Trihalomethanes (THMs) are a by-product of water disinfection; they form when chlorine and other disinfectants react with organic and inorganic elements already in the water.
THMs consist of several compounds (such as bromoform, chloroform, bromodichloromethane, fluoroform, iodoform, dibromochloromethane), in which halogen atoms replace most of the hydrogen atoms of methane.
Some trihalomethanes find uses in industry as solvents (chloroform) or refrigerants (trifluoromethane and chlorodifluoromethane). However, most are simply by-products of disinfection, and their presence in the environment is unintentional.
THMs are only one group out of hundreds of disinfection by-products. Much more research is needed to isolate the exact compounds that pose a serious health risk, however, for safety reasons, THMs are deemed carcinogenic at high levels, and constitute an environmental pollutant. They are also believed to cause reproductive system issues. For this reason, the EPA regulates total THMs to 80 parts per billion, and advises the use of active carbon systems and cartridges for home water disinfection.