Trihalomethanes (THMs) are a group of chemicals that can contaminate drinking water. THMs are formed when the chlorine used to disinfect water reacts with natural organic matter (vegetation, dead leaves). THMs contamination occurs especially in drinking water systems supplied by surface water, such as lakes and rivers. THM concentration tends to increase in the summer and at the beginning of fall.

The standard for THM concentration in Québec is 80 micrograms/litre (80 µg/L). This is the annual average that should not be exceeded. Drinking water distribution system officials, including those of municipalities, are responsible for applying this standard.

Health risks

You can be exposed to THM in running water these three ways:

  • Drinking contaminated water
  • Inhaling the vapours of contaminated water
  • When skin comes into contact with contaminated water, such as during a shower or a bath

Prolonged exposure (over a period of at least 20 years) to a high concentration of THMs in water could slightly increase the risk of bladder cancer.

Protection and prevention

If your municipal water utility officials inform you that THM levels in the water system are high, you should not necessarily stop using tap water. However, you can take preventive steps and reduce your exposure to THMs:

  • Fill a pitcher with water and place it in the fridge for 24 hours without covering it before consuming the water. This allows the THMs to partially evaporate.
  • When you bathe or shower, open a window or run the fan to ventilate the bathroom properly.
  • If the THM contamination is significant, use bottled water instead of tap water.
  • You may also use a device to treat the water, such as a pitcher water filtration system, or install a filter on the faucet or under the sink. These devices must conform to NSF/ANSI standards for the elimination of volatile organic substances. Follow the manufacturer’s instructions to maintain these devices regularly.

Even when water is contaminated with THMs, you can continue using tap water to:

  • Brush your teeth
  • Wash fruits and vegetables
  • Prepare foods with water, such as soup and rice
  • Wash dishes and clothes

You can prepare baby formula with tap water provided that you first bring the water to a rolling boil for at least 1 minute. The THMs will evaporate and any microbes will be destroyed.