Disinfection Byproducts (THMs & HAA5) in Drinking Water

What Are Disinfection Byproducts?

Disinfection byproducts (DBPs) are chemical compounds that form when disinfectants used to treat drinking water react with naturally occurring organic matter. When water utilities add chlorine or chloramine to kill bacteria and viruses, those disinfectants interact with decaying plant material, algae, and other organic substances present in the source water. The reaction produces a range of unintended chemical compounds — most of them not present in either the source water or the disinfectant alone.

The two most regulated groups of DBPs are trihalomethanes (THMs) and haloacetic acids (HAA5). THMs include four compounds: chloroform, bromodichloromethane, dibromochloromethane, and bromoform. HAA5 refers to five haloacetic acids: monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid. Together, these account for the majority of measured DBPs in US drinking water systems, though researchers have identified more than 600 distinct DBP compounds in chlorinated water.

DBPs represent a unique regulatory challenge. Disinfection itself is essential — without it, waterborne diseases like cholera, typhoid, and giardiasis would pose immediate, acute threats to public health. No safe alternative fully replaces chlorine for large distribution systems. The regulatory goal is therefore to minimize DBP formation while maintaining effective disinfection, not to eliminate chlorination. Note that systems using chloramine instead of chlorine produce lower THM levels but form other byproduct classes, including nitrosamines like NDMA and inorganic byproducts like chlorate when hypochlorite is the disinfectant source.

How DBPs Get Into Drinking Water

DBPs form during and after the water treatment process. Several factors determine how much is produced and where concentrations will be highest by the time water reaches a tap:

• Organic matter levels: Water drawn from rivers, lakes, and reservoirs with higher concentrations of natural organic matter (NOM) produces more DBPs when chlorinated. Seasonal changes — fall leaf decay, spring algae blooms, post-storm runoff — can sharply increase NOM levels and cause DBP spikes that persist for weeks.
• Chlorine dose and contact time: Higher chlorine doses and longer contact times lead to greater DBP formation. The chemistry is cumulative: water that travels long distances through the distribution system has more time to generate byproducts as residual chlorine continues reacting with organic matter in the pipes.
• Water temperature: Warmer water accelerates the chemical reactions that produce DBPs. Systems in southern states or those drawing from shallow surface water sources during summer months consistently see higher concentrations than the same systems in winter.
• Source water type: Surface water systems generally produce more DBPs than groundwater systems because surface water contains more organic matter and is more subject to runoff. Deep, confined aquifers with low NOM can produce negligible DBPs even after chlorination.
• pH levels: Higher pH levels increase THM formation while decreasing HAA5 formation, and vice versa. Treatment plants that raise pH to control lead and copper corrosion may inadvertently shift their DBP profile toward higher THMs.
• Distance from treatment plant: DBP concentrations are consistently higher at points far from the treatment plant, where chlorinated water has had the most time to react with residual organic matter in distribution system pipes. Dead-end mains and areas with long water age are particularly prone to elevated levels.

Health Effects

Long-term exposure to elevated levels of disinfection byproducts has been linked to several health concerns. The EPA classifies some THMs and HAAs as probable or possible human carcinogens based on animal studies and epidemiological data.

Cancer Risk

Multiple epidemiological studies have found associations between long-term DBP exposure and increased risk of bladder cancer. The EPA estimates that DBPs in drinking water may contribute to roughly 2—17% of bladder cancer cases in the United States — a wide range that reflects ongoing scientific uncertainty about the precise dose-response relationship.

Recent meta-analyses have sharpened those estimates. A 2023—2024 pooled analysis of cohort and case-control studies found that total trihalomethane (TTHM) exposure has a nearly linear association with bladder cancer risk, with a relative risk of approximately 1.08 (95% CI: 1.05—1.11) per 10 µg/L increase in TTHM concentration. HAA5 exposure showed a similar pattern, with approximately a 7% increase in bladder cancer risk per 10 µg/L. These associations persisted after adjusting for smoking, occupational exposures, and other confounders, lending greater confidence that the relationship is not entirely attributable to lifestyle factors.

Bromodichloromethane and dichloroacetic acid are among the individual compounds considered most concerning; both carry MCLG values of zero from the EPA, reflecting the position that no known safe threshold exists for probable carcinogens.

Reproductive and Developmental Effects

Research has linked high DBP exposure to increased risk of adverse pregnancy outcomes. Studies have found associations with miscarriage, low birth weight, preterm birth, and neural tube defects, though findings vary across studies and populations. The associations are strongest at TTHM levels consistently above 50 µg/L throughout pregnancy. Pregnant individuals in areas with known high DBP levels may want to discuss point-of-use filtration with their healthcare provider.

Liver and Kidney Effects

Animal studies have shown that certain HAAs — particularly dichloroacetic acid and trichloroacetic acid — can cause liver and kidney damage at high concentrations. Human epidemiological evidence for these endpoints is less well established than for bladder cancer, but the toxicological findings informed the EPA’s regulatory approach to HAA5.

It is important to note that the immediate risks of consuming unfiltered, unchlorinated water far outweigh the long-term risks posed by DBPs at regulated levels. Waterborne infections cause tens of thousands of hospitalizations per year in countries with inadequate disinfection. The public health priority remains effective disinfection, with DBP minimization as a critical but secondary objective.

EPA Regulation and Limits

The EPA regulates DBPs under the Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 DBPR), finalized in 2006. The key enforceable limits are:

Standard	MCL	MCLG	Measurement Basis
Total Trihalomethanes (TTHM)	80 µg/L (ppb)	0 (for individual probable carcinogens)	Locational running annual average (LRAA)
HAA5 (five haloacetic acids)	60 µg/L (ppb)	0 (for DCA and TCA)	Locational running annual average (LRAA)

The Stage 2 rule marked a significant improvement over the earlier Stage 1 rule. Under Stage 1 (effective 2002), compliance was based on a system-wide running annual average. That allowed utilities to mask elevated concentrations at specific distribution points by averaging them with lower values elsewhere — a practice that left some customers consistently drinking water above the MCL while the system technically passed. The Stage 2 rule requires each individual monitoring location to meet the MCL independently, preventing that averaging problem.

The MCLGs for several individual DBP compounds are set at zero because the EPA classified them as probable human carcinogens (Group B2). These include bromodichloromethane, chloroform, and the HAAs dichloroacetic acid and trichloroacetic acid. MCLGs are non-enforceable health-based goals; the enforceable MCLs are set higher to reflect what is feasible for water systems to achieve in practice.

How Widespread Are DBPs?

Disinfection byproducts are among the most universally present regulated contaminants in US drinking water, for the simple reason that they are an unavoidable byproduct of chlorination — and the vast majority of public water systems use chlorine or chloramine as their primary disinfectant. EPA data indicate that detectable TTHM levels are found in essentially all chlorinated surface water systems and in most chlorinated groundwater systems with any measurable NOM.

Concentrations follow predictable seasonal patterns. Summer is consistently the highest-risk season: warmer temperatures accelerate DBP-forming reactions, source water NOM peaks with algae growth and runoff, and distribution system water age increases as demand patterns shift. In many systems, Q3 (July—September) TTHM values run 30—50% higher than Q1 (January—March) values. Systems that barely comply in winter can exceed the MCL during summer.

Surface water systems are disproportionately affected. They draw from rivers and reservoirs that receive agricultural runoff, wastewater effluent, and decaying vegetation — all high-NOM sources. Groundwater systems, particularly those drawing from deep confined aquifers, generally produce much lower DBP concentrations unless the source water has elevated NOM or bromide.

Within any given system, the distribution system endpoints — taps farthest from the treatment plant, dead-end mains, buildings with internal storage tanks — tend to show the highest DBP concentrations. Utilities required to monitor under the Stage 2 LRAA standard must place sampling sites at locations with high water age, which means reported compliance data reflects the worst-case points in the system.

How WaterVerge Tracks DBPs

WaterVerge pulls disinfection byproduct data from the EPA’s Safe Drinking Water Information System (SDWIS), which compiles compliance monitoring results submitted by water systems across the country. Our platform tracks both TTHM and HAA5 levels for every community water system that reports to the EPA.

When a water system exceeds the MCL for THMs or HAA5, it triggers a violation in the SDWIS database. WaterVerge monitors these violations and flags affected systems so users can see whether their local water provider has a history of DBP compliance issues. We display the most recent reported DBP concentrations alongside the federal MCL to give residents a clear picture of where their water stands.

Because DBP levels fluctuate seasonally — often peaking in summer when source water temperatures rise — WaterVerge tracks results across multiple monitoring periods. Viewing a full year of quarterly data provides a more accurate picture of DBP exposure than any single test result, particularly for systems whose levels swing significantly across seasons.

How to Remove DBPs

If your water system has elevated DBP levels, several home treatment options can meaningfully reduce your exposure. The methods vary in effectiveness for THMs versus HAAs, since THMs are volatile while HAAs are not.

Method	THM Removal	HAA5 Removal	Certification	Best For
Carbon block / GAC filter	High (80—95%)	Moderate (50—70%)	NSF/ANSI 53	Most households; pitchers, under-sink
Reverse osmosis (RO)	High (>95%)	High (>95%)	NSF/ANSI 58	Comprehensive point-of-use removal
Aeration / off-gassing	Moderate (THMs only)	None	None	Not practical for household use
Boiling	Not effective	Concentrates HAAs	—	Not recommended for DBP removal

Activated carbon filters: Granular activated carbon (GAC) and carbon block filters are the most practical and widely available option for most households. They adsorb THMs effectively and provide meaningful HAA5 reduction. Pitcher filters, faucet-mounted filters, and under-sink systems using carbon filtration can all reduce DBP levels. For certified THM reduction, look for NSF/ANSI Standard 53 certification. Carbon filter performance degrades over time as the media saturates, so replacing filters on the manufacturer’s schedule is important. For more on choosing a filter, see our guide to the best water filter pitchers.

Reverse osmosis (RO): RO systems remove a broad range of contaminants, including both THMs and HAAs, along with most other regulated contaminants. These systems are typically installed under the kitchen sink and filter water at the point of use. They produce slower flow rates and waste some water in the process, but provide the most comprehensive DBP reduction of any household technology. Look for NSF/ANSI 58 certification.

Aeration and off-gassing: Because THMs are volatile organic compounds, they can be partially removed by allowing water to sit in an open container or by vigorous aeration. This is impractical for routine household use and does not address HAAs.

Boiling is not effective: A common misconception is that boiling removes DBPs as it does pathogens. The opposite is true: boiling can concentrate non-volatile HAAs (which make up much of the HAA5 group), and while THMs will eventually volatilize, temporary concentration during boiling may briefly increase THM levels before they off-gas. Carbon filtration or reverse osmosis remain the only reliable and practical household solutions for DBP reduction.

Check Your City

Disinfection byproduct levels vary widely between water systems depending on source water quality, treatment methods, and distribution system characteristics. Even neighboring cities served by different utilities — or drawing from different source water — can have very different DBP profiles.

Search your city on WaterVerge to look up the most recent TTHM and HAA5 levels reported by your water provider. You can see how your system’s concentrations compare to the federal MCL, review any past violations, and understand whether DBPs are a meaningful concern in your area. Knowing your local data is the first step toward making informed decisions about filtration and water safety for your household.