Haloacetic acids (HAA5)

WTR

0.2 µg/L California's health goal for dichloroacetic acid

It is one of five acids counted inside a single 60 µg/L limit, and that limit is what your report is graded against.

Emerging evidence

Stasis Home Research Every claim on this page is graded against peer-reviewed health research and linked to its primary source.Updated August 2026

Haloacetic acids are the second regulated family of disinfection byproducts, formed when chlorine reacts with organic matter in the source water. Five are regulated as one number, capped at 60 µg/L. Inside that sum California sets health goals as low as 0.03 µg/L for single members, and no US agency sets a health line for the group at all. Trihalomethanes reach you through shower air; these reach you only by drinking.

Haloacetic acids (HAA5): what is it, and where does it come from?

Haloacetic acids are acetic acid with one, two, or three hydrogens replaced by chlorine or bromine. Five are federally regulated as a single group: monochloroacetic, dichloroacetic, trichloroacetic, monobromoacetic, and dibromoacetic acid. Four more complete the nine that labs commonly measure, and none of those four is regulated; up to 13 haloacetic acids have been identified in disinfected water. They form the same way trihalomethanes do, when the chlorine used to disinfect drinking water meets natural organic matter from lakes and rivers, so every home on a chlorinated or chloraminated public system carries some level. Levels run higher on surface water than groundwater, higher in warm months, and higher where the source carries more organic matter. One thing runs opposite to trihalomethanes: as pH rises, trihalomethanes increase and haloacetic acids decrease. A utility raising pH to control lead pushes the two in opposite directions. On a steady, low-bromide system they usually rise and fall together, but you cannot read one off the other.

Haloacetic acids (HAA5): why does it matter?

The problem is what a single 60 µg/L number hides. That limit sums five acids with genuinely different toxicity. EPA's own health goal for dichloroacetic acid is zero and for trichloroacetic acid is 20 µg/L, and the two brominated members have no federal health goal at all. California went further in December 2022 and set health goals for each of the five, landing at 0.2 µg/L for dichloroacetic acid and 0.03 µg/L for dibromoacetic acid. Then it declined to publish a combined number, because the five have different potencies and different critical effects. So a utility can sit at 55 µg/L, fully compliant, with a large share of that 55 being dichloroacetic acid, the member EPA's own health goal puts at zero. Nationally, dichloroacetic acid is the largest single contributor to the sum. The cancer findings behind those goals are animal studies, and human epidemiology specific to haloacetic acids is sparse and has not established the association that the broader disinfection-byproduct literature shows. The exposure route is the one useful simplification here. These are charged, non-volatile molecules that do not leave the water in a hot shower and cross skin poorly, so from your tap water, drinking is the dominant route by a wide margin. Skin and shower air are treated as negligible rather than zero, and how much you get from food and air is not well enough measured for anyone to put a number on your total.

California adopted public health goals for all five regulated haloacetic acids in December 2022, ranging from 0.03 µg/L for dibromoacetic acid to 53 µg/L for monochloroacetic acid, and explicitly declined to set one for the group, stating that a single goal for the five combined was not developed because of the differing potencies and the distinct critical effects of the individual chemicals.CA OEHHA. Public Health Goals for Haloacetic Acids in Drinking Water, December 2022 (per-compound goals; the declined group goal; the shower-inhalation and boiling findings)
EPA's own non-enforceable health goal for dichloroacetic acid is zero, and for trichloroacetic acid is 20 µg/L, while the two brominated members carry no federal health goal, yet all five are enforced together against a single 60 µg/L limit.US EPA. 40 CFR 141.53, maximum contaminant level goals for disinfection byproducts (dichloroacetic acid zero; trichloroacetic acid 20 µg/L; monochloroacetic acid 70 µg/L)
The National Toxicology Program lists six haloacetic acids as reasonably anticipated to be human carcinogens. Only two of them, dichloroacetic and dibromoacetic acid, are among the regulated five; the other four are all four of the unregulated relatives that complete the nine, so the listed acids sit disproportionately outside what your utility is required to measure. NTP evaluated thirteen in total and declined to list the other seven, including trichloroacetic and monochloroacetic acid, and found the data inadequate to evaluate haloacetic acids as a class. Four of the six listings rest on animal cancer studies and two on metabolism and mechanism. NTP separately records that the one available human study of haloacetic acids and kidney cancer found no increased risk.US National Toxicology Program. Report on Carcinogens, haloacetic acids found as water disinfection byproducts
How HAA5 compliance is judged depends on how often the system monitors. For systems on quarterly monitoring it is a locational running annual average: the last four quarters averaged at one specific sampling point, evaluated separately at each point rather than pooled across the system. Systems monitoring yearly or less often follow a separate rule, where each sample must come in below the limit and a non-exceeding result is treated as that location's annual average.US EPA. 40 CFR 141.620, the locational running annual average that HAA5 compliance is judged on
Inhalation is not a meaningful route: in a shower-stall study summarized by California's health hazard assessment office, at 250 µg/L combined haloacetic acids in water the inhalation dose was under 1% of the corresponding oral dose, and the office concluded inhalation is not an important exposure route and did not quantify it.CA OEHHA. Public Health Goals for Haloacetic Acids in Drinking Water, December 2022 (per-compound goals; the declined group goal; the shower-inhalation and boiling findings)
Nationally, dichloroacetic acid is the largest single contributor to the regulated sum. In US utility compliance testing for 2014 to 2017 its population-weighted average was 7.8 µg/L across 29,673 community water systems, ahead of trichloroacetic acid at 6.4, against California health goals of 0.2 and 0.1 µg/L. The aggregation was published by authors on the staff of the Environmental Working Group and is used here as dated national context only, never as a household figure.Evans et al., 2020 (Int J Environ Res Public Health 17(6):2149). Analysis of Cumulative Cancer Risk Associated with Disinfection Byproducts in United States Drinking Water; source of the 2014 to 2017 national occurrence figures, used as dated national context only. All three authors are Environmental Working Group staff
Haloacetic acid formation moves opposite to trihalomethane formation with pH: Health Canada records that while trihalomethane formation increases with increasing pH, haloacetic acid formation decreases, with hydrolysis likely a significant factor.Health Canada. Guidelines for Canadian Drinking Water Quality, Guideline Technical Document: Haloacetic Acids (maximum acceptable concentration 80 µg/L; formation chemistry; pH direction; treatment)
Boiling is not a reliable control and can move one member the wrong way. California's review of three primary studies found boiling decreased trichloroacetic acid and increased dichloroacetic acid, and that the effect on dichloroacetic acid was specific to the disinfection method, suggesting interaction with the residual disinfectant.CA OEHHA. Public Health Goals for Haloacetic Acids in Drinking Water, December 2022 (per-compound goals; the declined group goal; the shower-inhalation and boiling findings)

Haloacetic acids (HAA5): what do we grade it against?

Haloacetic acids (HAA5) is held to two different numbers, and they do not agree. The health-based level for HAA5, the regulated sum (µg/L) is none set (no US group health goal; California declined to set one), from CA OEHHA Public Health Goals for haloacetic acids, 2022. The enforceable legal limit is 60. The table below lists every level on this page next to the source that set it.

Where the health research draws the line, versus the legal limit.
Contaminant	Health-based level	Legal limit	Source (health-based)
HAA5, the regulated sum (µg/L)	none set no US group health goal; California declined to set one	60 EPA MCL (40 CFR 141.64(b)(2)), sum of five, judged on an annual average at each sampling point	CA OEHHA Public Health Goals for haloacetic acids, 2022
Dichloroacetic acid (µg/L)	0.2 California health goal; EPA's federal goal is zero	60 no limit of its own; it counts toward the 60 sum	CA OEHHA Public Health Goals for haloacetic acids, 2022
Trichloroacetic acid (µg/L)	0.1 California health goal; EPA's federal goal is 20	60 no limit of its own; it counts toward the 60 sum	CA OEHHA Public Health Goals for haloacetic acids, 2022
Dibromoacetic acid (µg/L)	0.03 California health goal; no federal goal exists	60 no limit of its own; it counts toward the 60 sum	CA OEHHA Public Health Goals for haloacetic acids, 2022
Monochloroacetic acid (µg/L)	53 California health goal; EPA's federal goal is 70	60 no limit of its own; it counts toward the 60 sum	CA OEHHA Public Health Goals for haloacetic acids, 2022
Monobromoacetic acid (µg/L)	25 California health goal; no federal goal exists	60 no limit of its own; it counts toward the 60 sum	CA OEHHA Public Health Goals for haloacetic acids, 2022

Your report cannot be graded against the health column on this one. Utilities publish HAA5 as a single sum, and no US agency sets a health line for that sum. California declined to set one on purpose, because the five acids differ in potency and in the harm they cause. The per-compound goals above are real and they are what the sum is hiding, but your utility reports one combined number rather than a breakdown, so we can never tell you how much of your reading is which acid. Your number is compared to the 60 µg/L legal limit, and the distance between that limit and the per-compound goals in the table is the thing to understand.

What helps against Haloacetic acids (HAA5)?

4 approaches below reduce Haloacetic acids (HAA5) in a home, ordered from the most direct fix to the smallest change in habit. The most direct is Reverse osmosis at the kitchen tap. Each entry also names what it cannot do, because no single fix covers every exposure route.

Direct fixes

Reverse osmosis at the kitchen tap
Reverse osmosis rejects charged molecules efficiently, and haloacetic acids are charged, so on mechanism it is the better-founded of the two household options here.
In our 2026-08-11 check of the certified-product listings we found no reduction claim for HAA5 or for any of the five regulated acids, and we did not identify a primary household performance study in the sources we reviewed. So the verb is may reduce, not removes, and the only way to know is a paired test before and after the unit.
Point-of-use carbon block at the kitchen tap
Activated carbon is EPA's named best available technology for haloacetic acids at utility scale, so carbon does adsorb them given enough contact time.
The phrase doing the work there is enough contact time. These are hydrophilic charged molecules with much weaker affinity for carbon than the neutral chloroform that the carbon-block standard is built around, so a cartridge sized and certified for trihalomethanes should not be assumed to do the same job here.

Free and behavioral

Asking your utility about pH and precursor removal
The real control on this contaminant sits at the plant, not the house. Removing the organic matter before chlorination is the fix EPA names, and haloacetic acid formation falls as pH rises.
Asking is the part you control; the pH dial is the utility's. It also trades against trihalomethanes, which rise as pH rises, so it is not a free win, and Health Canada's rule holds: nothing done to control haloacetic acids may compromise disinfection.
Boiling
Boiling is not a control here and can push one member the wrong way. This is the sharpest divergence from the trihalomethane advice, where five minutes of boiling drives off most of the chloroform.
Haloacetic acids are not volatile, so boiling does not drive them off, and as water evaporates it can leave what remains more concentrated. In chlorinated water dichloroacetic acid can rise while trichloroacetic acid falls, and which way it goes depends on the disinfectant, the residual, and the heating time.

Two things we will not sell you on this one. A shower filter does nothing for haloacetic acids, because unlike trihalomethanes they barely reach you through steam or skin; from tap water, drinking is the route that counts. Whole-house carbon is a different case. It would treat the water arriving at your kitchen tap, but nobody has shown that household carbon removes these acids, so the objection is twofold: it may not do the job, and you would be paying to treat every shower, toilet and hose bib for a contaminant that reaches you essentially through the glass. If a whole-house system is right for your home, let the reason be trihalomethanes, chlorine and how the water feels, and take any haloacetic acid benefit as a bonus rather than the justification. And we will not put a removal percentage on a filter here, because when we checked the certified-product listings on 2026-08-11 we found no reduction claim for HAA5 or for any of the five regulated acids, and we did not identify a household performance study in the sources we reviewed. We would rather tell you the mechanism and where it stops than invent a number. What is worth doing, if this is your finding: read your number against 60, ask your utility which disinfectant it uses and what its running annual average was, and if you want it lower, put reverse osmosis on the kitchen tap and test before and after. Do not buy a shower filter for this.

Common questions about Haloacetic acids (HAA5)

What are haloacetic acids in drinking water?

Haloacetic acids are disinfection byproducts, formed when the chlorine used to make drinking water safe reacts with natural organic matter from the source. Five of them are regulated together as HAA5: monochloroacetic, dichloroacetic, trichloroacetic, monobromoacetic, and dibromoacetic acid. They are the second-largest byproduct family after trihalomethanes, and every home on a chlorinated public system carries some level.

What is a safe level of HAA5?

In the US there is no published safe level for the group, only a legal one. EPA caps the sum of the five at 60 µg/L. No US agency sets a health-based number for that sum, and California explicitly declined to, because the five acids differ in potency and in the harm they cause. What California did publish are goals for each acid separately, as low as 0.2 µg/L for dichloroacetic acid and 0.03 for dibromoacetic acid. Health Canada is worth reading because it shows its work. It calculated a health-based target of 10 µg/L for dichloroacetic acid but found that level cannot be reached in a distribution system without weakening disinfection. It set its limit at 80 on achievability instead, and recorded that the lifetime cancer risk at 80 is higher than the range it normally treats as negligible, while noting it calculates that risk conservatively. None of these group numbers is a safety number. Treat 60 as compliance, not as safety.

Are haloacetic acids worse than trihalomethanes?

Nobody can answer that for your household. The one modeling study that compares the two families across national US data, published in 2020 by authors on the staff of the Environmental Working Group, does not settle it. Its ranking flips with the assumptions: haloacetic acids come out ahead under its defaults and trihalomethanes ahead once childhood sensitivity is factored in, and the larger haloacetic burden in it comes mostly from the unregulated brominated members rather than the regulated five. What is solid is that these are different problems with different fixes: trihalomethanes reach you through the shower, and these do not.

Does a shower filter help with haloacetic acids?

No. This is the useful difference from trihalomethanes. Haloacetic acids sit in water as charged molecules with almost no vapor pressure, so they do not leave the water into shower air, and they cross skin poorly. In a shower-stall study summarized by California's health hazard assessment office, inhalation came to under 1% of the dose from drinking the same water. If someone sells you a shower filter for these, that is not what it does.

Does boiling water remove haloacetic acids?

No, and it can make one of them worse. Boiling works on trihalomethanes because chloroform is volatile and leaves as steam. Haloacetic acids are not volatile, so boiling leaves them behind and can concentrate what is left. Research summarized by California found boiling lowered trichloroacetic acid and raised dichloroacetic acid, the member with a federal health goal of zero, with the direction depending on which disinfectant the utility uses.

Is there a filter certified to remove haloacetic acids?

Not for the regulated five. When we checked the certified-product listings on 2026-08-11 we found no reduction claim for HAA5 as a group or for any of the five regulated members. One unregulated relative, tribromoacetic acid, does appear, so the statement is narrow, not absolute. A filter can still be a reasonable choice on mechanism, and reverse osmosis is the better-founded one, but nobody can show you a certification for it and we will not imply one exists. Those listings are a live database, so the check is a snapshot of that date.

Why do my haloacetic acids and trihalomethanes not track each other?

Usually they do, on a typical low-bromide system with steady treatment. They come apart when the source water carries bromide or the plant changes how it operates, and most of all with pH. Trihalomethane formation rises as pH rises; haloacetic acid formation falls. A utility raising pH to protect against lead is pushing the two numbers in opposite directions at once.