Trihalomethanes in Water: What You Need to Know in 2024

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Trihalomethanes (THMs) are chemical compounds that are often found in tap water disinfected by chlorine or chloramine. Trihalomethanes have a number of known health risks if high THM levels are consumed regularly for a long time.

This glossary discusses the origin and health effects of trihalomethanes. We’ve also shared how to detect THMs and how to protect your family from these contaminants in tap water.

❔ What are Trihalomethanes?

Trihalomethanes are a type of chemical disinfection byproduct (DBP) that occur in disinfected drinking water supplies.

💡 There are several kinds of THMs in water: chloroform, bromodichloromethane, bromoform, and dibromochloromethane. The concentration of THMs depends on the amount of chlorine added to water, the temperature of the water supply, and the concentration of organic material in the water supply.

Trihalomethanes are some of the most well-known and researched of all 600 known disinfection byproducts. Since THMs were discovered and their health effects became known, some action has been taken to reduce their concentration in tap water. However, trihalomethanes are still widely present in drinking water supplies today.

Continue Reading: How to Remove Chloroform from Water

In Water AsChloroform
SourcesDisinfection of public drinking water supplies
Contaminant Levels (MCLs)
US EPA: 0.080 mg/L
Potential Health RisksHigh risk on pregnant women
Colon cancer and bladder cancer

TreatmentsGranular Activated Carbon Filters
Reverse Osmosis
Water Distillers:

🩺 What are the Potential Health Effects of Trihalomethanes?

The health effects of THMS depend on the types of THMs present in water. The most common THMS and their potential health effects are:

  • Chloroform: nausea, drowsiness, dizziness, liver and kidney damage, fitting, unconsciousness, irritated skin
  • Bromodichloromethane: kidney damage, liver damage, decreases in immune response, cancer
  • Bromoform: liver and kidney injury, slowing down of brain functions, increased risk of developing bladder cancer and other cancers
  • Dibromochloromethane: problems during pregnancy, increased risk of cancer

Scientists are still researching the potential health effects of trihalomethanes, but we know for certain that all THMs have something in common: they have a likely cancer risk. Colon cancer and bladder cancer are two of the most common cancers caused by trihalomethanes.

Pregnant women are particularly vulnerable to the effects of THMs in water. Many trihalomethanes have been linked to reproductive problems, like miscarriage and low birth weight. There is no official recommendation that pregnant women shouldn’t drink tap water due to its THM content. Speak to your physician if you’re concerned.

A person drinking tap water that contains THMs under the guidance value shouldn’t experience any of the health effects listed above, regardless of age or gender.

Pregnant woman

🚰 How Do Trihalomethanes Get Into Drinking Water?

Trihalomethanes (THMs) are disinfection byproducts, which means that they are formed in drinking water that is treated with disinfectants (typically chlorine). Disinfectants are used to control microbial contaminants in public drinking water supplies. THMs occur when chlorine produces a free chlorine residual that reacts with organic substances in water.

📌 Chlorine-treated water is most likely to form trihalomethanes because it’s highly reactive with organic material in water. If your water distribution system uses chloramine as a disinfectant, your water’s THMs level will be much lower than a chlorinated water supply.

The more chlorine your water contains, the higher the potential concentration of THMs. Water with a high concentration of organic material will also enable more reactions with chlorine residual, resulting in a higher level of THMs. THM absorption is greater in hot water than in cold water, so the temperature of your water will also determine the concentration of trihalomethanes.

Trihalomethanes aren’t found naturally in groundwater or surface water supplies. If you have a private well supply, it’s unlikely that your water will contain these contaminants unless you chlorinate your water yourself.

Municipal water treatment with chloramine

📉 Do Water Treatment Facilities Monitor Levels of Trihalomethanes in Drinking Water?

Yes, water treatment facilities monitor levels of trihalomethanes in their drinking water.

The Environmental Protection Agency (EPA) has established Disinfection Byproducts Rules to protect the public from disinfection byproducts with adverse health effects, including trihalomethanes.

There are two Disinfection Byproducts Rules that water treatment facilities must comply with:

  • Stage 1 Disinfectants and Disinfection Byproducts Rule (DBPR): reduces public exposure to disinfection by-products in drinking water systems
  • Stage 2 Disinfectants and Disinfection Byproducts Rule: tightens compliance monitoring requirements for haloacetic acids and total trihalomethanes

📌 The EPA’s Maximum Contaminant Level for total trihalomethanes, based on the known information on their potential health risks, is 0.080 mg/L. This means that water suppliers must ensure that their water’s THMs level doesn’t exceed a maximum of 0.080 mg/L.

It’s important to note that drinking water disinfection is incredibly beneficial. Chlorination has virtually eliminated typhoid fever and other diseases caused by bacterial contamination and other waterborne pathogens. However, chlorine disinfection is still monitored for public health protection because of the known risks of disinfectants and their byproducts, including THMSs.

laboratory water testing

🔎 How Can I Tell if Trihalomethanes Are in My Drinking Water?

Your drinking water won’t look, smell, or taste any different if it contains trihalomethanes. THMs are invisible, odorless, and tasteless, and the only way to detect them in your water is to conduct a water test.

Laboratory testing is the best form of testing for THMs. Some lab tests will detect disinfection byproducts separately, while others will provide results for a combined concentration of DBPs, including THMs. You’ll need to send a water sample to the laboratory for professional testing, and you’ll usually receive results within 1-2 weeks.

Water testing with tap score

Don’t want to pay to get your water tested? Contact your local municipality and ask to see your Water Quality Report or Consumer Confidence Report. Your report should note the method used to disinfect water in your region. This will give you an indication of whether or not your water is likely to contain THMs.

👩🏽‍⚕️ How Can I Protect My Family from Trihalomethanes in Drinking Water?

The best way to protect your family from trihalomethanes in tap water is to install a water treatment system in your home.

Some of the most capable water treatment methods for removing THMs are:

  • Granular activated carbon filters: GAC filters are most commonly known for their chlorine removal ability, but granular activated carbon media can also remove most chlorine disinfection byproducts, including THMs. GAC filters may be used alone (such as in a drinking water filter pitcher or a faucet filter) or as a filter stage in a point of entry filtration system.
  • Reverse osmosis systems: Reverse osmosis is highly effective in removing virtually all contaminants from water, including metals, chemicals, volatile organic compounds, microbes, and disinfection byproducts, including THMs. RO sends water through a semipermeable membrane, which has tiny pores that trap THMs and other impurities before they’re flushed down a drain. Under-sink, countertop, and whole house RO water filters are all available.
  • Nanofiltration: Nanofilters are similar to reverse osmosis systems, using a membrane to separate water particles from contaminants. Nanofiltration is capable of removing tiny particles, including chemicals, metals, pathogens, and trihalomethanes. Nanofilters are typically installed underneath the kitchen sink.
  • Water distillers: Water distillation purifies water by boiling and evaporating the water into a separate container. The majority of contaminants end up left behind in the boiling chamber. Any lingering contaminants are removed by an activated carbon filter in the distiller’s spout. Most distillers are installed as countertop systems.

Look for a water filter that has been certified by the Water Quality Association (WQA) or the National Sanitation Foundation (NSF), which means it has been tested to industry standards and deemed capable of contaminant removal.

If you discover exceptionally high levels of trihalomethanes in your tap water, switch to bottled water while you consider methods of long-term protection. Some bottled water products contain chlorinated tap water, so check the label before you buy.

Additionally, we know that THMs are caused when chlorine chemicals in drinking water react with natural organic matter. Other disinfectants, like chloramine (a combination of chlorine and ammonia), also produce THMs, but aren’t as reactive with organic material as chlorine.

If you chlorinate your own well water supply, switch to chloramine or consider UV purification to reduce the THMs level in your water. If your water supplier uses chlorine to disinfect your water, contact them and express your concerns about disinfection byproducts. There’s no guarantee that speaking to your supplier will do anything, but it’s good to make your voice known.

Usws defender whole house ro system after installation

⚠️ How Else Can I Be Exposed to Trihalomethanes?

Aside from being exposed to elevated levels of THMs in drinking water, other sources of exposure are:

  • Eating foods that have been stored, grown, prepared, or cooked in water containing chlorine or other disinfection chemicals
  • Breathing in air from vaporized disinfected water supplies (such as when you’re swimming, showering, or bathing)
  • Absorbing THMs through your skin when swimming in chlorinated water

Installing a water filtration system at your home’s point of entry will reduce your risk of inhaling trihalomethanes.

📝 Where Can I Get More Information?

Click on the links below to learn more about trihalomethanes in drinking water, including their origin and potential health effects.

  • Jennifer Byrd
    Water Treatment Specialist

    For 20+ years, Jennifer has championed clean water. From navigating operations to leading sales, she's tackled diverse industry challenges. Now, at Redbird Water, she crafts personalized solutions for homes, businesses, and factories. A past Chamber President and industry advocate, Jennifer leverages her expertise in cutting-edge filtration and custom design to transform water concerns into crystal-clear solutions.

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