Trihalomethanes (THMs)

THMs consist of a group of four chemicals - chloroform, bromodichloromethane, chlorodibromomethane and bromoform. These are halogen-substituted single carbon compounds and are formed primarily as a result of chlorination. High levels of THMs can be dangerous. Some studies have suggested a small increase in the risk of bladder and colorectal cancers. Other investigations have found that these chlorination by-products may be linked to heart, lung, kidney, liver and central nervous system damage. Pregnant woman appear to be at the greatest risk, as some studies have linked THMs to reproductive problems, including miscarriage. An Interim maximum acceptable concentration of 0.1 mg/L is established as a running annual average. It is based on the risk associated with chloroform, the trihalomethane most often present and in greatest concentration in drinking water.