What is Monochloroacetic acid* and Why Does It Matter?

Monochloroacetic Acid: What It Is and Why It Matters

Monochloroacetic acid (MCA) is a disinfection byproduct — a chemical that forms when chlorine used to treat drinking water reacts with naturally occurring organic matter. Think of it this way: the same chlorine that kills harmful bacteria can also create unintended chemical compounds. MCA belongs to a family of byproducts called haloacetic acids. Water systems across the country have detected it in tap water, and right now, 1,336 public water systems report measurable levels of MCA in their supply.

The average detected level sits at 0.685 parts per billion (ppb), which sounds small. But the maximum recorded level reaches 11.6 ppb — more than 16 times that average. MCA forms most readily in water with high levels of organic material, like water drawn from rivers or lakes surrounded by farmland or decaying vegetation. Systems that rely heavily on surface water and use chlorine-based disinfection tend to see higher concentrations. Seasonal changes matter too — warm months often bring more organic matter into source water, which can push MCA levels higher.

Health concerns around MCA center on long-term, repeated exposure rather than a single glass of water. Animal studies have linked MCA exposure to liver and kidney damage, and some research points to potential reproductive effects at higher doses. The International Agency for Research on Cancer (IARC) classifies related haloacetic acids as possible human carcinogens. The EPA sets a maximum contaminant level (MCL) for the total haloacetic acid group — which includes MCA — at 60 ppb. None of the 1,336 systems reporting MCA currently exceed that legal limit. However, the Environmental Working Group (EWG) argues that the EPA's standard is outdated and doesn't fully reflect the latest science on cancer risk. EWG recommends a much stricter health guideline for this class of compounds, and the gap between legal compliance and health-protective levels is worth understanding.

Geographically, Texas leads the country with 157 systems reporting MCA, followed by Illinois (126), North Carolina (98), New York (94), and California (74). That pattern isn't random. Texas and Illinois both rely heavily on surface water sources — rivers and reservoirs that carry high organic loads, especially in agricultural regions where runoff adds nutrients and organic compounds to source water. North Carolina's mix of river-fed municipal systems and warm temperatures creates similar conditions. These states also tend to have large, aging water infrastructure, which can complicate treatment. If you live in any of these states, your water is more likely to contain detectable MCA than water from states that rely primarily on groundwater wells, which naturally contain less organic matter.

The good news is that effective filtration options exist. Activated carbon filters — the kind found in many under-sink and countertop systems — can reduce haloacetic acids including MCA. Reverse osmosis (RO) systems go further, removing up to 99% of a wide range of contaminants, including disinfection byproducts. An Echo Water reverse osmosis system is one of the most reliable ways to reduce MCA and similar compounds at the tap where your family actually drinks. If you're not ready for a full RO system, a high-quality activated carbon block filter is a meaningful step up from no filtration at all. Either way, start by pulling your local Consumer Confidence Report — your water utility is required to publish it annually — to see exactly what's been detected in your specific supply. Knowing your numbers is always the first move.

Regulatory Standards for Monochloroacetic acid*

Cities With the Highest Monochloroacetic acid* Levels

States Most Affected by Monochloroacetic acid*

Frequently Asked Questions

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