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PFOS (perfluorooctane sulfonic acid) and PFOA (perfluorooctanoic acid) are two man-made PFAS chemicals that do not break down in the environment. The US EPA’s drinking water limit for each is 4.0 parts per trillion (ng/L), with a health-based goal of zero. Labs measure them with solid-phase extraction and LC-MS/MS, for example under EPA Method 537.1.
Definition and scope
The CDC’s Agency for Toxic Substances and Disease Registry (ATSDR) describes per- and polyfluoroalkyl substances (PFAS) as man-made chemicals used in industry and consumer products since the 1940s. PFOA and PFOS are the most commonly studied PFAS. They have been phased out of production in the United States, although other countries may still make or use them (ATSDR, “About PFAS”).
EPA notes that there are thousands of PFAS chemicals, so PFOS and PFOA are two named members of a much larger group, not a synonym for PFAS as a whole.
How they are used and how people are exposed
ATSDR lists water-repellent clothing, stain-resistant fabrics and carpets, some cosmetics, some firefighting foams, and grease-, water- and oil-resistant products among the places PFAS are found. Exposure can come from contaminated drinking water, food grown near PFAS sites, and fish or wild game. Most PFAS, including PFOA and PFOS, do not break down, and ATSDR says they can build up in the body with each exposure over time.
ATSDR states that studies suggest exposure to some PFAS may be linked to harmful health effects, and that it is difficult to show that these substances directly cause health conditions in humans.
PFOS and PFOA compared
The table is limited to what the EPA and ATSDR pages state.
| Feature | PFOA | PFOS |
|---|---|---|
| Full name | Perfluorooctanoic acid | Perfluorooctane sulfonic acid |
| EPA drinking water MCL | 4.0 ppt (ng/L) | 4.0 ppt (ng/L) |
| EPA MCL goal (MCLG) | Zero | Zero |
| US production | Phased out | Phased out |
| Breaks down in the environment | No (most PFAS, including PFOA, do not) | No (most PFAS, including PFOS, do not) |
How they are regulated in US drinking water
EPA’s final rule sets a maximum contaminant level (MCL) of 4.0 ppt for each of PFOA and PFOS. A separate Hazard Index MCL applies to mixtures of two or more of PFHxS, PFNA, HFPO-DA and PFBS. EPA states that compliance with the MCLs is determined by running annual averages at the sampling point.
Regulatory dates
- 2024 rule: initial monitoring by 2027, public reporting beginning 2027, corrective action by 2029, and enforcement beginning 2029.
- Proposed, 18 May 2026 (not final, open for comment): keep the PFOA and PFOS standards but let systems request two additional years, to 2031. A separate proposal would rescind the PFHxS, PFNA, HFPO-DA and Hazard Index regulations.
EPA’s page was last updated on 18 May 2026; it was checked on 29 October 2026. Check it for later changes before relying on any date.
Where it matters for labs
An MCL expressed in ng/L sets the concentration range a drinking water method has to report on. EPA Method 537.1 (Revision 2.0) includes PFOA and PFOS among its analytes. It uses a 250 mL sample passed through a solid-phase extraction (SPE) cartridge, concentrates the extract 250-fold, and measures by LC-MS/MS. The SPE sorbent in the method is styrene-divinylbenzene (SDVB), not C18; C18 appears only as the LC column.
- For the method in detail, see the guide to EPA Method 537.1 for PFAS in drinking water.
- For how the extraction step works, see SPE cartridge selection and method optimization.
- For the detector, see what a mass spectrometer is and GC-MS versus LC-MS.
Common misunderstandings
- “PFOS and PFOA are the same thing.” They are two separate chemicals with different names, though EPA gives them the same MCL of 4.0 ppt and the same MCLG of zero.
- “PFAS means PFOS and PFOA.” They are the most commonly studied PFAS, but EPA says there are thousands of PFAS chemicals.
- “Phased out means gone.” The phase-out refers to US production. ATSDR says other countries may still make or use them, and the chemicals do not break down.
FAQ
What is the difference between PFOS and PFOA?
PFOS is perfluorooctane sulfonic acid and PFOA is perfluorooctanoic acid. Both are PFAS, both are persistent, and EPA regulates both at 4.0 ppt in drinking water.
Are PFOS and PFOA banned?
ATSDR says both have been phased out of production in the United States, while other countries may still make or use them. Separately, EPA limits them in public drinking water.
What is the EPA limit for PFOS and PFOA?
The final MCL is 4.0 ppt (ng/L) for each, with an MCLG of zero, assessed as a running annual average at the sampling point.
How are PFOS and PFOA measured in water?
EPA Method 537.1 uses solid-phase extraction followed by LC-MS/MS and lists both compounds among its analytes.
Sources
- US EPA, Per- and Polyfluoroalkyl Substances (PFAS), page updated 18 May 2026, checked 29 October 2026.
- CDC ATSDR, About PFAS, checked 29 October 2026.
- US EPA, PFAS Explained, checked 29 October 2026.
- US EPA, Method 537.1: Determination of Selected Per- and Polyfluorinated Alkyl Substances in Drinking Water by Solid Phase Extraction and Liquid Chromatography/Tandem Mass Spectrometry (LC/MS/MS), checked 29 October 2026.
