How to Remove Trihalomethanes (THMs) from Drinking Water

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Community water disinfection protects our tap water supplies from harmful microorganisms, but chemical disinfectants have their disadvantages, too.

In this guide, we’ve answered the question, “How do you remove trihalomethanes from drinking water?” We’ve discussed and compared the most effective methods of removing THMS with water treatment methods at home.

📌 Key Takeaways:

  • Trihalomethanes (THMS) are byproducts produced when water is disinfected with chlorine or chloramine.
  • The best ways to remove THMs from water are granular activated carbon and RO filtration.
  • Other effective ways to reduce or remove THMs are by boiling the water or purifying the water in a distillation system.

💡 What Are Trihalomethanes?

Trihalomethanes (THMs) are a class of chemical compounds that are produced when chlorine or chloramine (a combination of chlorine and ammonia) is used to disinfect drinking water. THMs are known as disinfection byproducts because they occur as a byproduct of disinfection, when chlorine or chloramine reacts with the naturally occurring organic matter present in the water.

THMs are so-called because of their chemical structure. They consist of a single carbon atom bound to three halogen atoms (bromine, chlorine, or iodine). The most common types of trihalomethanes are:

  • Chloroform (CHCl3): The most well-known and prevalent THM in water
  • Bromodichloromethane (CHCl2Br): Contains bromine and chlorine atoms.
  • Bromoform (CHBr3): Contains only bromine atoms.
  • Dibromochloromethane (CHBr2Cl): Contains bromine and chlorine atoms.

THMs are commonly found in drinking water that has been treated with chlorine or chloramine, as well as in swimming pools and any other water bodies that are disinfected with chlorine or chlorine-based products. Emerging research into these chemicals tells us that they have a number of dangerous health effects when consumed over extended periods at elevated levels.

municipal city water treatment plant

❓ Are Trihalomethanes in Water Harmful?

Yes, trihalomethanes in water can be harmful, especially if you’re exposed to these chemicals over an extended period, and if they’re present in high concentrations in your drinking water.

A number of studies have highlighted several possible health risks associated with exposure to trihalomethanes. We’ve discussed these risks later in this article.

How harmful are trihalomethanes? That depends on several factors, including the THMs levels in the water, the frequency of exposure to these disinfection by-products, and for how long exposure continues. Regulatory authorities are aware of the risks of THMs, and many water treatment plants are actively working to minimize the levels of THMs that are produced during chemical water treatment.

🤔 How Do Trihalomethanes Get Into Drinking Water?

Trihalomethanes get into water that has been disinfected with chlorine or chloramine.

Let’s take a look at the three stages involved in trihalomethane production in drinking water supplies:

  • Stage 1: Disinfection – Many public drinking water systems disinfect water with chlorine or chloramine. These chemicals are affordable and easily accessible, and have the advantage of providing extended disinfection all the way until the water reaches our homes. However, the use of chlorine and chloramine also results in the production of trihalomethanes and other byproducts.
  • Stage 2: Reaction with organic matter – Now that the water has been treated with disinfectants, these chemicals linger in the water, where they gradually react with the natural organic contaminants that are also present. The concentrations of organic materials, as well as the amount of chlorine or chloramine added to water, both determine the concentration of THMs produced due to these reactions.
  • Stage 3: THMs are formed – The reaction between these chemical disinfectants and organic substances results in the production of trihalomethanes. These byproducts remain in your water and are then consumed by you when you drink water from your faucet.
Trihalomethanes in water

📑 Possible Health Effects of Trihalomethanes

There are several possible health effects of ingesting trihalomethanes in drinking water:

  • Cancer – THMs are classed as “possibly carcinogenic” to humans.
  • Reproductive problems – Trihalomethanes have also been linked to reproductive issues, as well as miscarriage, low birth weight, and birth defects.
  • Organ damage – THMs may damage the liver, kidneys, heart, lungs, and central nervous system.

These health effects aren’t only caused by drinking water containing THMs – one 2005 study found that inhaling THMs while showering or bathing in disinfected water is another important source of trihalomethanes exposure.

✅ How to Remove Trihalomethanes from Drinking Water

Here are the best methods to reduce or remove THMs from drinking water.


Boiling water is an effective method of reducing THMs in drinking water if you don’t want to invest in a long-term solution.

A scientific review from 2004 found that boiling chlorinated water reduced trihalomethane concentrations by 64-98%, while boiling chloraminated water reduced THMs by 74-98%.

For this method to be effective, you need to bring water to a rolling boil for at least one minute, then allow the water to cool for consumption. The THMs will partially evaporate into the air, greatly reducing their concentration in the water.

The obvious benefit of boiling water is that it uses equipment that you probably already have at home. However, it’s a time-consuming process, and you have to boil every batch of water you want to drink, which is a bit of a hassle.

Boiling water

Granular Activated Carbon Filters

If you’re keen to invest in an affordable long-term solution to remove THMS from drinking water at home, granular activated carbon (GAC) filters are our top recommended choice.

These filters are highly effective in reducing trihalomethanes (one 2006 study found that GAC-sand filters at a water treatment plant could reduce up to 87% of THMS), and they’re also capable of removing chlorine dioxide and other chemicals (like pesticides and herbicides).

You can find granular activated carbon filters in a variety of water treatment systems, including water filter pitchers, under-sink water filtration systems, and whole home water filter systems.

The advantages of activated carbon are that it’s cheap and very reliable, but even the best filter probably won’t completely eliminate THMs from drinking water.

Reverse Osmosis Systems

The most comprehensive THMs removal solution in the water treatment industry is reverse osmosis.

Reverse osmosis water filters have proven capable of rejecting 89.0 to 98.8% of THMS (according to this 2021 test paper), depending on the types of trihalomethanes present. These systems also reduce up to a variety of other harmful contaminants – up to 99.99% of all total dissolved solids, in fact – so they’re a great choice if you want to purify your drinking water.

The most common RO systems are under-sink systems and countertop systems. Whole home RO systems are another option.

Reverse osmosis has the advantage of being the most thorough and comprehensive tap water treatment solution, but it also wastes water, and it removes everything, even healthy minerals like calcium and magnesium.

Reverse osmosis filter

Water Distillation

Water distillation is another purification method that can remove THMs and the majority of other impurities from tap water.

During the distillation process, water is heated until it boils and evaporates into steam. This steam is captured and carried into a separate container, where it condenses back into pure water. The majority of impurities are unable to vaporize with water and are left behind in the boiling chamber.

Water distillation systems for home use are typically quite small (with a holding capacity of 1 or 2 gallons) and are designed to sit on a kitchen countertop.

The major benefit of water distillers is that they’re very low-maintenance to run because they don’t need filter replacements, so there’s only the initial spend to save for. However, the distillation process is very slow, taking hours to produce a single 1-gallon batch of purified water.

❔ Frequently Asked Questions

Are trihalomethanes in water regulated?

Yes, trihalomethanes in water are regulated by the Environmental Protection Agency, which sets limitations on the concentration of THMs that can be present in public water supplies annually.

How do you remove trihalomethanes from drinking water?

The best way to remove trihalomethanes from water is with reverse osmosis. You can also reduce THMs with activated carbon filtration, water distillation, and boiling.

Can you boil out trihalomethanes?

Yes, you can boil out trihalomethanes. Boiling water causes the THMs to evaporate into the air. However, this process doesn’t guarantee the complete removal of trihalomethanes (it usually reduces THMs by around 75%.

What filters out trihalomethanes?

Carbon water filters and reverse osmosis filters are the best methods to filter out trihalomethanes. Carbon filters can reduce up to 87% of THMS, while RO filters can reduce up to 98.8% of these byproducts.

Can a UV water treatment system remove THMs?

No, UV water treatment systems can only kill microorganisms (like bacteria, viruses, and protozoa) by altering their DNA with UV rays. You can’t use UV light to remove THMS or any other physical particles from your tap water. UV purification is a great chemical-free alternative to disinfection and won’t product byproducts, but it also won’t remove byproducts that already exist in the water.

  • Brian Campbell
    President & CEO, CWS, CWR

    Brian Campbell, a WQA Certified Water Specialist (CWS) and Certified Water Treatment Representative (CWR) with 5+ years of experience, helps homeowners navigate the world of water treatment. After honing his skills at Hach Company, he founded his business to empower homeowners with the knowledge and tools to achieve safe, healthy water. Brian's tested countless devices, from simple pitchers to complex systems, helping his readers find the perfect fit for their unique needs.

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