Trihalomethanes are compounds consisting of a carbon atom bound to a hydrogen atom and three halogen atoms (i.e. like methane, but with three of the hydrogen atoms replaced with halogens such as fluorine, chlorine, and/or bromine). The halogen atoms can be of the same type or in different combinations. Chloroform is a well-known example of a trihalomethane that has many industrial and chemical applications; these compounds are not unique to drinking water and exposure to trihalomethanes can occur due to environmental contamination as well as through drinking water.
Trihalomethanes in Drinking Water
Chlorination and filtration of water supplies if often cited as one of the greatest public health achievements of the 20th century, greatly reducing the incidence of diseases such as cholera, dysentery, and typhoid (Calderon, 2000) where implemented. However, it was discovered in the 1970s that chlorine could react with naturally-occurring organic matter in the water to form trihalomethanes (THMs), especially chloroform.
THMs are sometimes also referred to as disinfection by-products, or DBPs. While THMs appear to be the first DBPs recognized, we are now aware of over 600 compounds that appear in drinking water as a result of a variety of disinfection processes (Richardson, 2012). However, only a few DBPs are regulated, and this discussion will be limited primarily to THMs formed as a result of chlorination.
There are four trihalomethanes that have historically been of concern in drinking water, which are sometimes grouped under the heading total trihalomethanes or TTHMs:
- Chloroform (CHCl3)
- Bromodichloromethane (CHCl2Br)
- Dibromochloromethane ( CHClBr2)
- Bromoform – (CHBr3)
Of these, it is chloroform that forms the basis for the minimum acceptable concentration of THMs in Canada (with respect to drinking water standards), because it the compound for which the most data on toxicological effects is available. The drinking water standard is based on the cumulative dose of all four THMs. Canada also has a separate standard for bromodichloromethane (even though it is also included within the trihalomethane guideline). The guidelines are primarily based on evidence from animal studies that identify chloroform as a potential carcinogen. Human studies have also suggested links to cancer, and at high levels reproductive effects. It has been suggested that trihalomethanes that contain bromine are more toxic than chloroform, and bromodichloromethane has been identified as a carcinogen in rodent studies (it is thought to be the most potent carcinogen of the four trihalomethanes being considered, at least in rodents)(Health Canada, 2006). Other guidelines are in effect elsewhere, as will be discussed in future reflections along with the rationale for these guidelines.
Other topics for reflection will included balancing the concern over trihalomethanes against the benefits of disinfecting water supplies, the importance of exposure routes, considering the risk of trihalomethanes relative to the risks of byproducts from alternate disinfection methods and relative to the hundereds of other disinfection by products.
Calderon, R. L. (2000). The epidemiology of chemical contaminants of drinking water. Food and Chemical Toxicology, 38, Supplement 1(0), S13–S20.
Government of Canada, H. C. (2006, May 2). Guidelines for Canadian Drinking Water Quality: Guideline Technical Document: Trihalomethanes – Health Canada. publication. Retrieved September 10, 2013, from http://www.hc-sc.gc.ca/ewh-semt/pubs/water-eau/trihalomethanes/index-eng.php
Richardson, S. D., & Postigo, C. (2012). Drinking water disinfection by-products. In Emerging Organic Contaminants and Human Health (pp. 93–137). Springer. Retrieved from http://link.springer.com/chapter/10.1007/698_2011_125