EPA legal limit
No federal limit
Maximum Contaminant Level
EWG health guideline
210 ppb
Science-based, stricter target
Health effects
Chlorate inhibits thyroid iodide uptake — the same mechanism as perchlorate. At high levels it disrupts thyroid hormone production. The primary health concern is in populations with already marginal iodine intake (pregnant women, infants) and in those with pre-existing thyroid conditions. Long-term effects at levels typically found in drinking water are still being characterised.
Where it comes from
Forms when sodium hypochlorite (bleach) used for water disinfection degrades over time. Concentrated hypochlorite solutions that are older or stored at higher temperatures produce more chlorate. Also a byproduct when chlorine dioxide is used as a disinfectant — chlorine dioxide is used by some utilities specifically to avoid TTHM and HAA formation.
How it's regulated
No federal EPA MCL as of 2024 — chlorate was a contaminant candidate for regulation but has not yet been regulated. EWG health guideline: 210 ppb, based on thyroid effects. Monitored nationally under UCMR3 (2013–2015) and appears at high detection rates.
How to filter chlorate
Not all filters address chlorate. Look for independently certified filters—NSF International certification means the removal claim has been independently verified.
Frequently asked questions
What is chlorate?
Chlorate is a disinfection byproduct that forms from the degradation of hypochlorite (liquid bleach) used in water treatment and from the use of chlorine dioxide as a disinfectant. It is one of the most widely detected unregulated contaminants in US drinking water, found in virtually all chlorinated systems.
What are the health effects of chlorate?
Chlorate inhibits thyroid iodide uptake — the same mechanism as perchlorate. At high levels it disrupts thyroid hormone production. The primary health concern is in populations with already marginal iodine intake (pregnant women, infants) and in those with pre-existing thyroid conditions. Long-term effects at levels typically found in drinking water are still being characterised.
Is chlorate regulated in drinking water?
No federal EPA MCL as of 2024 — chlorate was a contaminant candidate for regulation but has not yet been regulated. EWG health guideline: 210 ppb, based on thyroid effects. Monitored nationally under UCMR3 (2013–2015) and appears at high detection rates.
Where does chlorate come from?
Forms when sodium hypochlorite (bleach) used for water disinfection degrades over time. Concentrated hypochlorite solutions that are older or stored at higher temperatures produce more chlorate. Also a byproduct when chlorine dioxide is used as a disinfectant — chlorine dioxide is used by some utilities specifically to avoid TTHM and HAA formation.
How do I remove chlorate from tap water?
The most effective methods for removing chlorate are: Reverse osmosis (RO), NSF 58 certified RO system, Ion exchange resin. Look for NSF-certified systems—independent certification confirms removal claims have been verified.
Is chlorate in your water?
Enter your ZIP code to see the measured level in your specific utility.