Trihalomethanes (THMs)

THMs are trihalomethanes, chemical compounds that can be formed when water is disinfected with chlorine. THMs occur when chlorine reacts with organic matter in water. The four THMs are chloroform, bromodichloromethane, dibromochloromethane, and bromoform.

High levels of trihalomethanes can be dangerous. In fact, in December 2000, the U.S. Environmental Protection Agency lowered the maximum allowable annual average level for large surface water public water systems from 100 parts per billion (ppb) to 80 ppb.

Some scientific studies have linked THM to increased risk of cancer.  Several studies suggest a small increase in the risk of bladder cancer and colorectal cancer.  Beyond the cancer and reproduction concerns, some investigations have found that chlorination by-products may be linked to heart, lung, kidney, liver, and central nervous system damage. Other studies have linked THM to reproductive problems, including miscarriage.   A California study found a miscarriage rate of 15.7% for women who drank 5 or more glasses of cold water containing more than 75 ppb TTHM, compared to a miscarriage rate of 9.5% for women with a low THM exposure.