trihalomethanes

Trihalomethanes (THMs) are a group of chemical compounds in which three halogen atoms, typically chlorine or bromine, replace hydrogen atoms on a methane backbone. The four most common THMs found in water and air are chloroform (CHCl3), bromodichloromethane (CHBrCl2), dibromochloromethane (CHBr2Cl), and bromoform (CHBr3).

THMs form mainly as disinfection byproducts when chlorine- or bromine-based disinfectants react with natural organic matter, algae, and other organic material in surface water or groundwater. They can remain in treated drinking water and, because several THMs are volatile, may migrate into indoor air during activities such as showering or cooking.

Physically, THMs are relatively volatile organic compounds with varying water solubilities and modest vapor pressures. They tend to partition between water and air and can be transported through the environment, though their persistence depends on sunlight, temperature, and microbial activity.

Health and regulation: Long-term exposure to THMs in drinking water has been associated with an increased risk of cancer and other health effects in some studies, leading regulatory agencies to set limits on their levels. In the United States, the Environmental Protection Agency governs THMs in drinking water with a maximum contaminant level for total THMs (TTHMs) of 80 micrograms per liter (µg/L) as a running annual average. Measurements are reported as total THMs or as individual species.

Occurrence and management: THMs occur worldwide in chlorinated drinking-water supplies, with concentrations influenced by chlorine dose, contact time, water temperature, and the amount of natural organic matter. Water treatment strategies, such as removing precursors or using alternative disinfectants, can reduce THM formation.