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Drinking Water

Trihalomethanes in Drinking Water

Trihalomethanes (THMs) are a group of compounds that can sometimes be found in chlorinated water that is drawn from a source with high levels of organic materials. THMs are by-products of the process of disinfecting water (i.e. the addition of chlorine to water systems). THMs are produced when chlorine reacts with the naturally occurring organics in the source water. THMs are colourless, tasteless and odourless. It is very important (and legally required) that municipal water be chlorinated. Chlorine helps to prevent serious illness from organisms in water that can produce disease. Methods for treating drinking water must be designed and operated to remove or inactivate pathogens as a first priority; minimizing the formation of disinfection by-products is secondary. The health risks from drinking water that has not been disinfected are much higher than the perceived risks from disinfection by-products, including THMs.

Drinking water is carefully treated and monitored by the system operator, and the Ministry of the Environment, Conservation and Parks, to ensure it meets water quality standards. According to the Canadian Drinking Water Guidelines and the Ontario Drinking Water Quality Standards, the maximum acceptable concentration for THMs is a running annual average of 100 µg/L. However, Health Canada recommends drinking water operators strive to maintain THM concentrations as low as reasonably achievable without compromising the effectiveness of disinfection.

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 growth, biofilm formation and recontamination of water as it travels from the treatment plant to your home. The use of chlorine in the treatment of drinking water has virtually eliminated waterborne diseases because chlorine can kill or inactivate most micro-organisms commonly found in water. The majority of drinking water treatment plants in Canada use some form of chlorine to disinfect drinking water. 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.

Drinking water treatment is a balance between ensuring the proper disinfection of water to control the presence of pathogens that can cause severe acute illness and potentially death, while minimizing the formation of disinfection by-products that are associated with cancer risks over long-term exposures.

Can people receive more frequent results of municipal THM sampling?

More information on the Town of New Tecumseth's Tottenham monthly drinking water system sampling results are available on the Town's website. However, monthly results do not represent the overall level of risk for THMs. An annual running average provides a more accurate indication of THM levels within the drinking water system. THM levels within the New Tecumseth drinking water supply are presently below the Ontario Drinking Water Standard, but it is recommended that the Tottenham system maintain THM concentrations as low as reasonably achievable without compromising the effectiveness of disinfection. SMDHU would prefer the THM concentrations in the Tottenham drinking water system be even lower.

Research indicates that periodic increases in THMs above 100 μg/L have not been found to be associated with negative health impacts.

If you are interested in receiving more information about Tottenham's drinking water sampling results, please contact the Town of New Tecumseth.

What private home filtration is available to remove THMs from water?

Ideally, the THM levels in the water should be low enough that you don’t need to purchase additional home filtration units.

If you choose to add a treatment device to your home drinking water supply, it is strongly recommended to use equipment that has an approval stamp from NSF International or ANSI (American National Standards Institute). The NSF website contains information about home drinking water filters.

Health Canada has indicated that THMs may be reduced from the drinking water supply by both activated carbon filters and reverse osmosis systems. These systems can be installed at the faucet, or wherever water enters the home.

If you have treatment equipment installed in your home, carefully monitor and maintain the equipment according to the owner's manual. Filters may need to be replaced frequently when there are high levels of iron or other solids, sometimes referred to as turbidity. Failure to maintain filters in good working condition can result in bacterial overgrowth and contamination of drinking water.

There is more detailed information about THMs and the methods to reduce them in a federal document called Guidelines for Canadian Drinking Water Quality: Guideline Technical Document - Trihalomethanes

What is the provincial average for THMs, and how does that compare to Tottenham?

The Ministry of the Environment, Conservation and Parks reported that they do not have a readily available means of averaging THMs in all the drinking water systems within Ontario. Source water and drinking water treatment systems vary across the province, thus a provincial average would be an unreliable baseline for comparing local sample results. The health-based Ontario Drinking Water Standard for THMs is the only meaningful value for comparative purposes.

What are the health effects associated with exposure to THMs in drinking water?

The health unit requested Public Health Ontario (PHO) provide a technical review of the health effects associated with exposure to THMs in drinking water, and to consider the impact of periodic spikes in THM concentrations. PHO completed this technical review in March 2019 in response to our request. The report is available for review here.

This report found individual THMs are classified as "possibly carcinogenic to humans" or "not classifiable as to carcinogenicity to humans" by the International Agency for Research on Cancer. The review found that there are some studies that have associated exposure to THMs with adverse reproductive effects; however, this association is not consistently found and a dose-response pattern of increased risk with increasing THM concentrations has not been established.

What are the risks associated with THMs in Tottenham Drinking Water?

At the request of the health unit, a further risk assessment of the drinking water for the Town of Tottenham was done by PHO in 2020. The assessment identified potential cancerous hazards of the THMs already documented in the (North American and international) scientific literature and quantified the potential increased risk of cancer based on the documented local concentration of THM. This was done in order to answer questions raised by citizens living in this community regarding past THM exposures. The September 2020 report is available here.

Key Findings:

  • The potential incidence of cancer from Trihalomethane (THM) exposure from Tottenham's water is very small, especially when compared to the ongoing overall potential incidence of these cancers in the Canadian general population. The incidence of cancer from THM exposure to Tottenham's water would be too small to detect with a health study for a population of Tottenham's size.
  • Although the potential incidence is very small (estimated to be up to 2 to 3 cases per 10,000 population when consistently exposed over a 70 year period), the majority of this risk occurs at the Ontario Drinking Water Standards' (ODWS) maximum allowable concentration for THMs (an annual running average of 100 μg/L). This speaks to the importance of reducing THM concentrations to be as low as feasible, in keeping with Health Canada's recommendations.


Chlorine is important in drinking water treatment for controlling pathogenic organisms that can make us sick. Drinking water that is disinfected with chlorine to kill bacteria and viruses such as E. coli and cholera is a benefit that far outweighs the potential health risks at the present time.

Research shows there are no established adverse health effects associated with short-term variations in THM concentrations, however, research is continuing to further understand the potential association between THMs and adverse health outcomes.

If you are interested in receiving more information about public drinking water systems and their sampling results, please contact your local municipal drinking water provider.

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