Chlorate in Drinking Water: Complete Guide (2026)

What is Chlorate and Why Does It Matter?

CHLORATE: WHAT YOU NEED TO KNOW ABOUT THIS WIDESPREAD WATER CONTAMINANT

What Is Chlorate and How Does It Get Into Your Water?

Chlorate (ClO3-) is a chemical compound that enters drinking water primarily through water treatment itself. When water utilities use chlorine dioxide to disinfect water and remove harmful bacteria, chlorate forms as a byproduct. It can also come from industrial sources, bleach manufacturing facilities, and agricultural runoff. Once in the water supply, chlorate persists because it's highly soluble—it doesn't settle out or evaporate easily. This means once it's there, it stays there until removed by specific filtration methods.

The scale of chlorate contamination is substantial. Our analysis found chlorate in 5,585 water systems across the United States, with 1,412 systems (25%) exceeding health guidelines. The average concentration detected is 160.7 ppb (parts per billion), though some systems measured as high as 2,530 ppb—more than 15 times higher than safe levels.

Health Effects: What the Research Shows

Chlorate exposure primarily affects the thyroid gland. The thyroid regulates metabolism and is especially important for children's development. According to the Environmental Working Group (EWG), chlorate can interfere with iodine uptake in the thyroid, potentially disrupting thyroid function even at relatively low exposure levels.

The EPA currently allows up to 1,000 ppb of chlorate in drinking water. However, the EWG health guideline is significantly stricter at 100 ppb—a tenfold difference. This gap exists because the EPA's standard was set decades ago and hasn't been updated to reflect newer research on thyroid effects. At levels above the EWG guideline, regular exposure poses a measurable risk to thyroid health, particularly for pregnant women, infants, and young children whose thyroid development is still ongoing.

Most people exposed to chlorate at typical levels won't experience immediate symptoms. The concern is long-term exposure: repeated consumption of chlorate-contaminated water over months or years may subtly affect thyroid function. Families with young children or pregnant members should take this contaminant seriously.

The Regulatory Gap

This is where things get complicated. The EPA's 1,000 ppb standard is the legal limit, but it's outdated. The EWG's 100 ppb recommendation reflects current science better. Of the 1,412 water systems exceeding guidelines in our analysis, many are technically "compliant" with EPA rules but exceed the more protective EWG standard.

Your water utility must tell you if chlorate exceeds the EPA limit—that's the law. But they're not required to report levels between 100 ppb and 1,000 ppb. This means you could be consuming chlorate above what independent health experts consider safe, and you might not know it. Request your water quality report from your local utility, or check it online through your utility's website.

Geographic Hotspots: Where Chlorate Is Most Common

Chlorate contamination concentrates in five states: Texas (548 affected systems), California (524), North Carolina (320), Florida (286), and New York (242). These states don't share a single geographic feature—they're spread across different regions and climates. The common thread is that they all use chlorine dioxide extensively for water treatment, particularly in larger municipal systems.

Texas and California's high numbers reflect their large populations and numerous water systems. However, the concentration is also higher in states with older water infrastructure where chlorate byproducts have accumulated over decades of treatment.

How to Remove Chlorate From Your Water

Standard carbon filters won't remove chlorate effectively. You need either reverse osmosis or ion exchange systems. Reverse osmosis forces water through a membrane so fine that chlorate molecules can't pass through, removing up to 99% of chlorate along with other contaminants. Ion exchange systems use specially treated resin to swap chlorate ions for harmless ones.

If your water tests above 100 ppb for chlorate, a point-of-use reverse osmosis system under your sink provides reliable protection for drinking and cooking water. For whole-home protection, a reverse osmosis system combined with ion exchange offers the most thorough solution. Test your water first—many utilities provide free testing, or you can use a certified lab—so you know exactly what you're dealing with.

Regulatory Standards for Chlorate

Cities With the Highest Chlorate Levels

States Most Affected by Chlorate

Frequently Asked Questions

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