Bookmark this page

View Printer Friendly PDF Printer Friendly PDF

Facts About...


What are trihalomethanes?

Trihalomethanes (THMs) are a group of organic chemical compounds, which contain carbon hydrogen, and halogen atoms. THMs are found in chlorinated water supplies that draw their water from surface sources e.g., lakes and rivers, which possess high levels of organic materials such as decaying wood and leaves. Chlorine used in disinfecting the water reacts with these organic materials to produce THMs. Exposure to THMs can occur by drinking chlorinated water or inhaling vapours e.g., during showering or bathing.

Why are trihalomethanes 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.

Why chlorinate our drinking water supplies?

In the 19th century, hundreds of thousands of people died in North America and Europe as a result of water borne diseases. Durham Region uses chlorine for municipal water disinfection because it is the most effective means of reducing pathogenic bacteria, viruses, and parasites. 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 Durham Region Works Department and the Ministry of Environment to ensure strict compliance with accepted Provincial treated 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 Fg/L. In Durham Region, THM levels generally range from 10 Fg/L to 60 Fg/L.

How can I reduce THMs in drinking water?

The following method(s) can be used to further reduce 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


Chlorine aids the treatment process by 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 Durham Region Health Department 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 cancer.

More information can be obtained from

Durham Region Health Department, Environmental Help Line 905-723-3818 or 1-888-777-9613

April 22, 2013