Trihalomethanes are chemicals formed when chlorine or bromine reacts with organic matter in water, and are found in virtually all chlorinated tap water. They include chloroform and other compounds, some of which are possible carcinogens. While the health effects of trihalomethanes are controversial, the US EPA has established regulations on TTHM in drinking water due to evidence of increased cancer risk. Trihalomethanes can be removed using activated carbon filters, but not reverse osmosis filters. While there is concern about carcinogens in drinking water, the risk of not disinfecting water is high and must be balanced with the benefits of disinfection.
Trihalomethanes are a group of chemicals that include chloroform and structurally related compounds. Some are used industrially as refrigerants and solvents. The term is generally applied to a subset of these compounds that are possible carcinogenic byproducts of water chlorination.
Disinfecting water with chlorine or bromine eliminated many former waterborne diseases, such as dysentery and cholera, which used to kill large numbers of people. A side effect of these treatments, however, can be the formation of disinfection byproducts. These by-products are formed when natural decomposing organic matter in the water reacts with the chlorine or bromine used in disinfection. These can also be produced in swimming pools by the reaction of chlorine with sweat, skin particles or urine.
One group of chemicals produced is chloroform, bromoform, bromodichloromethane and dibromochloromethane. While there are many compounds that can, chemically, be considered trihalomethanes, these four compounds are usually what is meant when the term is used. Their sum is indicated as total trihalomethanes (TTHM). Chloroform is generally the most prevalent of the group and these are found in virtually all tap water that has been chlorinated.
The health effects of trihalomethanes are controversial within the scientific community. Studies with laboratory animals have shown an increase in tumors at high doses, but such studies cannot always be extrapolated to humans. Other studies have compared cancer rates with TTHM exposure levels in large numbers of people, and these results have also been inconsistent. There have also been conflicting studies on the effects of these substances on human development and reproduction.
The United States Environmental Protection Agency (EPA) has concluded that there is sufficient evidence to establish regulations on TTHM in drinking water. There may be a slight risk of increased colorectal or bladder cancer if you drink water contaminated with trihalomethanes at a level above 80 parts per billion (ppb) throughout your life. Some US states and bottled water companies limit levels down to 10 ppb. Many other countries also regulate levels in their water supplies.
Most trihalomethane exposure comes from consumption, drinking water, or cooking with it. Swimmers can absorb these compounds through the skin. Some trihalomethane compounds can also become volatile and evaporate into the air when you shower. These can then be inhaled.
Trihalomethane removal can be done using an activated carbon filter, of which there are many types. Coconut shell based carbons appear to be the best type of filter for removing small organic compounds like these. Reverse osmosis unit filters do not remove them, but reverse osmosis systems have additional carbon filters that remove trihalomethanes.
Chemically, a trihalomethane is a derivative of the carbon-containing compound methane (CH4), in which three of the hydrogen atoms have been replaced with a halogen atom. This can include chlorine, bromine, fluoride or iodine atoms. Trihalomethanes can have multiple halogens of the same type.
While there is concern about carcinogens in drinking water, it should be noted that TTHMs pose no immediate risk and may only show effects after decades of exposure. The risk of not disinfecting the water is very high, especially with the frequent pollution of fecal bacteria from wastewater. You have to balance the benefits and risks of these two factors.
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