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PFOS in drinking water

Also called Perfluorooctane sulfonic acid, Perfluorooctanesulfonic acid, Perfluorooctane Sulfonate (PFOS).

Legal limit
4.0 ppt
Health goal
Zero

Values in parts per trillion (ppt). Comes from the source.

Contaminant referenceLimits checked Oct. 4, 2026

Where the rules stand

Final MCL of 4.0 ppt with a health goal of zero, published April 26, 2024. Systems must comply by April 26, 2029. On May 20, 2026, EPA proposed keeping this MCL but letting systems that apply have until April 26, 2031. Comments closed July 20, 2026. As of Oct. 4, 2026, the Federal Register shows no final rule on that proposal.

The scale every TapBuff report uses for this contaminant: the legal line at the middle, twice it at the right.

Is it in your water?

Matched to your utility using EPA records

What EPA says about health

EPA classifies PFOS as likely to cause cancer in humans and found no known threshold below which there is no cancer risk. That is why its health goal is zero. EPA also identified non-cancer effects, such as lower immune response in children and lower birth weight.

Where it comes from

PFAS have been used since the 1940s in products like nonstick cookware, food packaging, waterproof clothing, stain-resistant furniture and firefighting foam. They last a long time in the environment and can reach drinking water sources.

What EPA found nationally

Under UCMR 5, EPA tested 9,944 utilities for PFOS. It was found at 1,320 of them, which report serving 68 million people. The highest single sample was 490 ppt.

Highest single results for PFOS
UtilityHighest sampleFound in
Town of NashvilleNashville, NC · serves 5,900490 ppt7 of 10
Running Springs Water DistrictRunning Springs, CA · serves 5,268237 ppt6 of 22
ArtondaleTacoma, WA · serves 5,828154 ppt2 of 18
Emmaus Borough Public WaterEmmaus, PA · serves 14,000153 ppt7 of 8
Gwa Northern Water SystemTalofofo, GU · serves 139,207135 ppt37 of 121
New MartinsvilleNew Martinsville, WV · serves 5,272122 ppt2 of 9
Yeshiva Farm SettlementNew Castle (T), NY · serves 300106 ppt4 of 4
City of Grand PrairieGrand Prairie, TX · serves 201,84382 ppt5 of 20
Orangewood Water SystemHoliday, FL · serves 6,46480.8 ppt10 of 10
City of OxfordOxford, MS · serves 28,50072.2 ppt2 of 10
Highest single sample per utility, EPA UCMR 5 file of Aug. 27, 2026. One high sample is not a violation; compliance is judged on averages over time.

What handles it

  • NSF/ANSI 53

    Claim to look for: PFOS Reduction

    Same claim, other certifiers’ wording: Total PFAS Reduction; Perfluorooctane Sulfonate (PFOS)

    NSF listing wording. NSF also files reverse osmosis systems' PFOS claims under 53. NSF says a PFAS claim means reducing PFAS below 20 ppt. EPA says current certifications do not yet show a filter reaches EPA's limits.

    WQA listing wording for the same claim.

    NSF listing footnotes include PFOS in Total PFAS.

  • NSF/ANSI 58

    Claim to look for: Perfluorooctane Sulfonate (PFOS)

    WQA listing wording for reverse osmosis systems. NSF's own listings show no PFAS claims under 58.

NSF’s PFAS claims are tested to below 20 ppt. EPA’s limit is 4.0 ppt, and EPA says no certification yet shows a filter reaching its levels.

Filters with that claim on a certifier’s listing

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Sources

  1. Per- and Polyfluoroalkyl Substances (PFAS): Final PFAS National Primary Drinking Water Regulation, U.S. EPA. Checked Oct. 4, 2026.
  2. PFAS National Primary Drinking Water Regulation (final rule, April 26, 2024), Federal Register. Checked Oct. 4, 2026.
  3. Extending the Compliance Deadline for the PFOA and PFOS Maximum Contaminant Levels (proposed rule, May 20, 2026), Federal Register. Checked Oct. 4, 2026.
  4. Federal Register: EPA documents mentioning PFAS, newest first (no final rule on either 2026 proposal), Federal Register. Checked Oct. 4, 2026.
  5. Maximum Contaminant Level Goals for PFOA and PFOS in Drinking Water (EPA-815-R-24-010, April 2024), U.S. EPA. Checked Oct. 4, 2026.
  6. Fact Sheet: PFAS National Primary Drinking Water Regulation (April 2024), U.S. EPA. Checked Oct. 4, 2026.
  7. Fact Sheet: Reducing PFAS in Your Drinking Water with a Home Filter (April 2024), U.S. EPA. Checked Oct. 4, 2026.
  8. The Fifth Unregulated Contaminant Monitoring Rule (UCMR 5) Data Summary: July 2026 (EPA 815-S-26-007), U.S. EPA. Checked Oct. 4, 2026.
  9. Search for NSF Certified Drinking Water Treatment Units (standards and reduction claims lists), NSF. Checked Oct. 4, 2026.
  10. Contaminant Reduction Claims Guide, NSF. Checked Oct. 4, 2026.
  11. PFAS in Drinking Water, NSF. Checked Oct. 4, 2026.
  12. NSF listings for the NSF/ANSI 53 "Total PFAS Reduction" claim (footnotes define Total PFAS), NSF. Checked Oct. 4, 2026.
  13. NSF listings search for NSF/ANSI 58 PFOA, PFOS and Total PFAS claims (no matching products), NSF. Checked Oct. 4, 2026.
  14. Find WQA Certified Products (Gold Seal listings by standard and claim), Water Quality Association. Checked Oct. 4, 2026.
  15. SDWA Dataset Download Summary (codes from SDWA_REF_CODE_VALUES.csv, 2026 Q2 extract), U.S. EPA ECHO. Checked Oct. 4, 2026.