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What Are Trihalomethanes and How Are They Formed?

What Are Trihalomethanes and How Are They Formed?


Trihalomethanes (THMs) are a group of halogenated chemicals that form as disinfection byproducts (DBPs) when chlorine is added to drinking water.[1] 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).[2]

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.

What Dangers Do Trihalomethanes (THMs) Present In Drinking Water?

While it is generally agreed that chlorine is a much needed addition to water, THMs can form when excess chlorine reacts with organic matter, which may present some serious health concerns with long term exposure. Studies report a connection between TTHMs and the following:[3]

  • Increased risk of cancer
  • Spontaneous miscarriage
  • Cardiovascular defects
  • Low birth weight
  • Liver and kidney damage
  • Central nervous system problems

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.

Disinfection Byproducts: The Adverse Effects of Water Chlorination—SimpleLab Tap Score

      What Concentrations of THMs Are Safe? 

      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) of 0.080 parts per million (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:

      • Bromodichloromethane: 0 PPM
      • Bromoform: 0 PPM
      • Dibromochloromethane: 0.06 PPM
      • Chloroform: 0.07 PPM

          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. It's best to assume that the lowest achievable value is safest. At SimpleLab, we consider 0 parts per billion THMs as the “best” safe level.

          The Difference Between MCLs and MCLGs—SimpleLab Tap Score

          What Type of Treatment Methods Are Effective Against TTHMs at Water Utilities? 

          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. 

          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. 

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          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.

          7 Pathogens That Contaminate Drinking Water—SimpleLab Tap Score

          How Can You Remove Trihalomethanes (THMs) at Home? 

          In general, the easiest way to reduce THM’s in your tap water is to consider buying a carbon filter for your drinking water at home, 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 while doing the dishes or showering.

          Top 5 Most Popular Water Filtration Technologies For Homes—SimpleLab Tap Score

          If you have a question about a particular treatment product, come to our website and ask us!

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