If you check the TapWaterSafety page for any major US city, two contaminants will almost certainly appear in the Top Concerns list: Total Trihalomethanes (TTHMs) and Haloacetic Acids (HAA5). These aren’t industrial pollutants or natural geological contaminants — they’re chemicals that form during the water treatment process itself, specifically when chlorine or chloramine reacts with naturally-occurring organic matter in the source water.
Disinfection byproducts (DBPs) are the most widespread contaminant class in US drinking water. They’re detected in essentially every municipal system that uses chlorine-based disinfection (which is nearly all of them). They’re regulated by the EPA but with limits that EWG and many independent researchers consider far too lenient.
What TTHMs and HAA5 actually are
TTHMs are a family of four chemicals: chloroform, bromodichloromethane, dibromochloromethane, and bromoform. The EPA regulates them as a group, with a total limit of 80 parts per billion. EWG’s health guideline is 0.6 ppb — a 133-fold difference.
HAA5 is a similar regulated group: monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid. EPA legal limit: 60 ppb. EWG health guideline: 0.1 ppb — a 600-fold difference.
Both form through the same basic chemistry: chlorine (a strong oxidizer) reacts with natural organic matter in the source water — decaying leaves, algae, soil organics — to produce these halogenated byproducts. The more organic matter in your source water and the more chlorine added, the more DBPs you’ll have.
This is why utilities drawing from surface water (rivers, lakes, reservoirs) typically have much higher DBP levels than utilities drawing from protected groundwater. The surface water has more organic content; the chlorine has more to react with.
Health effects of long-term DBP exposure
The health research on DBPs is extensive and consistent. The most established effects:
Bladder cancer. Multiple meta-analyses of epidemiological studies have found a 15-35% increased bladder cancer risk associated with long-term consumption of chlorinated tap water at typical US exposure levels. This is one of the best-documented carcinogenicity findings in environmental epidemiology.
Adverse pregnancy outcomes. DBPs are associated with increased risk of miscarriage, stillbirth, low birth weight, and certain birth defects. Pregnant residents are the most-cited at-risk population.
Colorectal cancer. Less well-established but emerging research suggests increased colorectal cancer risk at long-term DBP exposure.
Other suggested associations (less conclusive but documented in research): liver, kidney, and central nervous system effects.
The EPA has classified TTHMs and HAA5 as “probable human carcinogens.” The IARC classifies the most-studied member of the group (chloroform) as Group 2B (possibly carcinogenic).
How widespread are elevated DBP levels?
EWG’s 2021 analysis of US water utility data found:
- 57% of US utilities had TTHM levels above EWG’s health guideline of 0.6 ppb
- 78% had HAA5 levels above the 0.1 ppb guideline
- Median TTHM detection across US surface-water utilities was approximately 25 ppb (42x the EWG guideline)
- Highest reported levels exceeded 200 ppb in some smaller utilities, often during summer months when organic loading peaks
Examples from major cities:
- Tampa, FL: TTHM ~52 ppb (87x EWG guideline)
- Philadelphia, PA: TTHM ~40-60 ppb
- New Orleans, LA: TTHM consistently above 50 ppb due to Mississippi River source water
- Flint, MI: TTHM ~38 ppb (a contributing factor in the broader Flint water crisis)
For comparison, EWG’s guideline of 0.6 ppb is met only by utilities drawing from very protected source water with minimal organic loading — typically deep groundwater or pristine mountain watersheds (Portland, Seattle, parts of New York City).
Why aren’t EPA limits stricter?
The EPA’s 80 ppb TTHM and 60 ppb HAA5 limits were set in 2006 based on a balance of health protection and feasibility. Reducing DBP formation while maintaining adequate disinfection is technically difficult — utilities would need to upgrade treatment processes (typically adding ozonation or membrane filtration) at significant cost.
The regulatory tradeoff explicitly accepted some cancer risk in exchange for water that’s adequately disinfected to prevent acute waterborne disease. From a public health perspective, this was the right call when the rule was made — acute disease outbreaks kill faster than long-term DBP exposure causes cancer.
But the tradeoff is also why home filtration matters: utilities deliver water that’s safe enough by the regulatory standard, but you can easily do better at the tap.
How to filter TTHMs and HAA5
DBPs are some of the easiest concerning contaminants to remove. Activated carbon is highly effective for both TTHMs and HAA5, and the technology is widely available in consumer filters.
What works
Activated carbon block filters. Solid carbon block (not loose granular carbon) consistently removes 90%+ of TTHMs and 60-90% of HAA5. NSF/ANSI Standard 53 includes a specific TTHM reduction certification — look for this.
Reverse osmosis. RO membranes physically block most DBP molecules. Removal rates are typically 95%+ for both TTHMs and HAA5. Overkill if DBPs are your only concern; appropriate if you also want to address PFAS, heavy metals, or chromium-6.
Granular activated carbon (GAC). Loose carbon in pitchers and faucet-mounted filters works for chlorine but is less effective for DBPs than block carbon. Performance varies widely.
What doesn’t work
- Letting water sit out (DBPs don’t dissipate like chlorine does)
- Boiling water (some TTHMs evaporate but most DBPs remain or even concentrate)
- Water softeners
- UV disinfection
- “Alkaline” or hydrogen water systems
A timing note
DBP formation continues after the water leaves the treatment plant. Water sitting in distribution pipes for hours or days can develop higher DBP levels than freshly-treated water at the plant. The DBP level at your tap can be meaningfully higher than what the utility reports in its CCR — particularly in homes at the end of long pipe runs.
This is one reason filtering at the home tap is more reliable than relying on utility-level treatment alone.
Top filter recommendations for DBPs
Best pitcher: Clearly Filtered Water Pitcher. NSF/ANSI 53 certified for TTHM reduction (plus chlorine, lead, and PFAS). Among the few pitchers specifically tested for DBP reduction. $80-95.
Best under-sink: Aquasana AQ-5300. Three-stage activated carbon block with NSF/ANSI 42 + 53 certifications including TTHM reduction. 6-month filter life. $180-240.
Best comprehensive: AquaTru Countertop or Aquasana OptimH2O. Reverse osmosis handles DBPs along with everything else of concern. $429-599.
Strategic priorities by utility type
If your water comes from surface water (river, lake, reservoir): DBPs are very likely a top concern. A carbon block filter is almost certainly warranted. RO if you also have other concerns.
If your water comes from protected groundwater: DBPs are less of a concern. A basic carbon pitcher may be sufficient for taste improvement; full filtration is less urgent.
If your CCR shows TTHM above 40 ppb or HAA5 above 30 ppb: You’re well above health guidelines. Filtration is worthwhile.
If you’re pregnant or have young children: Filter regardless of utility profile. DBP risk is most pronounced for these populations.
The bottom line
Disinfection byproducts are the most common water quality concern in US tap water — present in essentially every chlorinated system, often at levels 50-500x above health-protective guidelines. The good news is they’re among the easiest contaminants to remove, with affordable carbon filter options widely available.
If your TapWaterSafety utility page lists TTHM or HAA5 as a top concern (it probably does), a properly certified activated carbon filter is the practical fix.
Find your water by ZIP code to see your specific DBP levels and get a filter recommendation matched to your water profile.