Total trihalomethanes (TTHMs) are a group of chemicals that can form in drinking water when chlorine or other disinfectants react with organic matter.

Exposure to high levels of TTHMs over a long period of time may increase the risk of certain health problems, such as cancer and reproductive issues.

Public water systems in Massachusetts are required to monitor TTHM levels and take action if they exceed the maximum contaminant level set by the Environmental Protection Agency. Consumers can also take steps to reduce their exposure to TTHMs, such as using a home water treatment system or letting tap water sit in an open container before using it.