What is Haloacetic acids (HAA5)* and Why Does It Matter?

Haloacetic Acids (HAA5): A Common Byproduct Hiding in Your Tap Water

When water treatment plants add chlorine to kill bacteria and viruses, a chemical reaction happens. Chlorine meets naturally occurring organic matter — things like decaying leaves, algae, and soil — and forms a group of chemicals called haloacetic acids, or HAA5. These aren't contaminants that spill into water from a factory. They're created during the treatment process itself. The five acids in this group include dichloroacetic acid and trichloroacetic acid, among others. In other words, the very process designed to make your water safe can introduce a new problem.

The health concerns around HAA5 are well-documented. The Environmental Working Group (EWG) classifies these chemicals as probable human carcinogens, meaning long-term exposure may increase cancer risk. Studies have also linked HAA5 exposure to reproductive problems, including an increased risk of miscarriage and low birth weight (EWG). These risks grow with cumulative exposure — meaning the longer you drink water with elevated HAA5 levels, the greater the concern. Children and pregnant women face the highest vulnerability.

Here's where the numbers get important. The EPA's legal limit for HAA5 is 60 parts per billion (ppb). The EWG's health guideline, based on current research, is just 0.1 ppb — 600 times stricter. Of the 545 water systems in the U.S. that tested positive for HAA5, 540 of them exceeded the EWG health guideline. The average level detected was 13.342 ppb. One system recorded a peak of 68.5 ppb — above even the EPA's legal limit. A water system can be fully compliant with federal law and still deliver water that the EWG considers a health risk. That gap matters.

Geographically, HAA5 shows up across the country, but some states carry a heavier burden. New York leads with 59 affected water systems, followed by Illinois (44), Massachusetts (41), Colorado (38), and Iowa (33). These states share a few common factors. Older water infrastructure means longer travel time through pipes, which gives HAA5 more time to form and accumulate. Areas with high organic matter in source water — like rivers near agricultural land or heavily forested watersheds — tend to produce more of these byproducts. Seasonal changes also play a role. Warmer months bring algae blooms and higher organic content in reservoirs, which drives HAA5 levels up in summer and early fall.

The good news is that HAA5 is one of the more manageable contaminants to address at home. Activated carbon filters can reduce HAA5 levels, but the most effective solution is a reverse osmosis (RO) system. Reverse osmosis forces water through a semi-permeable membrane that blocks contaminants at the molecular level, removing up to 99% of HAA5. An Echo Water reverse osmosis system installed under your kitchen sink gives you filtered water on demand — without relying on your utility to close the gap between the EPA limit and the EWG guideline. If you're a renter or prefer a simpler setup, a high-quality pitcher or countertop filter with activated carbon can still provide meaningful reduction. Check that any filter you choose is NSF/ANSI certified for HAA5 specifically — not all filters are rated for this contaminant. Whatever you choose, acting on your water quality is straightforward. You don't need to wait for regulations to catch up.

Regulatory Standards for Haloacetic acids (HAA5)*

Cities With the Highest Haloacetic acids (HAA5)* Levels

States Most Affected by Haloacetic acids (HAA5)*

Frequently Asked Questions

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