommend any change to this guideline technical document that it deems necessary.