Contaminants

What PFAS are, and how they got into drinking water

A class of chemicals invented to be indestructible, used for eighty years, and now present in the water supplied to a majority of tested American water systems.

What the acronym covers

PFAS stands for per- and polyfluoroalkyl substances. It is not one chemical but a family of several thousand, united by a structural feature: chains of carbon atoms bonded to fluorine.

That bond is the strongest single bond in organic chemistry. It is what gives PFAS the properties industry wanted. A surface coated in them repels water, oil and grease. A foam containing them smothers a fuel fire in seconds. A fabric treated with them shrugs off rain. Nothing else does all of that as cheaply or as well.

It is also why they persist. Bacteria cannot digest a carbon-fluorine bond. Sunlight does not break it. Conventional water treatment, which is built to remove particles and kill organisms, does not touch it. A PFAS molecule released in 1970 is, for practical purposes, still a PFAS molecule.

How they reached the water supply

Four routes account for most contamination, and they explain the geography of the results.

• Firefighting foam. Aqueous film-forming foam was used for decades at military installations, commercial airports and industrial fire training grounds. It was sprayed directly onto the ground, repeatedly, by design. Groundwater near those sites is among the most heavily contaminated in the country.
• Manufacturing discharge. Plants producing fluoropolymers, or applying them to textiles, paper and carpet, discharged process water into rivers that other cities drew from downstream.
• Landfills. Consumer products containing PFAS were thrown away for eighty years. Rainwater moving through a landfill produces leachate, and leachate carries PFAS into groundwater.
• Biosolids. Treated sewage sludge is applied to farmland as fertilizer. Where that sludge contained PFAS, the compounds entered soil and, from there, shallow groundwater.

This is why contamination clusters rather than spreading evenly, and why our analysis of the federal data found that larger water systems tested worse than smaller ones. Population and industry occupy the same map. The cities are near the airports, the plants and the landfills.

Which compounds are regulated

In April 2024 the EPA set the first enforceable national limits for PFAS in drinking water.

Two proposals are pending. On 18 May 2026 the EPA proposed rescinding the limits for PFHxS, PFNA, HFPO-DA and the Hazard Index. Two days later it proposed extending the compliance deadline for PFOA and PFOS from April 2029 to April 2031 for systems that apply and qualify. Neither is final. The 4.0 ppt limits for PFOA and PFOS are not part of the rescission.

Why the limits are set so low

Four parts per trillion is roughly one drop in twenty Olympic swimming pools. Limits are not usually written at that scale, and the reason has less to do with acute toxicity than with accumulation.

Most contaminants pass through the body. PFAS do not. They bind to proteins in blood and are cleared slowly, over years. The relevant exposure is therefore not the dose in one glass of water but the total accumulated over a lifetime of drinking from the same tap.

The health evidence is what drives the number. In 2023 the International Agency for Research on Cancer classified PFOA as carcinogenic to humans (Group 1), citing renal cell carcinoma and testicular cancer, and PFOS as possibly carcinogenic to humans (Group 2B). Beyond cancer, the EPA's assessments associate exposure with reduced antibody response to vaccination, elevated cholesterol, changes in liver enzymes, lower infant birth weight and pregnancy-induced hypertension.

These are population-level associations across a range of exposures. A hazard classification describes how strong the evidence is, not how large the risk is at any particular concentration. Water slightly above 4.0 ppt is not equivalent to an occupational exposure. What the evidence supports is the conclusion the regulation itself reflects: less is better, and the level in your own supply is worth knowing.

What the national testing found

The Fifth Unregulated Contaminant Monitoring Rule required systems serving more than 3,300 people, plus a sample of smaller ones, to test for 29 PFAS compounds and lithium. Sampling ran from January 2023 to December 2025.

Of 10,313 systems, 6,203 detected at least one substance. In 1,734 systems, serving about 81 million people, a sample came back above one of the five health-based limits. The most frequently detected regulated compounds were PFOS, in 1,352 systems, and PFOA, in 1,305.

A detection above a limit is a measurement, not a violation. Compliance is judged on a running annual average at each entry point, and is not required until 2029 at the earliest.

What removes PFAS

Conventional municipal treatment does not. Utilities that need to remove PFAS install granular activated carbon, ion exchange resin, or reverse osmosis, and all three are expensive at municipal scale. That cost is the reason the compliance deadline is measured in years.

At household scale the same three technologies work, and certification is the only reliable way to confirm a product uses them properly. We cover what to look for in which filters actually remove PFAS.