Glossary · What's in the water

What Are PFOA and PFOS? The Two PFAS Limited to 4 ppt

PFOA and PFOSPerfluorooctanoic acid and perfluorooctane sulfonic acid

The two most studied PFAS: eight-carbon fluorinated acids that the EPA's 2024 rule limits to 4.0 parts per trillion each in public water.

FFFFFFFFFFFFFFFCCCCCCCCOOH
PFOA 8 carbons, carboxylic acid head
FFFFFFFFFFFFFFFFFCCCCCCCCSO₃H
PFOS 8 carbons, sulfonic acid head
  1. The tail: a chain of carbon atoms (C) fully wrapped in fluorine (F). Carbon–fluorine bonds are very hard to break, which is why PFAS last so long in the environment and in the body.
  2. The head: an acid group, carboxylic in PFOA and sulfonic in PFOS. Other PFAS vary the chain length and the head, which is how there come to be thousands of them.
PFOA and PFOS both have eight carbons. In PFOA, seven carry fluorine and the eighth is part of the carboxylic acid head; in PFOS all eight carry fluorine and the chain ends in a sulfonic acid group.

At a glance

Legal limit (MCL)
4.0 ppt each, in the final April 2024 EPA rule
Health goal (MCLG)
Zero for both; not enforceable
Compliance due
2029; a May 2026 proposal would allow up to 2031 (not final)
Structure
Eight carbons each; PFOA has a carboxylic acid end, PFOS a sulfonic acid end
Cancer rating (IARC, 2023)
PFOA Group 1; PFOS Group 2B

If your utility’s report or its federal monitoring results list any PFAS, look for these two first: they are the ones with the firmest federal limit, and the ones the health research knows best.

What’s the difference between PFOA and PFOS?

Both are eight-carbon chains in which the hydrogen atoms on the carbons have been swapped for fluorine. They differ at one end: PubChem lists PFOA as perfluorinated octanoic acid, a carboxylic acid, and PFOS as a perfluoroalkanesulfonic acid.

Both were phased down by their main makers years ago. According to the EPA, 3M voluntarily phased out PFOS between 2000 and 2002, and a global stewardship program with eight major companies aimed to eliminate PFOA from emissions and products by 2015. Because they break down so slowly, they still turn up in water.

The EPA’s final April 2024 rule sets a legal limit (MCL) of 4.0 parts per trillion for each, with a non-enforceable health goal of zero. Compliance is judged on running annual averages, not one sample, and utilities have until 2029 to meet it.

In May 2026 the EPA proposed keeping both limits while letting water systems request two more years, to 2031. As of October 10, 2026, that is still a proposal. Our news story explains what it would change.

Are PFOA and PFOS harmful?

They are the best-studied PFAS, and the evidence is real but has limits. In 2023, the WHO’s cancer agency classified PFOA as carcinogenic to humans and PFOS as possibly carcinogenic. The PFOA rating rests on animal and mechanistic evidence; evidence of cancer in people was rated “limited” for PFOA and “inadequate” for PFOS. Our guide to PFAS in drinking water walks through the studies.

If your results are above 4 ppt, look for a filter with “PFOA Reduction” and “PFOS Reduction” on the certifier’s own listing, such as NSF’s NSF/ANSI 53 listings. If they’re below the limits, the EPA says a filter may not be useful to you.

Published by the Checked Water editorial team

Sources

  1. Perfluorooctanoic acid (CID 9554) (Source, PubChem, National Library of Medicine, 2026)
  2. Perfluorooctanesulfonic acid (CID 74483) (Source, PubChem, National Library of Medicine, 2026)
  3. Risk Management for Per- and Polyfluoroalkyl Substances (PFAS) under TSCA (Clinical guideline, US Environmental Protection Agency, 2026)
  4. Per- and Polyfluoroalkyl Substances (PFAS): Final PFAS National Primary Drinking Water Regulation (Clinical guideline, US Environmental Protection Agency, 2026)
  5. Proposed PFOA and PFOS Compliance Extension Rule (Clinical guideline, US Environmental Protection Agency, 2026)
  6. Carcinogenicity of perfluorooctanoic acid and perfluorooctanesulfonic acid (Review, The Lancet Oncology, 2024)
  7. Reducing PFAS in Your Drinking Water with a Home Filter (Clinical guideline, US Environmental Protection Agency, 2026)
  8. Drinking Water Treatment Units: NSF/ANSI 53 Certified Product Listings (Source, NSF, 2026)

Where PFOA and PFOS comes up