Trihalomethanes (THMs) are a group of halogenated chemicals that form as disinfection byproducts (DBPs) when chlorine is added to drinking water. There are four THMs that make up the Total Trihalomethane (TTHM) category: bromodichloromethane, bromoform, dibromochloromethane, and chloroform.

THMs are an unintended consequence of water chlorination. They are created when chlorine–a common disinfectant used by most water utilities–reacts in your pipes with organic matter. The term "organic matter" generally refers to a wide group of carbon-based compounds (like vegetation) called humic substances (or humic acids).

It is generally assumed that the risk from pathogens in your water is greater than the health risk from THMs. Therefore, water utilities continue to prioritize chlorine disinfection over THM elimination–almost everywhere. Because chlorine is a near-universal means of disinfection by U.S. public water systems, it is practically inevitable that some THMs will be in your drinking water supply. Other than minimizing and monitoring chlorine and humic acid concentrations, there is essentially no foreseeable way of removing THMs at the water treatment plant. Water system engineers want there to be a chlorine residual (often referred to as “the residual”) in your distribution pipes because this keeps your water safe from potentially harmful bacteria and viruses as it travels through the distribution system to your home. Unfortunately, it is this excess chlorine that reacts with organic matter present in your pipes to create THMs. Ultimately, this decision to chlorinate versus have no THMs is a tradeoff between safety from microbes and safety from THMs.

Because there are so many different treatment products on the market, it is difficult to give a blanket statement about removal of THMs with home treatment products.

That said, typically the best way to remove THMs from your water is to use a carbon filter.

Consider buying a National Science Foundation certified carbon filter for your drinking water at home.

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