What is Chlorate* and Why Does It Matter?

Chlorate is a chemical byproduct that forms when water utilities use chlorine-based disinfectants to kill bacteria and viruses. It also enters drinking water through the use of chlorine dioxide, a disinfectant increasingly popular with water treatment plants. Some chlorate comes from industrial sources, including the manufacturing of paper, textiles, and herbicides. Once it enters the water supply, it's colorless, odorless, and impossible to detect without laboratory testing.

Right now, chlorate shows up in 2,374 water systems across the United States. The average detected level is 157 parts per billion (ppb), but some systems have recorded levels as high as 1,244.5 ppb. That top number is roughly 8 times higher than the average. The Environmental Working Group (EWG) has set a health guideline of 210 ppb, and 579 of those systems — nearly 1 in 4 — are already delivering water above that threshold.

The health concern with chlorate centers on the thyroid gland. Chlorate interferes with the thyroid's ability to absorb iodine, which it needs to produce hormones that regulate metabolism, brain development, and growth. Children, pregnant women, and people with existing thyroid conditions face the greatest risk (EWG). At high exposure levels, research in animals has also linked chlorate to red blood cell damage, a condition called methemoglobinemia, which reduces the blood's ability to carry oxygen. The EPA has not yet set a legal limit for chlorate in drinking water, which means utilities aren't required to remove it or even report it as a violation when levels climb.

That regulatory gap is significant. The EPA has been studying chlorate for years but has not finalized a maximum contaminant level (MCL) — the legal ceiling for how much of a substance is allowed in tap water. Without an MCL, water utilities face no enforcement pressure to reduce chlorate levels, even when they exceed the EWG's health guideline. The EWG's guideline of 210 ppb is based on protecting the most vulnerable populations, particularly developing fetuses and infants. Until the EPA acts, monitoring and filtration remain the most reliable ways to protect your household.

Geographically, Texas leads the country with 315 affected systems, followed by North Carolina (206), Illinois (131), New York (114), and Iowa (107). Texas and North Carolina both have large agricultural sectors and heavily chlorinated municipal systems, two factors that drive chlorate formation. Illinois and Iowa sit in the Midwest's industrial corridor, where both municipal water treatment and industrial discharge contribute to contamination. New York's high number reflects the sheer size of its water infrastructure — more systems mean more opportunities for chlorate to accumulate. If you live in any of these states, checking your utility's annual water quality report (also called a Consumer Confidence Report) is a smart first step.

The good news is that chlorate is very treatable at the household level. Reverse osmosis (RO) filtration is the most effective method, removing up to 99 percent of chlorate from drinking water. Activated carbon filters alone are not sufficient — chlorate requires a membrane-based system to achieve meaningful reduction. Echo Water's reverse osmosis systems are designed specifically to handle disinfection byproducts like chlorate, along with dozens of other contaminants commonly found in tap water. If you're on a well or a smaller municipal system, testing your water first gives you a clear picture of what you're dealing with before investing in filtration. For families with young children or anyone managing a thyroid condition, addressing chlorate in your drinking water isn't something to put off.

Regulatory Standards for Chlorate*

Cities With the Highest Chlorate* Levels

States Most Affected by Chlorate*

Frequently Asked Questions

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