Toxic Tuesdays

Trihalomethanes (THMs) are a class of chemical compounds which contain three halogen atoms. Common THMs include chloroform, fluoroform, and chlorodifluoromethane. While THMs are used in some industrial processes like refrigeration, people are most likely to be exposed to them through drinking water. Most water utilities use small amounts of chlorine as a disinfectant to keep water supplies clean. While adding chlorine is generally accepted to be an important practice to protect public health, this chlorine can react with organic matter in the water and create THMs. Drinking contaminated water is generally considered to be the most serious route of exposure to THMs, though one study has found that bathing with contaminated water causes even higher exposure. THMs are odorless and flavorless, so people may not know if they have been exposed.

Many types of THMs have adverse health effects upon exposure, and there are four that are known to be particularly harmful to human health:

1. Chloroform is known to cause lung, liver, and kidney damage, and in high doses can lead to death. In studies of laboratory animals, drinking chloroform-contaminated water caused liver and kidney cancer. The Environmental Protection Agency (EPA) classifies it as a likely cancer-causing agent in humans. 
2. Bromoform can cause neurological impairments, unconsciousness, and death. In studies of laboratory animals, drinking bromoform-contaminated water caused liver and kidney cancer. EPA classifies it as a probable cancer-causing agent in humans.
3. Dibromochloromethane is less well understood, but in studies of laboratory animals, drinking contaminated water caused liver and kidney cancer. EPA classifies it as a possible cancer-causing agent in humans.
4. Bromodichloromethane is known to cause liver, kidney, and immune system damage. It can also cause reproductive system damage and lead to miscarriage and low birth weight. In studies of laboratory animals, drinking bromodichloromethane-contaminated water caused intestinal, kidney, and liver cancer. EPA classifies it as a probable cancer-causing agent in humans.

Because of the danger of THM exposure, EPA regulates the maximum amount of total THMs allowed in tap water. While this can help keep people safe, regulating the total THMs may not necessarily be effective because each type of THM can cause health effects on their own. Furthermore, little is known about the health effects of simultaneous exposure to multiple THMs. EPA has maximum contaminant level goals (MCLGs) for each of the four THMs listed above, but these goals are not enforced by laws or regulations, so they have no power to keep people safe. Because most people get their water from chlorinated municipal water supplies, and because THMs have such serious health effects, more must be done to keep people safe from exposure. This includes more studies to understand the effects of simultaneous exposure to multiple THMs and enforceable standards for individual THMs in drinking water.