What Are PFAS? The Forever Chemicals in Some Tap Water

When people talk about PFAS in water, they usually mean a handful of well-studied compounds, mostly PFOA and PFOS, measured in parts per trillion. The family itself is much bigger: the EPA’s PFAS explainer says there are thousands of them, used in many consumer, commercial and industrial products.

Why are PFAS called “forever chemicals”?

What they share is a carbon chain that is partly or fully fluorinated. ATSDR, the CDC’s toxic substances agency, says in its chemical overview that the carbon-fluorine bonds are strong, so these compounds don’t fully break down in the environment or in the human body. Once in people, some leave within days, others take years: ATSDR gives half-lives from a few days to 8 years or more, depending on the PFAS.

Are PFAS in my tap water?

Maybe, at low levels. A US Geological Survey team sampled tap water at 716 homes and buildings between 2016 and 2021 and estimated that at least one PFAS could be detected in about 45% of US drinking-water samples, with similar profiles for private wells and public supplies. That’s a national estimate, not your tap. Our guide to PFAS in drinking water covers sources and what the health research shows.

Are there legal limits for PFAS in drinking water?

Yes, for six of them. The EPA’s April 2024 rule set legal limits for PFOA and PFOS (4.0 ppt each), PFHxS, PFNA and HFPO-DA (10 ppt each), plus a Hazard Index for mixtures. Utilities have until 2029 to comply.

In May 2026 the EPA proposed keeping the PFOA and PFOS limits with an option to extend the deadline to 2031 and rescinding the other four limits. As of October 10, 2026, both are still proposals, so the 2024 rule stands. Our news story tracks what the 2026 proposals would change.

Do I need to do anything?

Check before you buy anything. On city water, ask your utility or look up its PFAS results; on a private well, the EPA suggests a state-certified lab using EPA-developed methods. The EPA’s home filter guidance says that if PFAS haven’t been measured in your water, or were measured below federal limits, a filter may not be useful to you. If your results are higher, see which water filters remove PFAS.