Trihalomethanes

Trihalomethanes in Drinking Water

Trihalomethanes (THMs) are a by-product of the water treatment process.  They are formed when naturally occurring organic substances found in raw water, react with chlorine used to treat the water.  This reaction produces "disinfection by-products" the most common of which are THMs.

What are the THM Levels?

In the Ontario Drinking Water Standards, Objectives and Guidelines the THM level is 0.1 mg/L.

What are the Health Risks?

Current evidence is that THMs do not pose an immediate health risk.  Studies have explored two issues: cancer and pregnancy outcomes.  Some studies suggest that long-term exposure (e.g. 35 years) to high levels of THMs may be linked to a slightly increased risk of some types of cancer, particularly bladder cancer.  Studies exploring the link between high levels of THMs and adverse pregnancy outcomes are not conclusive.  While some studies have shown a weak association, others have not demonstrated any effect.  This is an issue that Health Canada is continuing to actively investigate.

Why do we use Chlorine if it Creates the By-Products?

Chlorination continues to be the best choice to treat drinking water.   It's use since the early 1900's has been a huge public health benefit in largely eliminating plagues such as cholera and typhoid and reducing other health problems caused by waterborne viruses and bacteria, i.e. E. coli.  The benefits of chlorinating drinking water are considered much greater than the risk of health effects from THMs.

I am Pregnant.  Should I Stop Drinking the Water?

It is very important to maintain fluid intake during pregnancy.  Based on current knowledge, the potential risks of adverse pregnancy outcomes associated with drinking water containing THMs are much lower than the risks of serious illness and death that could result from drinking water that has not been properly disinfected. Pregnant women may wish to speak with their doctor for advice.  It is important that pregnant women continue to drink sufficient amounts of water according to their doctor's recommendations.  If you are concerned about the level of THMs in tap water, you have several options as indicated below.

What Options Exist to Reduce THMs?

Several options are available to the homeowner concerned about the level of THMs in their tap water:

• Store tap water in a refrigerator for 24 hours, in an open jug

• Bubble air through tap water in a blender, for one minute

• Boil water and cool

• Use an activated carbon filter

• Buy bottled water.

What is Being Done to Reduce the THM Levels in the Drinking Water?

The Health Unit is actively involved in reviewing the remedial plan being implemented by the municipality and the Ministry of Environment to address the elevated THM levels.  The goal is to reduce the THM levels and maintain low levels over the long term.

Where Can I Get More Information?

You can get more information about THMs at the Health Canada website.

http://www.hc-sc.gc.ca

Specific resources include:

Chlorinated Water and Health Effects

It's Your Health:  Drinking Water Chlorination