Trihalomethanes (THMs): Health Risks, EPA Limit & How to Remove Them

Trihalomethanes (THMs) are disinfection byproducts that form when chlorine reacts with natural organic matter in water. The four regulated compounds (chloroform, bromodichloromethane, dibromochloromethane, and bromoform) are measured together as total trihalomethanes (TTHMs), which the EPA limits to 80 parts per billion. Long-term exposure is associated with an increased risk of bladder cancer. To remove THMs, use a filter certified to NSF/ANSI 53 for VOC reduction: a carbon block filter, whole-house activated carbon system, or reverse osmosis system reduces them effectively.

What is Trihalomethanes?

Trihalomethanes are a group of four chemical compounds (chloroform, bromodichloromethane, dibromochloromethane, and bromoform) that form as unintended byproducts when chlorine or chloramine reacts with naturally occurring organic matter in the water supply. They are the most common class of disinfection byproducts (DBPs) in chlorinated drinking water. Total trihalomethanes (TTHMs) is the sum of all four compounds.

Is Trihalomethanes Dangerous?

The EPA classifies several THMs as probable human carcinogens. Long-term exposure to THMs at elevated levels is associated with increased risk of bladder cancer. Some studies also suggest associations with colorectal cancer, adverse reproductive outcomes (including miscarriage and low birth weight), and liver and kidney effects. THM exposure occurs through ingestion, inhalation (during showers), and dermal absorption.

EPA Limit: 80 PPB
EPA MCL is 80 PPB for total trihalomethanes (TTHMs) as a running annual average.

How to Identify Trihalomethanes in Your Water

THMs cannot be reliably detected by taste or smell at regulatory levels. Laboratory testing is required for precise measurement. Your utility's CCR reports TTHM levels as a running annual average. This is the most practical way to assess your exposure.

How Trihalomethanes Is Regulated

The EPA MCL for total trihalomethanes is 80 PPB, measured as a running annual average across monitoring points in the distribution system. The EPA also regulates haloacetic acids (HAA5) at 60 PPB. Both are part of the Stage 2 Disinfectants and Disinfection Byproducts Rule. The MCLG for chloroform is 70 PPB; the MCLG for dibromochloromethane is 60 PPB; MCLGs for bromodichloromethane and bromoform are zero.

Standard	Limit	Notes
EPA MCL	80 PPB	EPA MCL is 80 PPB for total trihalomethanes (TTHMs) as a running annual average.
Health Guideline (MCLG)	0 PPB	Health-based target; not enforceable

How to Remove Trihalomethanes

Activated Carbon Filter Recommended — Effective removal, especially with carbon block · Whole house or point of use
Reverse Osmosis — 90%+ removal via carbon pre-filter and membrane · Single tap
Pitcher Filter — Moderate reduction with standard carbon · Drinking water only

Recommended Systems to Reduce Trihalomethanes

These treatment methods are proven to reduce trihalomethanes. Whole-house systems treat every tap; point-of-use units treat drinking water only.

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Frequently Asked Questions

Does boiling remove THMs?

Boiling can reduce some THMs through volatilization (THMs are volatile compounds), but it is not a reliable or practical treatment method. Boiling also removes protective chlorine residual.

Are THMs worse in hot water?

THMs volatilize more readily in hot water. Shower exposure (inhalation and dermal absorption) may contribute more to total THM exposure than drinking water alone. A whole-house carbon filter addresses this.

Do chloramine systems have lower THMs?

Yes. Chloramine produces significantly fewer THMs than free chlorine, which is one reason utilities switch to chloramine. However, chloramine produces its own category of byproducts (nitrosamines) that are also being studied.

What filter removes trihalomethanes?

Choose a filter certified to NSF/ANSI 53 for VOC reduction. NSF uses chloroform (a trihalomethane) as the surrogate test contaminant for VOC reduction, and certified filters must reduce it by at least 95%. In practice, a quality carbon block filter, a whole-house activated carbon system, or a reverse osmosis system (which includes carbon stages) all reduce THMs effectively. A whole-house carbon filter has the added benefit of addressing THM exposure from hot showers, not just drinking water.

Do Brita or PUR pitchers remove THMs?

Standard carbon pitcher filters provide only modest THM reduction and are not the most reliable option. For meaningful reduction, choose a filter specifically certified to NSF/ANSI 53 for VOCs, such as a dense carbon block filter or a reverse osmosis system. Always check the product's NSF listing for VOC or chloroform reduction rather than assuming a pitcher handles it.

Sources

EPA Stage 2 DBPR — EPA regulation
EPA Integrated Risk Information System — Cancer classification
WHO Trihalomethanes in Drinking-water — Health effects

Problems: Chlorine

Other Contaminants: Chlorine & Chloramine · PFAS

Treatment: Activated Carbon Filter · Reverse Osmosis · Pitcher Filter

Trihalomethanes