What are Trihalomethanes (THMs)?

Trihalomethanes (THMs) are a group of 4 chemical compounds that form in water disinfected with chlorine. They are known to be toxic but are not yet well understood by science. 

Of over 700 known chemicals that result from adding disinfectant to water, only 9 are currently regulated; four of these are THMs.

Trihalomethanes (THMs) are the most commonly known and researched group of disinfection byproducts. The THM group consists of:

• Chloroform
• Bromodichloromethane
• Dibromochloromethane
• Bromoform

Although these 4 types of THMs are monitored individually, they are regulated as a group. In other words, federal guidelines for the maximum acceptable concentration of THMs are based on one limit for all 4 THMs combined so you will sometimes see reference to TTHMs (total triahalomethanes).

Guidelines for total THMs in tap water

• HEALTH CANADA 
  0.1 milligrams per litre (100 parts per billion)
  In Canada, this is a recommended guideline, not an enforceable limit.
  
  
• EPA (USA)
  0.08 milligrams per litre (80 parts per billion)
  In the USA, this lower (than Canada) limit is enforced.

Trihalomethanes (THMs) are linked to cancers and birth defects.

Studies * indicate that negative health effects may occur from daily exposure to THMs at concentrations lower than the Health Canada and EPA levels listed above.

Trihalomethanes are suspected to damage the bladder, liver, kidneys and central nervous system. They are considered to be probable carcinogens and have been linked to bladder, kidney and colo-rectal cancers, miscarriage and birth defects.

Extra precautions are recommended during pregnancy.

Finding the balance between adequate disinfection of tap water and minimizing exposure to the chemical byproducts of disinfection at the municipal water treatment level is discussed further in The Chlorine Dilemma.

Some factors that influence the formation of THMs in tap water. 

• Higher concentrations of THMs tend to occur in municipalities supplied by surface water rather than by ground water. This is because surface water contains more organic material (such as leaves) with which the chlorine reacts resulting in the formation of THMs.
• Trihalomethane concentrations increase with heat and can form in hot water heaters used with chlorinated water.
• The formation of THMs is more likely to occur as the pH of water is increased.

How to minimize your exposure to THMs in chlorinated tap water.

• Use a point-of-use drinking water filterthat has been third party tested or is NSF certified for high capacity reduction of trihalomethanes (NSF/ANSI Standard 53 for Health Effects).
• Take shorter showers.
• Use cooler water.
• Install a whole house carbon filterto remove chlorine from incoming water before it gets to your hot water heater. Heat causes the formation of additional THMs. 

Showers, bathing and exposure to THMs.

THMs are volatile gases that vaporize at temperatures lower than turn water to steam. Showers concentrate these toxic gases because showering and bathing typically takes place in an enclosed space. Consequently, you may be inhaling THMs into your lungs and absorbing them through the opened pores of your skin. These pathways into the body introduce THMs directly into the bloodstream. 

Shower filters and bath dechlorinators are effective at reducing your exposure to chlorine. However, they have no ability to remove THMs and other disinfection byproducts. Carbon filtration is the most effective means for scavenging THMs however it is only effective with cooler water temperatures and at slower flow rates than are appropriate for shower and bath water.

Be wary of shower filters that use carbon as a medium of filtration. The effectiveness of carbon used in this way will be very short-lived. For this reason, we recommend shower filters that use Chlorgon, KDF or vitamin C.

....

References

Health Canada re THMs
https://www.canada.ca/en/health-canada/services/publications/healthy-living/guidelines-canadian-drinking-water-quality-trihalomethanes.html

EPA (USA) re THMs
https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations#Byproducts 

*  Overview of Disinfection By-products and Associated Health Effects
Current Environmental Health Reports, March 2015, Volume 2, Issue 1, pp 107–115 
Cristina M. Villanueva, Sylvaine Cordier, Laia Font-Ribera, Lucas A. Salas, Patrick Levallois