Guidelines for Canadian drinking water quality – Malathion: Overview
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Guideline value
The maximum acceptable concentration (MAC) for malathion in drinking water is 0.29 mg/L (290 μg/L).
Executive summary
This guideline technical document was prepared in collaboration with the Federal-Provincial-Territorial Committee on Drinking Water and is based on assessments of malathion completed by Health Canada's Pest Management Regulatory Agency and supporting documents.
Exposure
Malathion is a registered insecticide and acaricide used on a wide variety of sites including agricultural and non-agricultural sites. In 2018 (the most recent year for which data are available), over 25,000 kg of malathion was sold in Canada (Health Canada, 2020a). Malathion may be released into surface water or soil as runoff from the application site.
Malathion is not usually found in drinking water sources in Canada. Low levels of malathion have been found in several Canadian provinces. The maximum reported concentrations are well below the MAC. Malathion is rarely detected in foods.
Health effects
Animal studies indicate that the kidney is the most sensitive target organ for malathion toxicity. There are no human studies on the effects of malathion on the kidney. The MAC of 0.29 mg/L (290 µg/L) is based on an increase in severity of chronic kidney effects seen in a 2-year rat study.
Analytical and treatment considerations
The establishment of drinking water guidelines takes into consideration the ability to both measure the contaminant and remove it from drinking water supplies. Several analytical methods are available for measuring malathion in water at concentrations well below the MAC.
At the municipal level, treatment technologies are available to effectively decrease malathion concentrations in drinking water supplies. Activated carbon, membrane filtration, oxidation and advanced oxidation processes can all be used in the treatment of malathion in drinking water. Advanced oxidation processes achieve the highest removal, with lower removals achieved through oxidation. When using degradation processes like oxidation or advanced oxidation processes, water utilities should be aware of the potential for the formation of degradation by-products (for example, malaoxon). Pilot- and/or bench-scale testing are recommended prior to full-scale implementation.
In cases where malathion removal is desired at a small-system or household level, for example when the drinking water supply is from a private well, a residential drinking water treatment unit may be an option. Although there are no treatment units currently certified for the removal of malathion from drinking water, activated carbon adsorption and reverse osmosis technologies are expected to be effective. When using a residential drinking water treatment unit, it is important to take samples of water entering and leaving the treatment unit and send them to an accredited laboratory for analysis to ensure that adequate malathion removal is occurring.
Application of the guidelines
Note: Specific guidance related to the implementation of drinking water guidelines should be obtained from the appropriate drinking water authority.
The guideline value for malathion is protective against health effects from exposure to malathion in drinking water over a lifetime. Any exceedance of the MAC should be investigated and followed by the appropriate corrective actions, if required. For exceedances in source water where there is no treatment in place, additional monitoring to confirm the exceedance should be conducted. If it is confirmed that source water malathion concentrations are above the MAC, then an investigation to determine the most appropriate way to reduce exposure to malathion should be conducted. This may include the use of an alternate water supply or the installation of treatment. Where treatment is already in place and an exceedance occurs, an investigation should be conducted to verify the treatment and determine if adjustments are needed to lower the treated water concentration below the MAC.
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