Canada’s proposed update to its drinking water guidelines would keep the maximum acceptable concentrations for chlorite and chlorate at their current levels, maintaining limits that health officials say already protect against developmental and thyroid effects.
The draft guideline technical document from Health Canada proposes retaining the maximum acceptable concentration (MAC) of 1 milligram per litre for both chlorite and chlorate in drinking water. While the chlorite limit remains based on neurodevelopmental and liver effects observed in a two-generation rat study, the chlorate limit would now be supported by newer long-term research rather than the shorter 90-day animal study used in the 2008 guideline. The approach for chlorine dioxide itself, which does not require a MAC, would also remain unchanged, the department stated.
Chlorite and chlorate are disinfection byproducts formed when chlorine dioxide is used to treat drinking water. Chlorine dioxide, an unstable gas generated onsite from sodium chlorite or sodium chlorate, is used by some municipal systems to control microbes as well as taste, odour and colour. It quickly breaks down into chlorite and chlorate in water and in the body, which is why guidelines focus on those byproducts rather than chlorine dioxide directly.
Although chlorine dioxide is a strong disinfectant, Health Canada says its use in Canada is limited because its disinfectant residual does not last as long as other options and it rapidly decomposes in distribution systems through reactions with organic matter, pipe materials and exposure to light.
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Health Canada notes that drinking water is the primary source of exposure to chlorite and chlorate for Canadians. However, monitoring data show levels in Canadian drinking water are generally well below the proposed limits and are often below detectable levels.
The guideline values are based on animal studies showing chlorite can affect neurodevelopment, brain and liver weights, and thyroid hormone levels, while chlorate primarily affects the thyroid and blood. Thyroid effects were identified as the most sensitive endpoint for chlorate exposure.
The updated technical document is now open for a 60-day public consultation period before the guidelines are finalized.
