trihalometaner

Trihalomethanes (THMs) are a class of chemical compounds that derive from methane with three hydrogen atoms replaced by halogen atoms. The general formula is CHX3, where X is a halogen such as chlorine, bromine, fluorine, or iodine. The most common THMs encountered in environmental contexts are chloroform (CHCl3), bromodichloromethane (CHBrCl2), dibromochloromethane (CHBr2Cl), and bromoform (CHBr3). Fluorinated or mixed-halogen THMs are also reported, but are typically less prevalent in natural waters.

Formation and sources: THMs primarily form as disinfection by-products when chlorine or chloramines used in water treatment react with natural organic matter. They can also arise from certain industrial processes or the use of halogenated solvents. Because they are relatively volatile, THMs can partition into air from water surfaces and accumulate in indoor air during bathing or showering.

Environmental occurrence and regulation: In drinking water and wastewater, THMs are monitored as total trihalomethanes (TTHMs), which is the sum of the commonly regulated species. Concentrations vary with source water quality, treatment practices, and season. Many countries regulate TTHMs in drinking water and set maximum allowable concentrations to limit exposure.

Health and safety: THMs are of interest because some compounds within the group show toxicity in laboratory studies, and there is evidence linking long-term exposure to certain THMs with health risks. Chloroform, in particular, is classified by some agencies as possibly carcinogenic to humans based on animal studies. Health risk assessments generally focus on exposure via drinking water, inhalation of volatilized THMs during showering, and dermal contact.

Detection: THMs are typically measured using gas chromatography with detectors such as mass spectrometry or electron capture detectors, with results reported in micrograms per liter.