Are trihalomethanes in tap water dangerous?

Long-term exposure to THMs above the EPA limit has been associated with increased bladder cancer risk in epidemiological studies. The risk applies to sustained, above-limit exposure over years, not a single glass of water. Two of the four regulated THMs, bromodichloromethane and bromoform, have a federal health goal of zero. Regulators treat any detectable level of those two as a concern, even if the enforceable limit is higher.

How do I know if my tap water has high THM levels?

Your water utility is required to publish an annual Consumer Confidence Report (CCR) showing TTHM levels measured across the distribution system. If your system's CCR consistently shows TTHMs at or near 80 µg/L, or if your area draws from a river or lake with high organic content, that's worth paying attention to. You can also check the EWG Tap Water Database at ewg.org/tapwater for your utility's historical TTHM data.

Does a Brita filter remove THMs?

Standard Brita pitcher filters use activated carbon and do reduce chlorine taste and odor, but not all models are certified to NSF/ANSI 53 for THM reduction. Check the specific model's NSF certification before assuming it removes THMs. Brita's Longlast+ filter does carry NSF 53 certification for several contaminants. If THM reduction is your goal, look for NSF/ANSI 53 on the packaging or the NSF product database.

What water filter removes trihalomethanes?

Activated carbon filtration, either granular activated carbon (GAC) or solid carbon block, is effective at reducing THMs at the tap. Look for NSF/ANSI 53 or NSF/ANSI 42 certification for VOC and disinfection byproduct reduction. Reverse osmosis systems also reduce TTHMs along with many other contaminants. Boiling does not help. It worsens the problem by concentrating most THMs.

Why are THM levels higher in summer?

Warmer water speeds up the chemical reactions between chlorine and organic matter. Summer also brings algae blooms, increased runoff, and more decaying plant material in source water, all of which add organic matter for chlorine to react with. Most utilities that hit TTHM violations do so in the July-October window. Systems drawing from rivers and lakes are more exposed to this seasonal swing than those using groundwater.

Trihalomethanes (THMs) in Tap Water: The Byproduct of Disinfection

Disclaimer: WaterAnswer.com provides general information about water quality, not medical advice. If you have health concerns related to your drinking water, consult a healthcare provider.

Chlorine is one of the most important public health advances in modern history. Adding it to drinking water eliminated cholera and typhoid outbreaks that once killed tens of thousands of Americans every year. That’s the good part.

The catch is this: chlorine doesn’t just kill bacteria. It also reacts with naturally occurring organic matter in source water, things like leaves, algae, and soil runoff, and forms a family of chemical byproducts. The most regulated group is called trihalomethanes.

One problem solved, another created.

What Are Trihalomethanes?

The EPA regulates four specific trihalomethanes, measured together as total trihalomethanes (TTHMs):

Chloroform
Bromodichloromethane
Dibromochloromethane
Bromoform

Which of the four dominates in a given water supply depends on the source water chemistry, particularly how much bromide is present. Surface water from rivers and lakes tends to produce more of the brominated compounds. Chloroform is typically the most common in systems with low bromide.

THMs only form in chlorinated water. Private well water doesn’t contain THMs unless someone has added chlorine to the well.

The EPA Standard and What the Health Goals Reveal

The EPA’s Maximum Contaminant Level (MCL) for TTHMs is 80 micrograms per liter (µg/L), also written as 80 parts per billion (ppb). Utilities measure this as an annual average across sampling sites in the distribution system. One high reading in summer doesn’t automatically mean a violation, because the MCL applies to the annual locational running annual average.

But the MCLGs tell a different story. The MCLG, or Maximum Contaminant Level Goal, is the concentration at which EPA believes there’s no known or expected health risk. It’s the target, not the enforceable limit.

For bromodichloromethane and bromoform, the MCLG is zero. For chloroform, it’s 70 ppb. For dibromochloromethane, it’s 60 ppb.

The gap between the health goal and the enforceable limit exists because the MCL is set at what’s technically and economically feasible to achieve in real water systems, not at the zero-risk level. Source: EPA National Primary Drinking Water Regulations.

This matters. A utility can be in full compliance at 75 ppb while the federal health goal for two of the compounds in that reading is zero.

Health Effects

The health research on THMs focuses primarily on long-term, sustained exposure. Not short-term contact.

Long-term exposure to THMs above the EPA limit has been associated with increased bladder cancer risk in epidemiological studies. Multiple studies conducted in the US and Europe have found this association. The International Agency for Research on Cancer classifies chloroform as a possible human carcinogen (Group 2B). The EPA’s own MCLG of zero for bromodichloromethane reflects its classification as a probable human carcinogen.

Prolonged exposure at high concentrations is also associated with liver and kidney damage.

For pregnant people, some research has found associations between high THM exposure and adverse birth outcomes, including low birth weight and preterm delivery. The evidence here is less consistent than the bladder cancer literature. It’s worth knowing, but the findings aren’t conclusive.

To be clear: the risk applies to sustained, years-long exposure above the regulatory limit. Drinking a glass of water from a system with elevated TTHMs doesn’t cause cancer. The concern is for people on systems that consistently run near or above 80 ppb over years.

Where THM Levels Run Highest

Not every water system has the same THM exposure. Several factors drive higher concentrations:

Systems drawing from surface water, rivers, reservoirs, and lakes, have more organic matter to start with. The more organic matter in the source water, the more byproducts chlorine can form.

Older distribution systems with longer pipe residence times are a second factor. Water sitting in pipes for hours or days before reaching a tap gives THMs more time to form. Utilities with sprawling or aging pipe networks tend to show higher TTHM readings at the edges of the system.

Seasonality matters too. Warm water accelerates the reaction between chlorine and organic matter. Algae blooms in summer add more organic material. Most TTHM violations occur between July and October. The EWG Tap Water Database at ewg.org/tapwater tracks violation history by utility. Texas, Pennsylvania, and New York consistently appear among states with the most TTHM violations.

How to Check Your Own Tap Water

Every public water system is required to publish an annual Consumer Confidence Report (CCR) showing measured TTHM levels. Your utility mails it in the summer or posts it online. If your CCR shows TTHM levels consistently near 80 µg/L, or trending upward year over year, that’s a reasonable prompt to consider a point-of-use filter.

Check the testing section for guidance on reading a CCR and understanding what the numbers mean.

THMs vs. HAAs

THMs aren’t the only chlorination byproducts to know about. Haloacetic acids (HAAs) are the other major class. The EPA MCL for five regulated HAAs (HAA5) is 60 µg/L.

The practical difference is volatility. THMs, especially chloroform, partially off-gas if water sits open in a pitcher for 30 minutes or more. HAAs don’t off-gas because they’re not volatile. They stay in the water.

Both groups are reduced by activated carbon filtration. Reverse osmosis removes both as well.

How to Reduce THMs at the Tap

Activated carbon filtration is the most practical starting point. Both granular activated carbon (GAC) and solid carbon block filters reduce THMs at the tap. Look for NSF/ANSI 53 certification, which covers health-based contaminant reduction, or NSF/ANSI 42 for aesthetic reduction including chlorine and some organic compounds. Carbon pitcher filters can work, see best pitcher water filters, but confirm the model carries the NSF certification before buying.

Reverse osmosis is the more thorough option. An RO system at the kitchen sink reduces TTHMs along with dozens of other contaminants. Visit the treatment section for a breakdown of system types.

Letting water sit in an open pitcher is a partial measure, not a solution. Chloroform is volatile enough that it partially off-gases in about 30 minutes. But this doesn’t work for the brominated THMs, which are far less volatile. You’d be addressing one of four regulated compounds.

Don’t boil water to reduce THMs. Boiling evaporates water but leaves dissolved compounds behind at higher concentrations. For most THMs, boiling makes the problem worse, not better.

The Direct Recommendation

If your CCR shows TTHM levels above 40-50 µg/L, which is about half the EPA limit, a carbon block filter at the kitchen tap is a sensible step. Look for one certified to NSF/ANSI 53. It’ll reduce THMs in your drinking and cooking water without any major infrastructure change.

If your system runs near or above 80 ppb, or if you’re pregnant and using municipal water with elevated TTHMs, an under-sink RO system offers broader coverage.

Related: How chloramines compare as a disinfectant. Some utilities have switched from chlorine to chloramines specifically to reduce THM formation. That trade-off comes with its own byproducts worth understanding.

Disclaimer: WaterAnswer.com provides general information about water quality only. For health-specific concerns about THM exposure, consult a healthcare provider.