Perchlorate
Perchlorate is a soluble groundwater contaminant from rocket fuel, munitions, and fireworks that blocks the thyroid's ability to take up iodide, the raw material it needs to make thyroid hormone. California's health goal is 1 ppb, or parts per billion, six times below its legal limit of 6 ppb, and there is still no federal limit in force. Reverse osmosis removes it, and carbon does not.
Science and Health line for perchlorate
1 ppb
California caps it at 6 ppb. The federal government has no limit in force.
Perchlorate: what is it, and where does it come from?
Perchlorate is a stable, highly soluble inorganic anion most associated with solid rocket propellant, munitions, explosives, road flares, and fireworks. Decades of aerospace and defense manufacturing left perchlorate plumes in groundwater across much of the West, with Southern California especially affected.
It also enters water naturally and through Chilean-nitrate fertilizer. Because it dissolves readily and does not bind to soil, it travels through aquifers and persists. It is a source-water signal, like nitrate: utilities drawing on contaminated groundwater carry it, while imported or treated surface water carries little.
It is not a disinfection byproduct or a plumbing problem. It arrives in the source water, and standard municipal treatment does not remove it.
Perchlorate: why does it matter?
Perchlorate's effect runs through the thyroid.
It competitively inhibits iodide uptake by the thyroid gland, blocking the transport that pulls iodide in. Iodide is the raw material the thyroid uses to make thyroid hormone, so less uptake means less hormone production.
The people most exposed to that risk are pregnant women, fetuses, and infants, because thyroid hormone is essential for fetal and infant brain development. EPA's risk modeling is built around pregnant women who could be pushed toward low thyroid hormone.
Where it shows up, perchlorate is usually present but under the legal limit, the same compliant-but-detectable pattern as nitrate. In several Southern California groundwater districts, for example, it has been reported in the 1 to 3 ppb range, below the 6 ppb legal limit but at or above the 1 ppb health goal.
Perchlorate inhibits iodide uptake by the thyroid, lowering thyroid hormone production. OEHHA lists its health basis as endocrine toxicity (thyroid) and developmental toxicity (neurodevelopmental deficits), and set the public health goal at 1 ppb.
California OEHHA, Perchlorate Public Health Goal (1 ppb), updated February 2015
There is no federal limit in force. EPA made a 2011 determination to regulate, withdrew it in 2020, was reversed by the D.C. Circuit in 2023 (NRDC v. Regan), and signed a proposed federal rule on January 2, 2026.
Standard activated carbon filters improve the taste and smell of water. Reverse osmosis removes dissolved inorganic contaminants from drinking water.
Perchlorate: what do we grade it against?
Our Science and Health line for perchlorate is 1 ppb, California's public health goal, set on its effects on the thyroid and on development. The table below sets every published level for perchlorate, including the legal limit, beside the source that set it.
Science and Health line
1 ppb
California's public health goal, set on its effects on the thyroid and on development
Perchlorate
Health-based level 1 ppb thyroid and neurodevelopmental basisLegal limit 6 ppb California MCL, since 2007Legal limit 6x the health line
The levels for perchlorate, as a table
| Contaminant | Health-based level | Legal limit | Source (health-based) |
|---|---|---|---|
| Perchlorate (ppb) | 1 thyroid and neurodevelopmental basis | 6 California MCL, since 2007 | CA OEHHA Public Health Goal, 2015 |
| Perchlorate, federal (ppb) | None set no federal health-based number in force | no federal MCL; proposed rule signed Jan 2026 none set | US EPA, Perchlorate in Drinking Water |
The Science and Health line is where we flag. We set it from the health research, and it is never looser than any legal limit. Legal limits are enforceable compromises that also weigh what treatment costs. Your report grades to the Science and Health line.
What helps against Perchlorate?
The approaches below are the ones people use for Perchlorate at home. Each entry says what it does and what it cannot do, and not every one helps.
Direct fixes
Point-of-use reverse osmosis (NSF/ANSI 58)
Rejects perchlorate at the membrane as it does other dissolved inorganic anions. It is the same under-sink unit already used for PFAS, chromium-6, and nitrate, so perchlorate is covered with no new product. Exposure is through drinking, so the kitchen tap is the right scope.
Bigger retrofits
Perchlorate-selective or strong-base anion exchange
The standard utility-scale technology for perchlorate, used at affected wellheads in California. It targets the perchlorate anion directly, the same resin class used for chromium-6 and nitrate.
Free and behavioral
Carbon pitcher or carbon block
Improves taste and odor only. Plain activated carbon does not adsorb the perchlorate anion at meaningful efficiency.
Carbon does not remove perchlorate. It is the wrong tool for dissolved anions, the same limit that applies to nitrate and chromium-6.
Boiling
Does not remove perchlorate and concentrates it as water boils off, the same trap as nitrate.
Never boil to address perchlorate. It makes the level worse.
Perchlorate is usually a present-but-compliant story, rarely a violation.
California sets a health goal of 1 ppb and a legal limit six times higher, and the federal government still has no limit in force, so a level that clears every rule can sit well above the health-based number. The useful part: if your water already justifies reverse osmosis for PFAS, chromium-6, or nitrate, that same kitchen-tap unit handles perchlorate too. The first move is reading your utility's annual water quality report, the Consumer Confidence Report, so you know whether it is there.
Common questions about Perchlorate
Is perchlorate in my tap water?
It depends entirely on your water system's source. Perchlorate shows up where a utility draws on groundwater touched by aerospace, defense, or fireworks legacy contamination, or by Chilean-nitrate fertilizer. Imported or treated surface water carries little. Your annual water quality report, or a direct water test, is the only way to know your level.
Is there a federal limit on perchlorate?
No federal limit is in force. EPA decided to regulate it in 2011, withdrew that in 2020, was reversed by a federal court in 2023, and signed a proposed rule on January 2, 2026. California is one of the few states with its own enforceable limit, 6 ppb, alongside a health goal of 1 ppb.
Does a carbon filter remove perchlorate?
No. Activated carbon, including carbon pitchers and carbon blocks, does not remove perchlorate. Carbon improves taste and odor but does not capture dissolved anions like perchlorate, nitrate, or chromium-6. Reverse osmosis or anion exchange is what removes it.
Does boiling water get rid of perchlorate?
No, and it makes things worse. Perchlorate stays in the water while some of the water boils off, so boiling concentrates it. The same is true for nitrate. Use reverse osmosis instead.
Why is perchlorate a concern for pregnant women and infants?
Perchlorate lowers the thyroid's ability to make hormone, and thyroid hormone is essential for fetal and infant brain development. Pregnant women, fetuses, and infants are the most sensitive group, which is why EPA's risk modeling centers on them.
Sources cited for Perchlorate
Every number on this Perchlorate page traces to one of the 3 sources below: 3 government or regulatory.