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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 produced when chlorine reacts with the naturally occurring organics in the source water. The health risks from drinking water that has not been disinfected are much higher than the perceived risks from disinfection by-products, including THMs.

Quick facts about THMs, drinking water and risks.

Drinking water is carefully treated and monitored by the operator, the Ministry of the Environment, Conservation and Parks as well as the health unit, 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.
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 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.

Can people receive more frequent results of municipal THM sampling? i.e. on a monthly or weekly basis?

More information on the Town of New Tecumseth's drinking water system results can be accessed by visiting their website. If residents are interested in receiving more information about drinking water sampling results, they can contact the Town of New Tecumseth to request this information.  However, moment-by-moment results aren’t indicative of the overall level of risk for THMs; when brought into a larger set of data, the longer term running average is the real indicator of health risk/safety. Levels are presently below the Ontario Drinking Water Standard now, but SMDHU would prefer they be even lower.

It is important to know that the Ontario Drinking Water Standard requires the annual rolling average of THMs to be below 100 μg / litre (100 micrograms per litre, or 0.1 mg per litre) of drinking water but also as low as can reasonably be achieved, based on samples taken throughout the community. Periodic increases in THMs above 100 μg / L have not been found to be associated with negative health impacts.

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 we strongly recommend using only 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.

It is important that you carefully monitor and maintain the equipment as per 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.

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

The Ministry of the Environment, Conservation and Parks informs us they do not have a readily available means of averaging THMs in all the drinking water systems in Ontario. As source waters and drinking water treatment systems vary so much across the province, such a value would be an unreliable baseline to compare local results to. The health-based Ontario Drinking Water Standard for THMs is the only meaningful value for comparative purposes.

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.

The health unit requested that Public Health Ontario 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. That report is available for review here.


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 perceived negative health risks at the present time.

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

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