Trihalomethanes (THMs)

Trihalomethanes

Trihalomethanes are disinfection byproducts formed when chlorine reacts with organic matter in source water. They appear in essentially every chlorinated tap.

Trihalomethanes (THMs) are a family of four compounds, chloroform plus three brominated cousins, and the most-measured members of a larger group called disinfection byproducts. Chlorination is one of the great public health wins of the last century. By the time water reaches your home, the disinfectant has done its job, and the byproducts left behind can be removed at the point of use. THMs are present in some amount in nearly every chlorinated supply, and because they are volatile, a hot shower releases them into the air you breathe, so drinking water is only part of the exposure. A carbon-block filter at the kitchen tap handles the water you drink but not the bathing route; that takes a shower filter or whole-house carbon. Recent peer-reviewed evidence links higher THM exposure to elevated bladder and colorectal cancer risk, with effects measured below the federal limit, so the level your own home carries is the number worth knowing and acting on.