At a glance
| Chemical family | Chlorinated/brominated disinfection byproducts (the acetic-acid family). "HAA5" is the regulated group of five: monochloroacetic, dichloroacetic (DCA), trichloroacetic (TCA), monobromoacetic, and dibromoacetic acid. Non-volatile, unlike trihalomethanes. |
| CAS numbers | Group of five — e.g. dichloroacetic acid 79-43-6, trichloroacetic acid 76-03-9 (the two of most concern) |
| Classification | Dichloroacetic acid and trichloroacetic acid are IARC Group 2B — possibly carcinogenic to humans. As a class, HAAs are part of the disinfection-byproduct literature linking long-term exposure to a small excess bladder-cancer risk. EPA HAA5 MCL 60 µg/L. |
| Where you encounter it | Any chlorinated public water supply, especially systems drawing from surface water (rivers, reservoirs) rich in natural organic matter — which describes many Midwestern and Southern cities. Higher in warm weather and at the far ends of the distribution system. |
| Sleep micro environment relevance | Indirect — an ingestion exposure via drinking water. Unlike THMs, HAAs are non-volatile, so the shower-air route does not apply; this is a drinking-and-cooking exposure. |
| Activated carbon capture | Well removed by activated carbon and reverse osmosis. A carbon filter certified to NSF/ANSI 53 for HAA5/disinfection byproducts is the standard fix. Inferred from standard DBP water-treatment engineering; haloacetic acids adsorb onto activated carbon |
Regulatory & certification status
Haloacetic acids are regulated together as a group under the same disinfection-byproduct framework as trihalomethanes. The rows below give the standards and the classification of the two members that matter most.
| United States | EPA enforceable Maximum Contaminant Level of 60 µg/L for HAA5 (the sum of the five regulated acids), under the Stage 1 (1998) and Stage 2 (2006) Disinfectants and Disinfection Byproducts Rules; Stage 2 tightened compliance to a locational running annual average across the system. Regulatory — US EPA |
| Cancer classification | The International Agency for Research on Cancer classifies both dichloroacetic acid and trichloroacetic acid as Group 2B — possibly carcinogenic to humans — based largely on animal evidence. Regulatory — IARC |
| World Health Organization | WHO sets individual guideline values for dichloroacetic and trichloroacetic acid rather than a grouped HAA5 number, reflecting a substance-by-substance risk assessment. Regulatory — WHO |
| The 72-hour test window | Not applicable. Haloacetic acids are a waterborne ingestion exposure measured by a specialized water-lab DBP test, unrelated to the VOC-emission chamber tests used for mattresses and foam. Inferred — from the ingestion-via-water route versus the material/VOC focus of product emissions testing |
What it is
Haloacetic acids are acetic acid (vinegar's acid) with one, two, or three of its hydrogens swapped for chlorine or bromine atoms. They don't come from a factory or a spill — they are created inside the treatment process. When chlorine (or, to a lesser degree, chloramine) is added to disinfect water, it reacts with the dissolved natural organic matter that surface water always carries — decayed plant material, soil humics — and a fraction of that reaction produces haloacetic acids, right alongside trihalomethanes.
Of the five regulated acids, dichloroacetic acid (DCA) and trichloroacetic acid (TCA) are usually the most abundant and the ones driving the health classification; both are IARC Group 2B. Regulatory The amount that forms depends on how much organic matter is in the source water, the chlorine dose, temperature, and time in the pipes — so haloacetic acids tend to be highest in warm months and at the far reaches of the distribution system, and higher in surface-water systems than in clean groundwater ones.
One important contrast with trihalomethanes: haloacetic acids are not volatile. THMs evaporate, so a hot shower delivers them to your lungs and skin; haloacetic acids stay in the water, so they matter only through what you drink and cook with. That single difference changes how you manage them — filter the tap, not the shower.
Where you encounter it
Chlorinated surface-water systems
The classic setting. Cities drawing from rivers and reservoirs — which carry more natural organic matter than deep groundwater — form more disinfection byproducts, so many Midwestern and Southern surface-water utilities are where HAA5 runs highest. If your water comes from a river, HAAs and THMs are both part of your picture.
Warm weather and distribution-system ends
HAA formation increases with temperature and contact time, so levels rise in summer and toward the far ends of the pipe network, farthest from the treatment plant. Two homes on the same system can see different HAA5 depending on where they sit in the distribution grid. Inferred from DBP formation chemistry — temperature and residence time drive yield
What the research says
The disinfection-byproduct cancer signal
Haloacetic acids are part of the same body of epidemiology as trihalomethanes: pooled analyses of long-term exposure to chlorination byproducts, including work led by Cristina Villanueva, have found a modest association with bladder cancer. Peer-reviewed — Villanueva et al. DBP pooled analyses Because THMs and HAAs occur together, epidemiology often can't fully separate which byproduct drives the signal — they are best understood as markers of total disinfection-byproduct exposure.
The benefit that dwarfs the risk
The essential context: the small long-term risk from disinfection byproducts is set against the enormous, immediate benefit of disinfection — the near-elimination of waterborne cholera, typhoid, and dysentery that chlorination achieved. Inferred from the public-health history of drinking-water disinfection The correct response is never to stop disinfecting; it is to reduce byproduct formation at the plant and to filter at the tap.
What helps
Filter with certified activated carbon. A carbon block (pitcher, faucet, or under-sink) certified to NSF/ANSI 53 for HAA5/disinfection byproducts removes haloacetic acids; reverse osmosis does too. See water filters compared.
Read your Consumer Confidence Report. US utilities report HAA5 annually. If yours runs near the limit — often the case for warm-season surface-water systems — a tap filter is a reasonable step.
Filter the tap, not the shower. Because HAAs are non-volatile, a drinking-water filter is enough; you do not need a shower filter for them (that's a THM consideration).
What does NOT help
- Boiling. Boiling does not reliably remove haloacetic acids (unlike volatile THMs, which boiling can drive off) and concentrates non-volatile components as water evaporates.
- Stopping or avoiding disinfected water. The disinfection that forms HAAs is what makes the water microbiologically safe — the pathogen risk of undisinfected water is far greater.
- A shower filter. Useful for THMs, but unnecessary for non-volatile HAAs.
- A plain sediment filter. Only activated carbon (or RO) removes haloacetic acids; a mechanical cartridge does nothing for them.
Open questions
- How much of the disinfection-byproduct cancer signal is attributable to haloacetic acids specifically versus co-occurring trihalomethanes and unregulated byproducts. Speculation
- The health significance of the many unregulated disinfection byproducts that form alongside the regulated HAA5 and THM4. Speculation
Where you meet Haloacetic Acids across your home
The same compound turns up in more than one place you live. Here's where it shows up in Embr — each links to the full breakdown for that part of your home.
Citations
- US Environmental Protection Agency. Stage 1 & Stage 2 Disinfectants and Disinfection Byproducts Rules — HAA5 MCL 60 µg/L. epa.gov Regulatory
- IARC. Dichloroacetic acid and trichloroacetic acid — Group 2B, possibly carcinogenic. iarc.who.int Regulatory
- Villanueva CM et al. Disinfection byproducts in drinking water and bladder cancer — pooled epidemiological analyses. pubmed.ncbi.nlm.nih.gov Peer-reviewed
- World Health Organization. Guidelines for Drinking-water Quality — dichloroacetic/trichloroacetic acid guideline values. who.int Regulatory
Frequently asked questions
What are haloacetic acids in drinking water?
Haloacetic acids (HAAs) are a group of chemicals formed as a byproduct of disinfecting water — when chlorine reacts with the natural organic matter (decayed leaves, soil) present in the source water. They are the second major class of regulated disinfection byproducts, alongside trihalomethanes (THMs). The EPA regulates five of them together as "HAA5" with a combined limit of 60 µg/L. Two of the five — dichloroacetic and trichloroacetic acid — are classified as possibly carcinogenic. Like THMs, they are the price of safe, disinfected water, and a carbon filter removes them.
Are haloacetic acids the same as trihalomethanes?
No — they are the two different major families of chlorination byproducts, and both are regulated. They form from the same reaction (chlorine plus natural organic matter) and rise and fall together, which is why a system high in one is often high in the other. The key practical difference: trihalomethanes are volatile, so you also inhale and absorb them in a hot shower, whereas haloacetic acids are not volatile — they matter through drinking, not showering. Both are removed by activated carbon at the tap.
What is the safe level of HAA5 in water?
The US EPA sets a Maximum Contaminant Level of 60 µg/L for HAA5 — the sum of the five regulated haloacetic acids — measured as a locational running annual average across the distribution system. The WHO sets separate guideline values for individual haloacetic acids (for example, dichloroacetic and trichloroacetic acid). As with all disinfection byproducts, the limit balances a small long-term cancer risk against the far larger, immediate benefit of killing waterborne pathogens — which is why you never stop disinfecting; you filter at the tap instead.
How do I remove haloacetic acids from water?
Activated carbon removes haloacetic acids well: a carbon block filter (pitcher, faucet, or under-sink) certified to NSF/ANSI 53 for HAA5 or disinfection byproducts captures them, and reverse osmosis (which includes carbon stages) does too. Because they are not volatile, you do not need to worry about them in shower air the way you do with trihalomethanes — filtering your drinking and cooking water is the fix. Do not respond by avoiding disinfected water: the disinfection that creates HAAs is what keeps the water safe from pathogens.
Related compounds
Embr researches the chemistry of where you live — including the byproducts of making water safe. See the methodology page for how this Atlas tags claims by evidence strength, the tap-water hub for the other things in your water, and water filters compared for how to remove disinfection byproducts.
Last reviewed 2026-07-13. If you find a factual error, contact us.