When chlorine gas, bleach or other chlorine-containing disinfectant contacts water, it becomes hypochlorous acid. This acid readily reacts with organic matter contained in the water to form trihalomethanes.

These compounds are carcinogenic, meaning they are known to lead to cancer. In 1974, these effects were first acknowledged and in 1979, the US Environmental Protection Agency (EPA) began regulating allowable concentrations of THMs in drinking water, limiting it to 100 parts per billion.