Trihalomethanes are the most common type of “disinfection byproduct,” which is a substance created from the treatment of water with organic matter.

How They Form

Chlorine is the most popular water disinfectant, used widely since the beginning of the 20th century to kill viruses and microorganisms in water. It has had a major role in significantly reducing global instances of cholera and typhoid given its effectiveness and relatively low cost. 

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.

How to Reduce Their Effect

Some tactics adopted by water treatment plants to attempt to avoid producing THMs mostly revolve around limiting chlorine’s contact with organic matter in water.

One strategy involves delaying the application of chlorine, giving organic matter less time to react with the hypochlorous acid.

If possible, and if organic matter content is too high, some municipalities switch to different water sources entirely.

Coagulation methods are also being implemented, which allow organic matter to collect into particulates which can be removed from water before treating it with chlorine.

Still others are switching disinfectants to ozone or chloramines, which don’t convert to hypochlorous acid.

Many home water filters can remove most trihalomethanes from tap water.