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Trihalomethanes

What are trihalomethanes?
Trihalomethanes (THMs) may be found in chlorinated water supplies that draw their water from a source with high levels of organic materials. The chlorine used for disinfecting the water combines with these organic materials to produce THMs. You can be exposed to THMs by drinking chlorinated water or during showering or bathing.
Why are THMs a health concern?
The full impact of THMs on public health is currently unclear. A 1995 Health Canada study identifies the need for further examination of the relationship between THMs and cancer. From a public health perspective, these potential health risks must be assessed against the very real health protection benefits from disinfection of drinking water.
Why chlorinate our drinking water supplies?
Chlorine is routinely used for drinking water disinfection because it is a very effective way to reduce bacteria, viruses, and parasites. Chlorine also helps to reduce bacterial regrowth, biofilm formation and recontamination of the water as it travels from the treatment plant to your home. The success of chlorine use in Canada today can be measured by the dramatic reduction of waterborne diseases such as Typhoid Fever, Cholera, and Dysentery.
What level of THMs is acceptable in drinking water?
Drinking water is carefully treated and monitored by the operator, the Ministry of Environment and Climate Change and the health unit to ensure it meets water quality standards. According to the Canadian Drinking Water Guidelines and the Ontario Drinking Water Objectives (ODWO), the Maximum Acceptable Concentration for THMs is a seasonal average of 100 µg/L. To protect the safety of our drinking water supplies, however, it is critical that any method used to control THM levels does not compromise the effectiveness of disinfection.
How can I reduce THMs in drinking water?

The following method(s) can be used to reduce the THM levels in your drinking water:

  • Aerate tap water in a blender
  • Store water in the refrigerator for at least 24-hours
  • Use a water treatment device containing an activated carbon filter
  • Boil water for at least one minute then allow it to cool before drinking
Summary
Chlorine is important in drinking water treatment for controlling algae, reducing odours and pathogenic organisms. The benefit of using chlorine to disinfect our water supplies far outweighs the perceived negative health risk at the present time. Health Canada, The Ontario Ministry of Health and Long-Term Care, and the Simcoe Muskoka District Health Unit support the principle that chlorinating remains the most effective means of ensuring the safety of drinking water. Continued research is required to further understand the potential association between trihalomethanes and adverse health outcomes.

Content adapted with permission from Durham Region Health Department.

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