What are the health effects of THMs in drinking water?

The EPA and EPA's National Toxicology Program classify chloroform, one of the main THM compounds, as a likely human carcinogen at high doses. Epidemiologic research has found associations between long-term THM exposure and bladder cancer risk. The research doesn't show that any single exposure is dangerous, the concern is cumulative exposure over years and decades. The EPA's 80 ppb limit is set to keep that long-term risk at acceptable levels for public systems.

Can I reduce THMs by letting water sit uncovered?

To some extent. THMs are volatile, and the lightest compounds (chloroform and similar) will off-gas over time if water is left uncovered in a wide, shallow container. Boiling drives off some THMs more quickly. But these methods don't eliminate THMs completely. A granular activated carbon filter certified to NSF/ANSI 53 is more reliable and practical.

Does a Brita filter remove THMs?

A Brita with the Longlast+ filter (blue filter) is NSF/ANSI 53 certified for VOC reduction, which includes some THM compounds. The standard Brita filter (white) is NSF 42 only, which doesn't cover THMs. Check the specific filter's NSF certification, not just the pitcher brand, before assuming coverage.

Are THMs in well water?

Rarely. THMs form when chlorine disinfectants react with natural organic matter in water. Private wells aren't treated with chlorine, so the reaction that creates THMs typically doesn't occur. Well water may have other disinfection byproduct-like compounds from different reactions, but standard THMs from chlorination are a municipal water concern.

My CCR shows 60 ppb total THMs. Is that dangerous?

It's within the federal limit of 80 ppb, which means your utility is in compliance. Whether 60 ppb poses a risk depends on your total exposure, the duration of your exposure, and your individual health factors. The EPA designed the 80 ppb limit to keep long-term cancer risk at an acceptable level for the general population. If you're concerned, an activated carbon filter certified to NSF 53 for THM reduction will lower your intake from drinking water.

Trihalomethanes (THMs) in Tap Water: What They Are and Where Levels Are Highest

This page covers a regulated drinking water contaminant. The information here is for educational purposes only. It does not constitute medical advice. If you have health concerns related to your water supply, consult your physician and contact your local water utility.

Trihalomethanes form when chlorine used to disinfect drinking water reacts with naturally occurring organic matter in the water. They’re a byproduct of water treatment, not a source contamination problem. If your water comes from a chlorinated municipal system, your supply contains some level of THMs.

How THMs Form and Which Are Most Common

The four regulated trihalomethanes are chloroform, bromodichloromethane (BDCM), dibromochloromethane (DBCM), and bromoform. Chloroform is almost always the dominant compound. The others appear in smaller concentrations depending on the source water chemistry.

The reaction requires three things: chlorine, organic matter (mainly humic acids from decaying plant material), and contact time. Source water high in natural organic matter, typically water drawn from rivers and reservoirs with heavy vegetation in the watershed, produces more THMs after chlorination. The more organic matter in the source, the more precursor material for the reaction.

Bromide in source water shifts the balance toward bromine-containing THMs. Coastal utilities, estuarine systems, and water sources with naturally higher bromide tend to produce more BDCM and bromoform alongside chloroform. This matters because the bromine-containing THMs have somewhat different toxicity profiles.

Temperature also plays a role. The reaction runs faster in warm water, which is why THM levels in distribution systems tend to peak in summer. Water that was treated with the same chlorine dose in January will often leave the tap with higher THM concentrations in July.

The EPA Limit and What It Covers

The EPA’s Maximum Contaminant Level for total THMs (all four combined) is 80 parts per billion. This limit applies to public water systems serving more than 10,000 people under the Stage 2 Disinfectants and Disinfection Byproducts Rule, finalized in 2006.

Compliance is measured as a locational running annual average (LRAA). Each individual monitoring site in the distribution system must average 80 ppb or below over a 12-month rolling period. That’s more protective than the old system, which averaged across the whole distribution network and let high-exposure areas get masked by low-exposure areas.

The EPA’s Maximum Contaminant Level Goal (MCLG) tells a different story. Chloroform has an MCLG of 70 ppb. BDCM has an MCLG of zero. DBCM’s MCLG is 60 ppb. Bromoform’s is zero. An MCLG of zero means there’s no established safe level. The MCL is set where it is because of treatment feasibility, not because 80 ppb is considered risk-free.

The Environmental Working Group uses its own health guidelines, which are lower than EPA limits for several THM compounds. The EWG’s tap water database shows utility-level THM data by zip code. It’s worth checking even if your utility is in federal compliance.

Where THM Levels Tend to Be Highest

Geography and source water type drive the variation.

Utilities drawing from rivers with high organic load, particularly in the Southeast, mid-Atlantic states, and Texas, consistently show higher THM levels than utilities sourcing from deep groundwater or high-elevation reservoirs with low organic content. Louisiana, Georgia, and parts of Texas regularly appear in the higher ranges in EWG data.

Seasonal variation matters too. In summer, warmer source water contains more dissolved organic matter, the chlorination reaction runs faster, and water spends more time in warm distribution pipes before reaching your tap. Many utilities see their THM levels peak in August.

Within a distribution system, the endpoints matter. Water at the far end of the distribution network has been in pipes longer and has more time to react. A home served by an older, more complex distribution system may consistently see higher THMs than a home near the treatment plant, even on the same supply.

The EWG tap water database at ewg.org/tapwater lets you look up your utility’s reported THM levels. Your utility’s Consumer Confidence Report also lists total THM measurements, usually as an annual average.

How to Reduce THM Exposure at Home

The most effective point-of-use approach is an activated carbon filter certified to NSF/ANSI 53 for VOC reduction. Most THM compounds are classified as volatile organic chemicals, and NSF 53 certification covers their reduction. Not all activated carbon filters carry this certification. Check the specific model’s NSF listing before buying.

Reverse osmosis also removes THMs and is a good choice if you also want to address other contaminants like PFAS or nitrates. It’s a more thorough solution but more expensive to install and maintain.

Two low-tech methods help at the margins. Letting water sit uncovered in a wide container for a few hours will allow some of the more volatile THMs to off-gas. Boiling speeds this up but concentrates any non-volatile contaminants in the water. Neither method is a substitute for a certified filter, but they’re better than nothing for occasional use.

One practical point: hot tap water has spent more time in your pipes at higher temperatures, which accelerates the reaction. If you’re concerned about THMs, use cold tap water for drinking and filter from there.

Pitcher filter options and what they cover are detailed on our best pitcher water filters page.

How Chloramine Changed the Picture

Many utilities switched from free chlorine to chloramines specifically to reduce THM and HAA formation. The strategy works for those regulated byproducts. THM levels do drop substantially when a utility moves to chloramine disinfection.

But chloramine forms its own byproducts, including N-nitrosodimethylamine (NDMA) and iodoacetic acids. These are largely unregulated. Some have higher toxicity per microgram than the regulated THMs that chloramine was meant to eliminate.

This is a known tradeoff in the water treatment field. The utilities that switched did so because the regulatory calculus favored reducing THMs. What wasn’t fully understood at the time was the byproduct profile on the other side of the tradeoff.

The practical implication for households: if your utility uses chloramines, your THM levels are likely lower. But different concerns apply. See our chloramines contaminant page for what changes when your utility uses chloramines.

For help reading your utility’s CCR and finding your reported THM numbers, see our guide to reading water quality reports.

For a full overview of THMs including the EPA’s MCL, health effects, and how to reduce exposure at home, see the trihalomethanes contaminant page.

Sources

EPA. “National Primary Drinking Water Regulations: Disinfection Byproducts.” https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations
Environmental Working Group. “EWG’s Tap Water Database.” https://www.ewg.org/tapwater/
EPA. “Stage 2 Disinfectants and Disinfection Byproducts Rule.” https://www.epa.gov/dwreginfo/stage-2-disinfectants-and-disinfection-byproducts-rule