THM

Trihalomethanes (THMs) are chemical compounds in which three of the four hydrogen atoms of methane (CH₄) are replaced by halogen atoms.

 Many trihalomethanes find uses in industry as solvents or refrigerants. THMs are also environmental pollutants, and many are considered carcinogenic.

Trihalomethanes are formed as a by-product when chlorine or bromine are used to disinfect water for drinking (commonly known as disinfection by-products).

 They result from the reaction of chlorine and/or bromine with organic matter in the water being treated. The THMs produced may have adverse health effects at high concentrations, and many governments set limits on the amount permissible in drinking water. In the United States, the EPA limits the total concentration of chloroform, bromoform, bromodichloromethane, and dibromochloromethane to 80 parts per billion in treated water. This number is called "total trihalomethanes" (TTHM).

Chloroform is also formed in swimming pools which are disinfected with chlorine or hypochlorite in the haloform reaction with organic substances (urine, sweat and skin particles). The reaction to phosgene under the influence of UV is also possible.

Some of the THMs are quite volatile and may easily vaporize into the air. This makes it possible to inhale while showering, for example. The EPA, however, has determined that this exposure is minimal compared to that from consumption.

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