Are disinfection byproducts dangerous?

At the levels found in regulated public water systems, the health risk from DBPs is relatively low — and far lower than the risk from the pathogens that disinfection prevents. Long-term high-level exposure is associated with modest increases in bladder cancer risk. Prudent precautions include using a carbon filter, which significantly reduces DBP levels.

What filter removes disinfection byproducts?

Activated carbon filters (NSF/ANSI 53 certified) are highly effective at removing TTHMs and HAA5 — the two main regulated DBP groups. Reverse osmosis also removes DBPs but is less commonly installed just for this purpose. Letting tap water sit in an open pitcher for a few hours also allows volatile THMs to off-gas.

Why does my tap water smell like chlorine?

The chlorine smell comes from the disinfectant itself, which is added intentionally to kill bacteria. A stronger smell doesn't always mean higher DBP levels — freshly treated water often smells more strongly of chlorine but may have lower DBPs than older water in the distribution system. A carbon filter will eliminate the taste and odor.

Are DBPs worse in summer?

Generally yes. Warm weather increases algae growth and organic matter in source water, providing more precursor material for DBP formation. Warm water also speeds the chemical reactions. Many utilities see their highest DBP readings in late summer and early fall.

Do DBPs form in well water?

DBPs form specifically as a result of chemical disinfection. Private wells that use chlorine shocking or have inline chlorination can develop DBPs. Untreated wells do not contain DBPs but may have microbial risks instead.

All Contaminants

moderate risk levelDisinfection Byproducts

Disinfection Byproducts (DBPs)

Disinfection byproducts form when chlorine or other disinfectants react with naturally occurring organic matter in source water. The two main regulated groups are total trihalomethanes (TTHMs) and haloacetic acids (HAA5). They are an unavoidable tradeoff of water disinfection — the risk of not disinfecting far outweighs the risk of DBPs, but minimizing exposure is prudent.

Quick Answer

When utilities add chlorine to water to kill pathogens, it reacts with dissolved organic matter — leaves, algae, soil — to produce disinfection byproducts (DBPs). Over 600 DBPs have been identified. The EPA regulates two groups: total trihalomethanes (TTHMs, including chloroform) and haloacetic acids (HAA5). DBP levels tend to be highest in surface water systems and in warm months when organic matter is elevated.

Why Do People Care?

While DBPs are present at low levels in virtually all chlorinated water systems, long-term exposure has been associated with increased bladder cancer risk and reproductive effects. The EPA sets running annual averages to limit exposure, but individual quarterly samples can exceed averages.

Everyone on chlorinated public water has some DBP exposure. People who drink large amounts of tap water, swim in chlorinated pools, or take long showers in chlorinated water (which allows inhalation and skin absorption) have greater exposure. Pregnant women are advised to minimize exposure.

Known Health Effects

Increased bladder cancer risk with decades of high-level exposure

Adverse reproductive outcomes — miscarriage, low birth weight (at elevated levels)

Possible colorectal cancer association (research ongoing)

Liver and kidney stress at high concentrations

Common Sources

Reaction between chlorine disinfectant and natural organic matter

Surface water sources (rivers, reservoirs) with high organic content

Seasonal algae blooms increasing precursor material

Chloramines (alternative disinfectant) producing different DBP profile

Distribution system aging — water that sits longer forms more DBPs

Regulatory Limit

EPA Maximum Contaminant Level (MCL)

80 µg/L (TTHMs) / 60 µg/L (HAA5)

The EPA limits total trihalomethanes (TTHMs) to 80 micrograms per liter (µg/L) and haloacetic acids (HAA5) to 60 µg/L, measured as annual running averages across all monitoring points in the system. These limits were set under the Stage 2 Disinfectants and Disinfection Byproducts Rule (2006). Some international standards are more stringent.

How to Test for It

DBPs are routinely monitored by public water systems and reported in Consumer Confidence Reports. Results are reported as system-wide running annual averages. Individual samples at your tap may vary — distribution system age and distance from the treatment plant affect DBP levels.

Effective Treatment Options

These treatment methods have demonstrated effectiveness for DBPs.

Reverse Osmosis

Reverse osmosis (RO) is the most comprehensive point-of-use water treatment technology available for residential use. It removes 90–99% of dissolved contaminants including PFAS, lead, arsenic, nitrates, and disinfection byproducts by forcing water through a semi-permeable membrane with pores of approximately 0.0001 microns.

PFAS / PFOA / PFOS — 90–99% removalLead — 95–99% removal

Activated Carbon

Activated carbon is the most widely used residential water treatment technology. It removes chlorine, taste and odor compounds, disinfection byproducts, many volatile organic compounds (VOCs), and — with NSF/ANSI 53 certification — lead and some PFAS. It is available as pitcher filters, under-sink units, and whole-house systems.

Chlorine and chloramine — virtually complete removalTaste and odor compounds — highly effective

Frequently Asked Questions

Reverse Osmosis Filtration

Removes: DBPs

Activated Carbon Filtration

Removes: DBPs

Data Sources & Provenance

All data on this page is sourced from official U.S. government or public datasets.

EPA Drinking Water Contaminant InformationView source

ATSDR ToxFAQs / Toxicological ProfilesView source

EPA SDWIS — violation and detection dataView source

Last updated: 2025-01-15

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