Trihalomethanes are chemical byproducts formed when chlorine used to disinfect water reacts with organic matter. They are common in chlorinated water and may increase cancer risk.

Trihalomethanes (THMs) are a group of four chemicals that form when chlorine, used to disinfect water, reacts with naturally occurring organic matter. The main THMs are chloroform, bromodichloromethane, dibromochloromethane, and bromoform. These compounds are commonly found in chlorinated drinking water and are measured in parts per billion (ppb). The EPA regulates total trihalomethanes (TTHMs) due to potential health concerns. THMs can enter the body through drinking, inhaling, or absorbing chlorinated water.