What are trihalomethanes in water?

Trihalomethanes (THMs) are a group of four chemical compounds — chloroform, bromodichloromethane, dibromochloromethane, and bromoform — that form when the chlorine used to disinfect water reacts with naturally occurring organic matter in the source water. They're an unintended byproduct of the disinfection process, not something deliberately added.

How to reduce THMs in drinking water?

Activated carbon filters are the most cost-effective solution — a quality under-sink carbon block or even a good pitcher filter significantly reduces THMs. Running your tap for a few seconds before filling a glass also helps, as THMs concentrate in standing water. For whole-house protection, a carbon-based POE system reduces THMs at every tap, including showers where THMs are absorbed through skin and inhaled as vapor.

Are THMs dangerous at levels below the EPA limit?

Research suggests yes. The EPA MCL of 80 ppb was set considering feasibility of treatment, not solely on health effects. Epidemiological studies have found increased bladder cancer risk at levels well below 80 ppb. The EWG health guideline of 0.8 ppb reflects a more health-protective standard. Since virtually all chlorinated water contains some THMs, a filter is a practical precaution.

disinfection byproduct

Trihalomethanes (THMs) in Drinking Water

Updated: 2026-05-16Written by: TapWaterGuide Editorial Team

What is Trihalomethanes (THMs) and is it dangerous in tap water? Trihalomethanes (THMs) is a disinfection byproduct contaminant found in drinking water. The EPA maximum contaminant level (MCL) is 80 ppb. The stricter EWG health guideline is 0.8 ppb. Health effects include bladder cancer — strongest epidemiological link, estimated 2-17% of cases attributed to thm exposure and reproductive and developmental effects including miscarriage and low birth weight. The most effective removal methods are Activated Carbon, Reverse Osmosis.

Last updated: 2026-05-16 · Source: EPA, WHO, EWG

Regulatory Limits

EPA MCL

80 ppb

Measured as Total Trihalomethanes (TTHMs) — the sum of four compounds.

EWG Guideline

0.8 ppb

100x stricter than EPA MCL. Based on cancer risk from lifetime exposure.

MCLG (Goal)

0 ppb

The level at which no known health effects occur

Health Effects

●

Bladder cancer — strongest epidemiological link, estimated 2-17% of cases attributed to THM exposure

●

Reproductive and developmental effects including miscarriage and low birth weight

●

Liver and kidney damage with chronic exposure

●

Potential colorectal cancer risk (research ongoing)

●

THMs are absorbed through skin and inhaled during showering, not just through drinking

Especially vulnerable: Pregnant women (miscarriage risk), People with liver or kidney disease, Swimmers and people with high water exposure

How to Remove Trihalomethanes (THMs) from Water

Technology	Effectiveness	Notes
Activated Carbon	high	GAC and carbon block filters are very effective at removing THMs. Most cost-effective solution.
Reverse Osmosis	high	RO with carbon pre/post filters removes THMs comprehensively.
Aeration	moderate	THMs are volatile and can be partially removed by aeration, but this is impractical residentially.
Water Softener	none	Water softeners do NOT remove THMs.
UV Purification	none	UV does NOT remove THMs.

Where Trihalomethanes (THMs) Is Most Common

Warm-climate cities with surface water: Houston, Dallas, Phoenix, Miami, Tampa, Atlanta

Cities with long distribution systems where water travels far from treatment plants

Rural systems with limited treatment capacity

EWG found THMs exceeding their guideline in water serving over 250 million Americans

Common sources include: Formed when chlorine reacts with naturally occurring organic matter (leaves, soil, algae) in source water, Higher in warm climates (heat accelerates the reaction), Higher in systems with surface water sources (more organic matter than groundwater), Higher at the ends of distribution systems where water has the longest contact time with chlorine, Four THMs regulated: chloroform, bromodichloromethane, dibromochloromethane, bromoform.

Best Filters for Trihalomethanes (THMs) Removal

We've tested and compared the top water filters that are NSF-certified to remove Trihalomethanes (THMs). Each recommendation is matched to specific contaminant removal performance, not just marketing claims.

View our top trihalomethanes (thms) filter picks →