Perchlorate in Tap Water: Thyroid Risks and Where EPA's Rule Stands in 2026

Perchlorate is a chemical used in rocket fuel, fireworks, flares and explosives that also occurs naturally, especially in the arid Southwest. It can interfere with the thyroid gland’s use of iodide, which matters most during pregnancy because thyroid hormones are critical for a baby’s brain development. There is still no enforceable federal limit. After a 2023 court ruling forced the issue, EPA proposed one in January 2026, with a health goal of 20 µg/L and possible enforceable limits of 20, 40 or 80 µg/L. A final rule is due by May 21, 2027. Two states already have their own limits: Massachusetts at 2 µg/L and California at 6 µg/L.

What perchlorate is and where it comes from

EPA describes perchlorate as a chemical compound that occurs naturally and is also manufactured. It is used in:

Solid rocket propellants and munitions
Fireworks, signal flares and matches
Vehicle airbag initiators

It can also show up in some fertilizers and as a byproduct when hypochlorite solutions (a form of bleach used to disinfect drinking water) are stored or handled improperly. Natural perchlorate is found particularly in arid regions such as the southwestern United States.

EPA notes that once perchlorate reaches groundwater or surface water, it can persist for decades.

Drinking water is not the only source. EPA says people are exposed mainly through food and drinking water. In an FDA total diet study that EPA cites, 74% of the foods tested had at least one sample with detectable perchlorate.

How it affects health

Perchlorate’s main effect is on the thyroid. According to EPA, it reduces the thyroid’s uptake of iodide, which can lower production of thyroid hormones. Those hormones help regulate metabolism and are critical for development, including brain development.

EPA’s key concerns:

Pregnancy. Changes in thyroid hormone levels in pregnant women can harm brain development in their children. EPA identifies the most sensitive group as the developing babies of pregnant women who have low thyroid hormone (thyroxine) and low iodine intake, especially early in pregnancy.
Young children may also be at increased risk.
At any life stage, EPA lists possible effects including hypothyroidism, reproductive and developmental effects, and cardiovascular effects.

EPA’s advice for consumers: contact your water utility to learn your system’s perchlorate levels, and if you’re pregnant or planning a pregnancy and concerned, talk to your doctor.

The regulatory history: two decades of back and forth

Perchlorate is one of the most contested drinking water decisions EPA has made. The timeline, based on EPA’s records:

2006 and 2007: Massachusetts and California set their own drinking water standards, before any federal action.
December 2008: EPA issued an interim health advisory of 15 µg/L. It is non-enforceable guidance for states and local officials.
February 2011: EPA made a formal determination that it would regulate perchlorate under the Safe Drinking Water Act.
June 2019: EPA proposed a limit of 56 µg/L, with alternatives of 18 and 90 µg/L.
July 2020: EPA reversed course, withdrew its 2011 determination and decided not to regulate. It concluded a national rule would not present a meaningful opportunity to reduce health risk.
May 2023: In NRDC v. Regan, the U.S. Court of Appeals for the D.C. Circuit vacated the 2020 withdrawal. The court held that the Safe Drinking Water Act did not allow EPA to withdraw a determination to regulate, so EPA had to proceed with a rule.
January 2, 2026: Under a court-supervised consent decree, EPA signed a proposed regulation, published in the Federal Register on January 6, 2026.
February 19, 2026: EPA held a virtual public hearing. Public comments closed March 9, 2026.

What EPA proposed in 2026

The proposal includes:

A health goal (MCLG) of 20 µg/L (0.02 mg/L)
An enforceable limit (MCL) of 20, 40 or 80 µg/L. EPA co-proposed all three and asked for comment before choosing one.
Monitoring by community water systems and non-transient non-community systems (such as schools and workplaces with their own water supply)
Action and notice when a system exceeds the limit: treatment or a new source, notice to customers, and reporting to the state. EPA proposed the most urgent tier of public notice (Tier 1) for exceedances, because of the short-term risk to the most sensitive group.
A compliance date three years after the final rule is published

EPA estimates that about one tenth of one percent of regulated water systems would exceed the proposed limits. The proposal estimates roughly 100 systems may need new treatment at 20 µg/L, about 60 at 40 µg/L, and about 20 at 80 µg/L.

The proposal is unusual in one way. EPA stated in the notice that “the benefits of this regulation would not justify the costs,” and that it is issuing the rule because the D.C. Circuit’s decision requires it. EPA also wrote that without the court ruling, it would instead update its 2008 health advisory. Under the Safe Drinking Water Act, a limit is generally set as close to the health goal as feasible, but EPA can set a less strict level if it finds benefits don’t justify costs. Which level EPA picks is the key open question.

Current status (as of October 2026): EPA’s perchlorate page, last updated in July 2026, describes the rule as proposed. Under the consent decree, EPA must sign a final rule by May 21, 2027. We could not find any final rule or change to that deadline.

State limits: Massachusetts and California

Two states didn’t wait.

Massachusetts adopted the nation’s first perchlorate drinking water standard in 2006: 2 µg/L. MassDEP said its limit is lower than federal reference values partly because it accounts for perchlorate exposure from food, and partly because of perchlorate in disinfectant solutions used to treat drinking water. Find Massachusetts utilities on our Massachusetts page.

California set an enforceable limit of 6 µg/L in October 2007. California’s Office of Environmental Health Hazard Assessment lowered the state’s public health goal from 6 to 1 µg/L in 2015. The State Water Board has since lowered the level at which labs must report detections, from 4 µg/L to 2 µg/L in 2021 and to 1 µg/L on January 1, 2024, to gather better data before deciding whether to revise the limit. Learn how these goals work in California public health goals explained, or browse utilities on our California page.

Note how far apart the numbers are: 2 µg/L in Massachusetts, 6 µg/L in California, and a federal proposal that could land anywhere from 20 to 80 µg/L.

How to find out if it’s in your water

Check your utility’s water quality report. In Massachusetts and California, perchlorate is a regulated contaminant, so detections should appear with the other regulated results. Elsewhere, it may appear only if your system tested for it voluntarily.
Ask your utility directly. That is EPA’s first recommendation.
Look up your system through our ZIP code search.
Private wells near military, aerospace or fireworks sites, or in areas with known contamination, should be tested by a certified lab.

What removes perchlorate

At water treatment plants, EPA says ion exchange, reverse osmosis and biological treatment have been shown to reduce perchlorate to levels that would meet the proposed limit.

At home, EPA’s proposal notes that certified point-of-use reverse osmosis devices are available that reduce perchlorate. EPA also said it was not aware of any home devices certified for perchlorate that use other technologies, or any whole-house systems certified for it. So standard carbon pitchers and refrigerator filters should not be relied on for perchlorate.

If you buy an RO system for perchlorate, confirm that the exact model is listed for perchlorate reduction in the NSF (or other accredited certifier) product listings. A general NSF/ANSI 58 certification does not by itself mean perchlorate was tested. For example, the AquaTru countertop RO unit’s IAPMO listing includes perchlorate, while the Aquasana OptimH2O’s listing does not. See reverse osmosis vs. carbon filters for how RO works.

Bottom line

Perchlorate is a thyroid concern, and the people who most need to pay attention are pregnant women and those planning a pregnancy. Most US water systems are not expected to exceed even the strictest federal option. After 15 years of reversals, EPA proposed a national limit in January 2026 under court order, and the final rule is due by May 2027. Massachusetts and California already enforce much stricter limits. If you’re concerned, ask your utility for its results, test a private well, and use a reverse osmosis unit listed for perchlorate reduction if you need treatment.