Chlorate in Drinking Water: Thyroid Risk, EPA Levels & Removal

What Is Chlorate?

Chlorate (ClO3⁻) is an inorganic anion that forms as a byproduct when chlorine-based disinfectants are used to treat drinking water. Unlike most contaminants, chlorate is not naturally present in source water in significant amounts — almost all chlorate in tap water is created during the disinfection process itself. It belongs to the same family of disinfection byproducts as HAA5 and trihalomethanes, though its formation chemistry is distinct.

Chlorate forms most readily when:

• Chlorine dioxide (ClO2) is used for taste-and-odor control or pre-oxidation
• Sodium hypochlorite (bleach) solutions are stored for extended periods or at high concentrations, where they slowly decompose into chlorate
• Electrolytic on-site hypochlorite generators produce small amounts of chlorate as a side reaction

The EPA included chlorate in the Unregulated Contaminant Monitoring Rule 3 (UCMR 3, 2013–2015). Of all UCMR 3 analytes, chlorate had one of the highest detection rates — present in roughly 70–80% of large public water systems sampled, with about 12% exceeding the EPA lifetime health advisory.

EPA Health Advisory and Limits

There is no federal maximum contaminant level (MCL) for chlorate. The EPA has set two health advisories:

• Lifetime health advisory: 210 µg/L — based on chronic exposure for adults
• One-day health advisory: 1,500 µg/L — for short-term exposure

WaterVerge uses the 210 µg/L lifetime value to flag elevated chlorate on city pages, since this is the most health-protective benchmark for ongoing tap-water exposure.

Several states have adopted their own action levels, with California’s Notification Level of 800 µg/L being one of the more widely referenced.

Health Effects

The primary health concern with chlorate is thyroid disruption. Chlorate competes with iodide for uptake by the sodium-iodide symporter — the protein that allows the thyroid gland to absorb iodine for hormone production. This is the same mechanism that drives concern over perchlorate, which the EPA is now moving to regulate federally. At elevated chronic doses, this can:

• Reduce thyroid hormone production in iodine-sufficient individuals, with possibly larger effects in iodine-deficient populations
• Affect fetal development — pregnant women, infants, and young children are most sensitive because thyroid hormones drive neurological development
• Worsen existing hypothyroidism in adults already managing thyroid conditions

Additional concerns include possible methemoglobinemia (impaired oxygen transport in blood) at very high acute doses, though this is mainly a concern for short-term exposure spikes rather than typical drinking-water levels.

The EPA classified chlorate as a contaminant of concern under the Contaminant Candidate List but has not moved to formal regulation. The World Health Organization’s drinking-water guideline for chlorate is 700 µg/L, intentionally less stringent than the EPA lifetime advisory due to differences in disinfection practices internationally.

How to Reduce Chlorate Exposure

Chlorate is challenging to remove with common household filters:

• Reverse osmosis (RO) — typically reduces chlorate by 80–95%. The most reliable household option. See our guide to the best reverse osmosis systems.
• Anion exchange — works but requires regular regeneration and proper resin selection.
• Distillation — effective but energy-intensive.

What does NOT work:

• Standard activated carbon filters (pitchers, faucet-mount, fridge filters) provide minimal chlorate reduction.
• Boiling does not remove chlorate and can concentrate it slightly as water evaporates.
• Leaving water to sit (effective for some volatile DBPs) does not reduce chlorate, which is non-volatile.

If your utility uses chlorine dioxide and your area has elevated chlorate readings, RO is the most practical solution. Note that chlorate levels can vary seasonally — summer levels are often higher because warmer storage temperatures accelerate hypochlorite decomposition.

What Public Water Systems Are Doing

Many utilities have responded to chlorate concerns by:

• Switching from concentrated bulk hypochlorite to on-site generation
• Reducing hypochlorite storage time
• Storing hypochlorite at lower concentrations and cooler temperatures
• Optimizing chlorine dioxide dosing to minimize ClO2 → ClO3 conversion

These operational changes can substantially reduce finished-water chlorate without compromising disinfection.

Frequently Asked Questions

Is chlorate the same as chloride or chlorine? No. Chloride (Cl⁻) is the harmless ion in table salt. Chlorine is the gas/liquid disinfectant. Chlorate (ClO3⁻) is a specific oxidized byproduct that forms during disinfection.

Does my pitcher filter remove chlorate? Most carbon-based pitcher filters do not effectively remove chlorate. Check the manufacturer’s NSF certifications — chlorate is rarely listed.

Should I switch to bottled water? Most bottled waters are also disinfected and may contain chlorate. RO-treated tap water is generally a more reliable low-chlorate option.