Disinfection byproducts (DBPs) are one of the most widespread and least-discussed water quality issues in the United States. They're present in virtually every chlorinated municipal water system in the country -- which means almost everyone on city water is exposed to them daily.
They're also the clearest example of a regulatory trade-off in drinking water: the disinfection that makes water safe to drink (killing bacteria, viruses, and parasites) produces chemical compounds that carry their own long-term health risks.
Here's what they are, how they form, what the research shows, and what filters work.
What Are Disinfection Byproducts?
Disinfection byproducts form when chlorine (or other disinfectants) react with naturally occurring organic matter in source water -- decomposed plant material, humic acids, algae. The chemical reaction produces halogenated compounds that weren't present in the original source water.
The EPA currently regulates two main categories:
Total Trihalomethanes (TTHMs): A group of four compounds -- chloroform, bromodichloromethane, dibromochloromethane, and bromoform. The ratio of chlorinated to brominated species depends on the bromide content of the source water. Higher bromide (common in coastal areas and drought-reduced reservoirs) shifts production toward brominated THMs, which are associated with higher cancer risk than chlorinated THMs.
Haloacetic Acids (HAA5 and HAA9): Five federally regulated acids (HAA5) and a broader nine-compound group (HAA9) that includes brominated species not covered by the standard HAA5 metric. The EPA regulates only HAA5 despite growing research showing brominated HAAs are more potent carcinogens.
The regulatory gap: The EPA's TTHMs MCL is 80 ppb; the EWG health guideline is 0.15 ppb -- a 533x difference. For HAA5, the MCL is 60 ppb against an EWG guideline of 0.1 ppb. These gaps reflect limits set in the 1970s-1990s based on the technology and economics of that era. Legal compliance is not the same as zero health risk.
Where DBPs Are Highest
DBP formation depends on the disinfectant used and the organic content of the source water. The highest DBP cities share a common profile: surface water sources with high organic loads.
Highest DBP cities nationally: Atlanta (Chattahoochee River, elevated bromide -- HAA9 ~348x EWG guideline), San Diego (Colorado River supply -- TTHMs up to 75.6 ppb, HAA9 302x EWG guideline), New Orleans (Mississippi River), Houston (San Jacinto River), Phoenix and Scottsdale (Colorado River supply with agricultural return flows).
Lowest DBP cities: Seattle/Tacoma (clean mountain snowpack source), Denver (upstream reservoir filtration), Boston (Quabbin/Wachusett Reservoirs), Minneapolis/St. Paul (advanced treatment including membrane filtration).
Seasonal variation: DBP levels spike in late summer when algae blooms add organic matter and when drought-reduced reservoir volume concentrates organic content. A city complying with MCLs on annual averages may have individual seasonal samples near or above the MCL.
Want to Know Your Specific DBP Levels?
Your utility's annual Consumer Confidence Report shows system-wide averages, but levels can vary by season and location within the distribution system. A certified lab test measures what's actually coming out of your tap. The Tap Score Essential City Water Test covers disinfection byproducts alongside 50+ other parameters in a single test, with results in 5 business days and a clear digital report. See our complete water testing guide for more detail.
What the Research Shows on Health Effects
Bladder cancer: The most replicated association. Multiple large epidemiological studies have found elevated bladder cancer risk in people with long-term high TTHM exposure from chlorinated water. IARC has classified chloroform as a possible human carcinogen (Group 2B).
Pregnancy outcomes: Some studies have found associations between high DBP exposure and adverse pregnancy outcomes including low birth weight, particularly for brominated THM species. The research is inconsistent, but it's the reason pregnant women in high-DBP cities often receive specific filtration guidance.
Inhalation exposure: TTHMs are volatile -- they vaporize when water is heated or agitated. Studies have found that showering in high-TTHM water for 10 minutes produces measurable blood TTHM levels. Whole-house carbon filtration reduces inhalation exposure; under-sink filtration alone does not.
Brominated species: Cities receiving Colorado River water (San Diego, Phoenix, Las Vegas) and Chattahoochee River water (Atlanta) have elevated bromide, shifting their DBP profile toward brominated species that are more potent than chlorinated counterparts.
The Chloramine Difference
Many utilities switched from free chlorine to chloramine to reduce TTHMs and HAA5 -- and it works for those specific regulated compounds. However, chloramine produces its own DBP profile: iodoacids, nitrosamines (particularly NDMA, a probable human carcinogen), and haloacetonitriles. These aren't currently regulated under federal MCLs.
Cities using chloramine include Denver, Phoenix, Las Vegas, Houston, Seattle, Minneapolis, San Francisco, and many others. If your city uses chloramine, TTHM and HAA5 levels may be low, but the fuller DBP profile is not being fully captured in standard regulatory testing. See our chloramine guide for what filters actually address it.
What Removes DBPs
Activated carbon filtration is highly effective at removing TTHMs. The key limitation: carbon filtration does not remove haloacetic acids as effectively as it removes THMs. HAAs are more water-soluble and harder for carbon to capture. For the most comprehensive DBP reduction, RO is more complete.
Reverse osmosis removes both THMs and HAAs to non-detectable or very low levels, and simultaneously addresses PFAS, lead, arsenic, and other co-contaminants.
For inhalation exposure: Only whole-house filtration addresses shower and bath inhalation. The most practical solution is a whole-house carbon block or GAC system upstream of all water fixtures.
What doesn't work: Boiling concentrates THMs (it doesn't reduce them). Letting water sit open reduces volatile THMs slightly over several hours -- not a practical solution. Basic ceramic or sediment filters have no effect on DBPs.
Filters That Reduce DBPs
- ✓Removes both TTHMs and HAAs comprehensively -- carbon alone can't match this for HAA removal
- ✓NSF 58 certified; also removes PFAS, lead, arsenic, nitrates in one system
- ✓The right choice for high-DBP cities like Atlanta, San Diego, New Orleans, Houston
- ✓~$50-70/year in replacement filters
- Requires one drilled faucet hole and basic plumbing
- 4:1 waste water ratio
- Does not address inhalation exposure from showers -- only drinking water
The most thorough solution for high-DBP city residents. Addresses TTHMs, HAAs, and all the other contaminants that often accompany high-DBP source water in a single system.
- ✓Same comprehensive TTHM and HAA removal as standard RO
- ✓6th alkaline stage restores minerals after RO -- noticeably better taste than flat RO water
- ✓WQA certified; good for Atlanta or San Diego households who drink a lot of water at home
- ✓NSF 58 and 372 certified
- Slightly larger footprint than the APEC
- Higher price than the APEC ROES-50
- 4:1 waste water ratio
The upgrade pick if you want better-tasting drinking water alongside comprehensive DBP removal. A good choice for households in high-DBP cities where the RO water is drunk heavily.
- ✓Catalytic carbon block provides strong TTHM reduction
- ✓NSF 42, 53, and 372 certified -- also covers lead and chloramine
- ✓No drilling required -- T-valve connection is fully reversible when you move
- ✓Best value for TTHM reduction without full RO
- Less effective at HAA removal than RO -- HAAs are more water-soluble than THMs
- Does not remove PFAS, nitrates, or arsenic
- Renters need to confirm faucet compatibility
The right pick for renters or budget-conscious households who primarily want TTHM reduction. For high-DBP cities where HAA levels are also elevated, upgrade to RO for complete coverage.
- ✓Independently certified for TTHMs, HAAs, chloramine, and 360+ contaminants
- ✓No installation required -- good for renters in high-DBP cities like Atlanta or Houston
- ✓NSF 42, 53, 244, 401, and 473 certified -- the most comprehensive pitcher available
- ✓Better DBP coverage than any standard carbon pitcher
- Slower than a Brita -- around 10 minutes to filter a full pitcher
- Higher ongoing cost than standard pitchers
- Does not address inhalation exposure
The best no-install option for renters in high-DBP cities. Comprehensive DBP coverage plus lead, PFAS, and chloramine -- a meaningful upgrade over any standard carbon pitcher.
- ✓NSF 42 and 53 certified -- provides basic TTHM reduction
- ✓Lowest ongoing cost of any certified pitcher
- ✓Available everywhere -- good starting point for lower-risk cities
- ✓6-month filter life
- Limited HAA coverage compared to RO or Clearly Filtered
- Does not remove PFAS, chloramine, fluoride, or nitrates
- Not adequate for high-DBP cities like Atlanta or San Diego
An adequate starting point for lower-risk cities or households where taste improvement and basic TTHM reduction is the goal. Upgrade to Clearly Filtered or RO for high-DBP situations.
Practical Guidance by Risk Level
Low-DBP city (Seattle, Denver, Boston, Minneapolis): Standard NSF 42/53 carbon pitcher or under-sink filter is adequate. Primary concern is taste.
Moderate-DBP city (most major US cities): Carbon block filter (Frizzlife SK99, Clearly Filtered) addresses TTHMs well. For comprehensive coverage including HAAs, consider RO.
High-DBP city (Atlanta, San Diego, New Orleans, Houston, Phoenix): RO at the kitchen tap is the strongest recommendation. For shower/bath inhalation exposure -- particularly relevant if pregnant -- a whole-house carbon filter is worth considering.
Pregnant and in a high-DBP city: The precautionary case is strongest here. RO for drinking/cooking water plus a shower filter or whole-house carbon for inhalation exposure addresses all DBP exposure routes.
Enter your ZIP at PureCity to see your city's specific DBP levels and how they compare to federal and health-based thresholds.
Related Articles
- How to Test Your Home's Drinking Water
- Chloramine in Drinking Water: What Standard Carbon Filters Miss
- Best Under-Sink Reverse Osmosis Systems
Sources: EPA: Disinfection Byproducts Rule · IARC Monograph: Chloroform · EWG Tap Water Database · NSF: Water Treatment Technologies · Cantor KP et al. "Drinking water source and chlorination byproducts in Iowa. Risk of bladder cancer." Epidemiology 1998 · Nieuwenhuijsen MJ et al. "Chlorination disinfection byproducts in water and their association with adverse reproductive outcomes: a meta-analysis." Occup Environ Med 2000