TTHMs

TTHMs, or total trihalomethanes, are a class of disinfection byproducts formed when disinfectants such as chlorine react with natural organic matter in water. The group commonly monitored includes four compounds: chloroform, bromodichloromethane, dibromochloromethane, and bromoform. In drinking-water systems, TTHMs are created primarily during chlorination or chloramination as precursors in the water interact with halogens, with higher levels of natural organic carbon, higher bromide content, warmer temperatures, and longer chlorine contact times increasing formation. Brominated species tend to form where source water contains bromide.

Health and regulation: Long-term exposure to elevated TTHMs has been linked in some studies to an increased risk of cancer, particularly bladder cancer, and to potential reproductive effects; the strength of evidence for non-cancer outcomes is less clear. Exposures can occur by ingestion, inhalation, and dermal absorption, with higher risk during showers and hot water use because volatile TTHMs can volatilize into the air.

Measurement and limits: TTHMs are typically reported as a sum of the four species and are measured by trace-level gas chromatography methods with detectors such as electron capture or mass spectrometry. Many countries regulate TTHMs in drinking water, often with a maximum contaminant level around tens to hundreds of micrograms per liter; in the United States the MCL is 80 µg/L as a running annual average.

Mitigation: Reducing formation includes removing natural organic matter ahead of disinfection, optimizing chlorine dose and contact time, and managing pH and temperature. Utilities may also switch or supplement disinfectants with ozone, ultraviolet irradiation, or chloramines, and employ granular activated carbon or other treatment to remove precursors.