Trihalomethanes (THMs) are chemical compound
s in which three of the four hydrogen atoms of methane
) are replaced by halogen
atoms. Many trihalomethanes find uses in industry as solvent
s or refrigerant
s. THMs are also environmental pollutants, and many are considered carcinogen
ic. Trihalomethanes with all the same halogen atoms are called haloforms. Several of these are easy to prepare through the haloform reaction
Trihalomethanes were the subject of the first drinking water regulations issued after passage of the U.S. Safe Drinking Water Act
Table of common trihalomethanes
Only chloroform has significant applications of the haloforms. In the predominant application, chloroform is required for the production of tetrafluoroethylene
, precursor to teflon
Chloroform is fluorinated by reaction with hydrogen fluoride
to produce chlorodifluoromethane
(R-22). Pyrolysis of chlorodifluoromethane (at 550-750 °C) yields TFE, with difluorocarbene
as an intermediate.
+ 2 HF → CHClF2
+ 2 HCl
+ 2 HCl
are both used as refrigerant
s. Trihalomethanes released to the environment break down faster than chlorofluorocarbons
(CFCs), thereby doing much less damage to the ozone layer
. Chlorodifluoromethane is a refrigerant HCFC
, or hydrochlorofluorocarbon
, while fluoroform is an HFC, or hydrofluorocarbon
. Fluoroform is not ozone depleting.
Chloroform is a common solvent
in organic chemistry.
Trihalomethanes are formed as a by-product predominantly when chlorine
is used to disinfect drinking water
. They are generally referred to as disinfection by-product
s. They result from the reaction of chlorine or bromine with organic matter
present in the water being treated. The THMs produced have been associated through epidemiological studies with some adverse health effects. Many governments set limits on the amount permissible in drinking water. However, trihalomethanes are only one group of many hundreds of possible disinfection by-products—the vast majority of which are not monitored—and it has not yet been clearly demonstrated which of these are the most plausible candidate for causation of these health effects. In the United States
, the EPA
limits the total concentration of the four chief constituents (chloroform
, and dibromochloromethane
), referred to as total trihalomethanes (TTHM), to 80 parts per billion
in treated water.
Traces of chloroform
is also formed in swimming pools
that are disinfected with chlorine
in the haloform reaction
with organic substances (e.g. urine
particles). Although it is possible to inhale THMs, the U.S. EPA has determined that this exposure is minimal compared to that from consumption. In swimmers, uptake of THMs is greatest via the skin with dermal absorption accounting for 80% of THM uptake.
Exercising in a chlorinated pool increases the toxicity of a "safe" chlorinated pool atmosphere
with toxic effects of chlorine byproducts greater in young swimmers than older swimmers.
Studies in adolescents have shown an inverse relationship between serum testosterone levels and the amount of time spent in public pools. Chlorination by-products have been linked as a probable cause.
National Pollutant Inventory - Chloroform and trichloromethaneEPA - Trihalomethanes in Drinking Water: Sampling, Analysis, Monitoring and Compliance (August 1983)