HAA5 (Haloacetic Acids) in Drinking Water

What Is HAA5?

HAA5 refers to the five haloacetic acids regulated together under a single EPA maximum contaminant level: monochloroacetic acid (MCAA), dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), monobromoacetic acid (MBAA), and dibromoacetic acid (DBAA). They are disinfection byproducts (DBPs) — compounds that form unintentionally when chlorine-based disinfectants react with naturally occurring organic matter in source water. Utilities that use chloramine instead of free chlorine produce less HAA5 but generate different DBP species like NDMA.

The presence of HAA5 in drinking water is a direct consequence of the disinfection process that makes tap water safe to drink. Chlorination kills pathogens — bacteria, viruses, protozoa — that historically caused mass disease outbreaks. But when chlorine meets humic acids, fulvic acids, and other organic compounds leached from decaying vegetation and soil, it produces a family of byproducts including HAA5 and total trihalomethanes (TTHMs). Managing DBPs means balancing two real risks: the immediate danger of microbial contamination against the long-term health effects of chronic low-level chemical exposure.

HAA5 are colorless, odorless, and tasteless. They cannot be detected without laboratory testing.

How HAA5 Gets Into Drinking Water

The Chemistry of Chlorination

When chlorine is added to water during treatment, it reacts with natural organic matter (NOM) — dissolved plant and soil-derived compounds present in virtually all surface water sources. The reaction produces dozens of DBP species, of which HAA5 and TTHMs are the most abundant and the ones EPA currently regulates.

The amount of HAA5 that forms depends on several interacting factors: the concentration of organic precursors in the source water, the dose of chlorine applied, the contact time between chlorine and water, water temperature, and pH. Warm water with high organic content and long distribution system residence times produces the highest HAA5 concentrations. Surface water systems — rivers, lakes, reservoirs — generally have higher NOM loads than groundwater systems and therefore generate more HAA5 during treatment.

Seasonal and Geographic Variation

HAA5 levels fluctuate seasonally. Concentrations typically peak in summer and early fall when water temperatures are highest, organic matter from runoff and algae is elevated, and longer distribution system residence times allow more reaction time. Agricultural regions see HAA5 spikes following planting and harvest seasons when runoff carries organic material into source water bodies.

Systems serving communities in the Midwest, Southeast, and mid-Atlantic states — where surface water sources are common and organic content is high — tend to report higher HAA5 levels. Drought conditions concentrate organic matter in reservoirs, driving DBP formation higher even as utilities try to maintain adequate disinfection.

Distribution System Formation

A portion of HAA5 formation occurs not at the treatment plant but within the distribution system itself. Residual chlorine reacts with organic matter throughout the pipe network. This means that water at the far end of a distribution system — after sitting in pipes for hours or days — can contain measurably higher HAA5 concentrations than water leaving the plant.

Health Effects

Cancer Risk

The EPA classifies DCAA as a probable human carcinogen (Group B2) and TCAA as a possible human carcinogen (Group C) based on animal studies. The International Agency for Research on Cancer (IARC) has assessed individual HAA species as potential carcinogens at elevated concentrations. Long-term exposure to HAA5 above the MCL is associated with increased risk of bladder cancer and colorectal cancer — the same cancer endpoints associated with TTHMs.

The cancer risk estimates for HAA5 at the MCL of 60 µg/L are relatively low for any individual, but because DBP exposure is population-wide and continuous over a lifetime, the aggregate public health burden is meaningful. EPA’s 1998 Stage 1 DBP Rule was explicitly designed around reducing lifetime cancer risk from DBP exposure.

Reproductive and Developmental Effects

Animal studies have associated high-dose exposure to individual HAA species — particularly DCAA and TCAA — with adverse reproductive outcomes including reduced sperm motility, developmental toxicity, and fetal abnormalities. Epidemiological studies on human populations have produced mixed results, with some showing associations between DBP exposure during pregnancy and adverse birth outcomes including low birth weight and neural tube defects. The evidence is not conclusive but has been sufficient for EPA to maintain MCLs that account for reproductive risk.

Who Is Most at Risk

Infants fed formula made with tap water receive proportionally higher DBP doses relative to body weight than adults. Pregnant women are advised by some health organizations to use filtered water if their utility’s HAA5 or TTHM levels are consistently elevated. People who shower in heavily chlorinated water absorb DBPs through skin and inhalation — a route of exposure not reflected in the drinking water MCL.

EPA Regulation and Limits

EPA regulates HAA5 under the Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules. The current standards:

Standard	Value	Notes
HAA5 MCL	60 µg/L	Running annual average at each monitoring location
HAA5 MCLG	n/a (mixture)	No MCLG for the mixture; individual MCLGs vary
TTHM MCL	80 µg/L	Regulated alongside HAA5
DCAA MCLG	0 µg/L	Probable carcinogen
TCAA MCLG	0.02 mg/L	Possible carcinogen

The Stage 2 DBP Rule (2006) tightened compliance requirements by requiring utilities to monitor at the locations in their distribution system with the highest DBP concentrations — the locational running annual average (LRAA) — rather than averaging across all sites. This change exposed higher-exposure locations that had previously been masked by low-DBP sampling points.

Systems with more than 10,000 people must monitor quarterly. Smaller systems monitor less frequently. Violations trigger public notification requirements and, if persistent, require corrective action plans.

How Widespread Is HAA5?

EPA’s Unregulated Contaminant Monitoring Rule 4 (UCMR 4), conducted 2018–2020, required approximately 4,900 public water systems to monitor for HAA5 and related compounds. The results showed HAA5 detections across every region of the country, with higher average concentrations in systems using surface water with elevated organic content.

Surface water systems consistently report higher HAA5 levels than groundwater systems. Large systems serving dense urban populations face the compounding challenge of long distribution networks where water ages and residual chlorine continues reacting with organic matter. Systems that rely on chloramine disinfection rather than free chlorine tend to produce less HAA5, though they generate different DBP species.

Violations of the 60 µg/L MCL occur primarily in smaller systems with limited treatment capacity and in systems that have not implemented optimized disinfection practices — see the Elm City, NC TTHM violations for a recent example of how prolonged DBP exceedances unfold.

How WaterVerge Tracks HAA5

WaterVerge displays HAA5 data from the EPA UCMR 4 monitoring program (2018–2020). The data is stored in the ucmr_results table and aggregated to the city level: we report the average HAA5 concentration across all monitoring results for each public water system, and flag any city where that average exceeds the 60 µg/L MCL.

Not every city has UCMR 4 data — the program covered approximately 4,900 systems, prioritizing larger utilities. Cities without UCMR 4 data are noted as untested in our database. HAA5 data is distinct from the lead and copper data (sourced from EPA’s Lead and Copper Rule monitoring) and the PFAS data (sourced from UCMR 5, 2023–2025).

For current compliance status, check your utility’s most recent Consumer Confidence Report (CCR), which must disclose HAA5 and TTHM levels annually.

How to Remove HAA5

HAA5 can be effectively reduced with the right treatment approach. Standard pitcher filters with basic activated carbon provide limited removal — choose filters specifically certified for DBP reduction.

Method	Removal Rate	Certification	Best For
Reverse osmosis (RO)	90–99%	NSF/ANSI 58	Comprehensive DBP removal at tap
Activated carbon block	50–90%	NSF/ANSI 53	Under-sink, countertop systems
Granular activated carbon (GAC)	30–70%	NSF/ANSI 53	Whole-house pre-filtration
Aeration / letting water stand	10–50%	n/a	Volatile species only (partial)
Standard pitcher filter	Varies widely	Verify NSF 53	Check certification before buying

Activated carbon works by adsorption — HAA5 molecules bond to the carbon surface as water passes through. Block carbon is more effective than granular carbon because it forces water into closer contact with the carbon matrix and has no channeling. Replace carbon filters on schedule; a saturated filter releases trapped compounds back into the water.

Reverse osmosis is the most reliable option for households in areas with consistently elevated HAA5. RO systems remove HAA5 along with hundreds of other contaminants including lead, nitrate, PFAS, and arsenic. The tradeoff is water waste (2–4 gallons rejected per gallon produced) and slower flow rate. For households that want effective DBP filtration without RO, see the best under-sink water filters certified to NSF/ANSI 53.

Aeration — running water through a faucet aerator or letting it stand in an open pitcher — causes some volatile HAA species to off-gas. This is a partial measure and does not substitute for certified filtration.

Check Your City

HAA5 levels vary significantly by utility, season, and distribution system. Use WaterVerge to search your city, review available UCMR 4 data, and see whether your system has reported HAA5 exceedances. If your utility’s levels are consistently elevated, consider an NSF 53-certified activated carbon filter or a reverse osmosis system for drinking and cooking water.

Frequently Asked Questions

Is HAA5 in tap water dangerous?

HAA5 are associated with increased cancer risk and reproductive effects at chronic exposure levels above the EPA’s 60 µg/L MCL. Below the MCL, the risk to any individual is low, but long-term cumulative exposure is a legitimate concern — particularly for infants, pregnant women, and people whose tap water is consistently in the upper range of the limit.

What causes high HAA5 levels?

HAA5 form when chlorine used to disinfect water reacts with naturally occurring organic matter. Surface water systems with high organic content, warm temperatures, and long distribution networks produce the most HAA5. Seasonal spikes occur in summer when organic loads are highest.

How do I know if my water has high HAA5?

Your utility’s annual Consumer Confidence Report (CCR) must disclose HAA5 and TTHM levels. WaterVerge also displays UCMR 4 data for cities where monitoring was conducted. You can also request detailed water quality results directly from your utility or test your tap water through a certified laboratory.

Does boiling water remove HAA5?

No. Boiling does not remove HAA5 and may actually increase concentrations by reducing water volume through evaporation. Use activated carbon filtration or reverse osmosis for effective HAA5 reduction.

Which filter best removes HAA5?

Reverse osmosis systems provide the highest removal rates (90–99%) and are certified under NSF/ANSI 58. Activated carbon block filters certified under NSF/ANSI 53 are effective for point-of-use reduction. Verify that any filter you purchase carries a current NSF certification specifically listing HAA5 or DBP reduction.