EPA legal limit
0.056 ppb
Maximum Contaminant Level
EWG health guideline
0.001 ppb
Science-based, stricter target
Why does California show high perchlorate?
Southern California has significant historical perchlorate contamination from decades of aerospace and rocket propellant manufacturing—facilities operated by Lockheed, Aerojet, and others discharged perchlorate into groundwater and surface water over several decades. The Colorado River, a primary source for much of Southern California, carries perchlorate from upstream contamination sites in Nevada and Utah.
The 0.056 ppb MCL is new and exceptionally strict. The EPA finalised this limit in 2024 (effective 2026), making it one of the most stringent drinking water standards ever set. Many utilities that were fully compliant under previous guidance now technically exceed this threshold. Federal regulations allow a multi-year compliance transition period—utilities showing alert status for perchlorate are not necessarily in violation; they may still be within the compliance window for treatment upgrades.
The health concern is real but targeted. Perchlorate interferes with thyroid iodide uptake, which matters most during fetal brain development and early infancy when thyroid hormones are critical. The highest-risk groups are pregnant women, infants, and people with pre-existing thyroid conditions or iodine-deficient diets. For healthy adults without thyroid issues, the risk at levels typically found in California utilities is low—but not zero at chronic exposures, which is why the EPA set the MCL where it did.
Health effects
Perchlorate interferes with thyroid hormone production by blocking iodide uptake. This disrupts thyroid function and is particularly concerning during pregnancy (maternal thyroid hormones are critical for fetal brain development) and in infants.
Where it comes from
Military and industrial sites where rocket propellant and explosives were manufactured or tested. Also from Chilean nitrate fertilisers used historically. The western US (particularly California, Nevada, and Utah) has the highest prevalence.
How it's regulated
EPA MCL: 0.056 ppb (effective 2026) — one of the most recently regulated contaminants. The EWG health guideline of 0.001 ppb is based on thyroid effects during pregnancy.
The EPA vs EWG gap
The legal limit (0.056 ppb) is 56× higher than the EWG health guideline (0.001 ppb). Water can be legally compliant while still exceeding the science-based threshold.
How to filter perchlorate
Not all filters address perchlorate. Look for independently certified filters—NSF International certification means the removal claim has been independently verified.
Frequently asked questions
What is perchlorate?
Perchlorate is a chemical used in rocket propellant, fireworks, flares, and some fertilisers. It has contaminated groundwater and surface water near military facilities, aerospace sites, and agricultural areas that used Chilean nitrate fertilisers historically.
What are the health effects of perchlorate?
Perchlorate interferes with thyroid hormone production by blocking iodide uptake. This disrupts thyroid function and is particularly concerning during pregnancy (maternal thyroid hormones are critical for fetal brain development) and in infants.
Is perchlorate regulated in drinking water?
EPA MCL: 0.056 ppb (effective 2026) — one of the most recently regulated contaminants. The EWG health guideline of 0.001 ppb is based on thyroid effects during pregnancy.
Where does perchlorate come from?
Military and industrial sites where rocket propellant and explosives were manufactured or tested. Also from Chilean nitrate fertilisers used historically. The western US (particularly California, Nevada, and Utah) has the highest prevalence.
How do I remove perchlorate from tap water?
The most effective methods for removing perchlorate 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 perchlorate in your water?
Enter your ZIP code to see the measured level in your specific utility.