Trihalomethanes in Public Water and Health

trihalomethanes in public water

Trihalomethanes are a family of chemicals formed when disinfectants used to kill viruses and bacteria in community water supplies react with naturally occurring organic matter and other substances in the source water. The risk of illness from trihalomethanes is much lower than the risk of illness from drinking most surface water and some groundwater sources that have not been disinfected.

Trihalomethanes include four related chemicals and have a combined maximum contaminant level (MCL) of 80 mcg/L:

  • chloroform
  • bromodichloromethane
  • dibromochloromethane
  • bromoform

Exposure and Risk

When people consume trihalomethanes at high levels over many years, they increase their risk of developing bladder cancer. Other health effects that may be associated with trihalomethanes include rectal and colon cancer, and adverse developmental and reproductive effects during pregnancy. They have been studied with mixed results; however, the weight of evidence of the health effects data suggests a potential association.

There are several ways that trihalomethanes can get into your body.


Community Public Water Supplies are already being tested for trihalomethanes, and are required to provide that information each year to consumers in the annual Consumer Confidence Report. If your Public Water System has notified you of a trihalomethanes violation, it does not mean that the people who consume the system's water will become sick.

The U.S. Environmental Protection Agency (EPA) requires that water systems use treatment methods to reduce the formation of trihalomethanes and to protect people from waterborne disease and the potential harmful effects of trihalomethanes.

  • Ingestion (through your mouth): drinking water with trihalomethanes
  • Inhalation (through your nose): Some trihalomethanes can be released into the air in your home when you use your tap water. This can happen when you are taking a shower or washing dishes. The hotter the water is, the more likely it is that trihalomethanes will be released into the air. Trihalomethanes can also get into the air when you boil your tap water, such as when you make tea or soup.
  • Dermal (through your skin): You can be exposed to trihalomethanes when your skin comes into direct contact with water, like when you are bathing or showering. But for most people, only very small amounts of trihalomethanes get into the body through the skin. However, much higher levels of trihalomethanes can get in your body when your contact time with water increases. This can happen if you typically take long baths or swim frequently in public pools.