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). These humic substances are generated by the decay of soil, leaves, and other microbiology–and frequently occur naturally in water supplies. They are also usually present by the time water enters your home’s pipes–albeit in low concentrations. When water system engineers add chlorine to your tap water at the water treatment plant, it eventually reacts with humic substances and THMs form.

It is widely agreed upon that chlorine is a much needed addition to water. However, it is also commonly understood that THMs can form when excess chlorine reacts with organic matter, which–with long-term exposure–may present some serious health concerns. You cannot taste THMs. Nor can you smell them. But, THMs are potentially dangerous to your health. Some THMs are better studied than others, but exposure pathways are primarily through drinking water and showering. Studies report a connection between TTHMs and the following:

If you are a customer of any U.S. water system, then the U.S. Environmental Protection Agency (EPA) currently regulates the Total THMs (TTHMs) in your water with a Maximum Contaminant Level (MCL). The MCL for all four THM's (i.e. TTHMs) is 0.080 parts per million (which is equivalent to 0.08 mg/L). This means that all THMs in your tap water are not allowed to exceed 0.080 parts per million (PPM). Meanwhile, because there are four types of THMs with their own distinct health effects, they are also monitored for their individual concentrations with a Maximum Contaminant Level Goal (MCLG). The MCLG for each TTHM is as follows:

Note: MCLGs are different from MCLs.

An MCLG is the level of a contaminant in drinking water below which there is no known or expected health risk to humans. MCLGs are not enforced via health regulations and are more like health goals. For now though, it's best to assume that the lowest achievable value is safest. We, at SimpleLab, consider 0 parts per billion THMs as the “best” safe level.

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. In general, the easiest way to reduce THM’s in your tap water is to consider buying a National Science Foundation certified carbon filter for your drinking water at home; ensure that it is certified specifically under the NSF/ANSI Standard 53 Certification for THMs. Point-of-entry systems are recommended in order to protect you both from ingesting THMs as well as inhaling them, for example while doing the dishes or showering.