Trihalomethanes (THM)

A group of four chemicals that are formed along with other disinfection byproducts when chlorine or other disinfectants used to control microbial contaminants in drinking water react with naturally occurring organic and inorganic matter in water.

The trihalomethanes are chloroform, bromodichloromethane, dibromochloromethane and bromoform.

EPA has published the Stage 1 Disinfectants and Disinfection Byproducts Rule to regulate total trihalomethanes (TTHM) at a maximum allowable annual average level of 80 parts per billion.