What is Hexachlorocyclopentadiene* and Why Does It Matter?

Hexachlorocyclopentadiene, often called "hex" or HCCP, is a yellow-green liquid chemical with a sharp, unpleasant odor. It doesn't occur naturally in the environment. Instead, it's a byproduct of manufacturing chlorinated compounds, including certain pesticides and flame retardants. HCCP can enter drinking water through industrial discharge, chemical plant runoff, or the breakdown of related compounds in soil and groundwater. It's persistent, meaning it doesn't break down quickly once it's in the environment.

The EPA has set a legal limit — called a Maximum Contaminant Level (MCL) — of 0.05 parts per billion (ppb) for HCCP in drinking water. Right now, none of the 100 water systems where HCCP was detected are exceeding that legal limit. The average detected level sits at 0.026 ppb, and the highest recorded amount was 0.145 ppb. That said, detection at any level is worth paying attention to. The EPA's legal limit was set decades ago and doesn't always reflect what modern toxicology considers safe for long-term, low-level exposure.

Even at low concentrations, HCCP raises legitimate health concerns. Animal studies show it can damage the kidneys and liver with repeated exposure (EPA). There's also evidence it may affect the thyroid and disrupt hormonal function. Children and pregnant women face the greatest risk from chronic low-level exposure because their bodies are still developing or supporting development. The EPA classifies HCCP as a possible human carcinogen, meaning there's enough concern to warrant caution, even if the science isn't fully settled (EPA Integrated Risk Information System). Drinking water at or near the detected average of 0.026 ppb over many years is not the same as a single high-dose exposure — the risk is cumulative and slow-moving.

Geographically, HCCP shows up most often in states with heavy chemical manufacturing histories. Arkansas leads with 30 affected water systems, followed by Louisiana with 19 and Indiana with 13. North Carolina and Colorado each have 8. Arkansas and Louisiana sit along the Mississippi River corridor, which has long been home to petrochemical and agricultural chemical production. Industrial facilities in these regions have historically discharged into waterways that feed municipal water supplies. Indiana's manufacturing belt tells a similar story. Colorado's presence on this list is less obvious but may reflect legacy contamination from older industrial sites or agricultural chemical use in specific watersheds.

The good news is that HCCP is highly removable with the right filtration. Activated carbon filtration — especially granular activated carbon (GAC) — is effective at reducing HCCP levels. Reverse osmosis (RO) systems go further, removing up to 99% of a wide range of chemical contaminants, including chlorinated compounds like HCCP. If you live in Arkansas, Louisiana, Indiana, or another state where HCCP has been detected, a point-of-use RO system installed at your kitchen tap is one of the most reliable ways to reduce your family's exposure. Echo Water's systems use multi-stage filtration that includes both activated carbon and reverse osmosis, specifically designed to handle the kinds of industrial chemical contamination common in these regions. Checking your annual Consumer Confidence Report — the water quality report your utility is required to send every year — is also a smart first step to see whether HCCP has been detected in your specific water supply.

Regulatory Standards for Hexachlorocyclopentadiene*

Cities With the Highest Hexachlorocyclopentadiene* Levels

States Most Affected by Hexachlorocyclopentadiene*

Frequently Asked Questions

Related Contaminant Guides