s) are chemical compounds
in which three of the four hydrogen atoms of methane
) are replaced by halogen
atoms. Many trihalomethanes find uses in industry as solvents
. THMs are also environmental pollutants, and many are considered carcinogenic
. Trihalomethanes with all the same halogen atoms are called haloforms
Table of common trihalomethanes
Common trihalomethanes (ordered by molecular weight)
||CAS registry number
||Freon 23, R-23, HFC-23
Trifluoromethane and chlorodifluoromethane are both used as refrigerants
in some applications. Trihalomethanes released to the environment break down faster than chlorofluorocarbons
(CFCs), thereby doing much less damage to the ozone layer
(if they contain chlorine). Chlorodifluoromethane is a refrigerant HCFC
, or hydrochlorofluorocarbon, while fluoroform is an HFC, or hydrofluorocarbon
. Fluoroform is not ozone depleting.
Unfortunately, the breakdown of trihalomethane HCFCs does still result in the creation of some free chlorine radicals in the upper atmosphere and subsequent ozone destruction. Ideally, HCFCs will be phased out entirely in favour of entirely nonchlorinated refrigerants.
Chloroform is a very common solvent
used in organic chemistry. It is a significantly less polar
solvent than water, well-suited to dissolving many organic compounds
Although still toxic and potentially carcinogenic, chloroform is significantly less harmful than carbon tetrachloride. Because of the health and regulatory issues associated with the use of carbon tetrachloride, in modern chemistry laboratories chloroform is used as a cheaper, cleaner alternative wherever possible.
Trihalomethanes are formed as a by-product when chlorine
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.