Government of Nova Scotia Government of Nova Scotia Nova Scotia, Canada


Drinking Water

Reducing or eliminating trihalomethanes (THMs) in drinking water

What are THMs?

THMs are trihalomethanes, chemical compounds that can be formed when water is disinfected with chlorine. THMs occur when chlorine reacts with organic matter in water, and for this reason are more common in surface water supplies throughout Canada.

Is there a drinking water guideline set for THMs in drinking water?

The acceptable level, or guideline, for THMs in drinking water in Canada is set by Health Canada in consultation with the provinces and territories. The Canadian drinking water guideline for total THMs is 100 micrograms of THMs per litre of water ( µg/L). The guideline is based on a locational running annual average of a minimum of four quarterly samples. An updated guideline for total THMs was released by Health Canada in May 2006. For more information, go to: Guidelines for Canadian Drinking Water Quality - Supporting Documents.

Why was the guideline updated in 2006?

New studies have been published since the last Canadian guideline on THMs was established. In response to these new findings, Health Canada established a National Chlorination Disinfection By-Products Task Group. It assessed the health risks from THMs and developed risk management options. In 2003, its recommendations were incorporated into the decision process. In the fall of 2005, public consultation began. The guideline for total THMs was approved in May 2006. Health Canada will continue to monitor new research in this area and recommend any changes to the guidelines when necessary.

What are the potential problems of having high THMs in drinking water?

There is concern among experts in Canada that THMs may pose a risk in the development of cancer. And though there is presently insufficient evidence to establish a causal relationship, some studies report an association between THMs and adverse birth outcomes

How are Canadians exposed to THMs in drinking water?

Canadians can be exposed to THMs when drinking tap water containing the chemical compounds. Showering, bathing and other water use activities can also contribute to an individual's total exposure. For example, THMs can evaporate from the tap and be inhaled during showering, or they can be absorbed through the skin during bathing. These routes of exposure have been considered in the guideline setting process. Water that meets the total THMs guideline is considered safe for all domestic uses, including drinking, bathing, showering and food preparation.

Should pregnant women take special precautions?

While THMs may be a contributing factor to birth outcomes, there are several other risk factors involved. However, Nova Scotia's Medical Officer of Health still recommends that pregnant women be aware of the THM levels in their water, and take steps to reduce their THM intake.

If I'm worried about THM levels in my water supply, what can I do?

The easiest way to reduce or eliminate THMs in drinking water is to use a water pitcher with a carbon filter, install a tap-mounted carbon filter, or to use bottled water. When using a filter, check to verify that it is certified to remove THMs and follow replacement instructions recommended by the manufacturer. When using bottled water, check to verify that the supplier is a member of the Canadian Bottled Water Association or the International Bottled Water Association. Individuals may also want to keep the length of time spent in showers or baths to a minimum in areas with elevated levels of THMs in drinking water.

What if I have my own water well?

Most private well water supplies are not disinfected. The lack of chlorine disinfection means there should be no THMs present.

Why don't we simply stop chlorinating our drinking water?

Without adequate disinfection of our water supplies, the health risks from micro-organisms would far outweigh the risks from THMs. Drinking water is disinfected with chlorine to kill bacteria and viruses that can cause serious illnesses and deaths. In fact, chlorination of drinking water has virtually eliminated typhoid fever, cholera and many other diseases; it represents one of the greatest achievements of public health protection.

Can THMs be eliminated at the source?

The province is working with municipalities to develop strategies for reducing THMs, including further treatment at the source. Research continues on new treatment methods that will reduce the levels of by-products while maintaining sufficient levels of disinfection to kill micro-organisms.

Which communities have the highest levels?

Levels normally vary within a single water supply depending on the season, water temperature, amount of natural organic matter in the water, pH, amount of chlorine used, time in the distribution system and other factors. For information regarding the THM levels in your community's drinking water supply, please contact your local water utility or municipality. A listing is available at: Municipal Contacts.

How do I test for THMs in my drinking water?

Individuals on municipal water supplies do not need to have their water supply tested. Nova Scotia Environment requires municipalities to regularly monitor water quality. If you have a private well that does not mix chlorine with the water, there should be no need to test for THMs. However, if you wish to have your water tested, please contact laboratories offering chemical analyses to find out what kind of bottle to use and how to properly collect a water sample. A laboratory listing is available at: Water Testing Labs in Nova Scotia.