Contaminant · PFAS
PFOS in drinking water
Perfluorooctanesulfonic acid, a legacy PFAS from stain repellents and firefighting foam. One of the two PFAS with an enforceable federal drinking water limit.
highest complete sampling-location average; sampled 2023–2025. Source: EPA records; Nashville, Town of (NC0464020).
What the rules say
EPA set an enforceable Maximum Contaminant Level of 4.0 ng/L (ppt) for PFOS in April 2024, with compliance required from 2029; in May 2026 EPA proposed keeping this limit and letting systems request until 2031 to comply. Compliance is judged on running annual averages at each sampling point, so a single UCMR 5 result above 4.0 ng/L is not a violation.
What EPA's assessment says
EPA set the health goal (MCLG) for PFOS at zero. EPA's 2024 rule cites its health effects assessment linking PFOS exposure to certain cancers, immune and developmental effects, and changes in cholesterol.
Across the country
EPA tested 10,307 water systems for PFOS and found it in 1,352. Half of the systems that found it measured less than 6.8 ppt, and one in ten measured more than 17.3 ppt.
EPA's final UCMR 5 occurrence file contains 66,358 PFOS results from 10,307 systems. 4,468 results were at or above the minimum reporting level, across 1,352 systems. Among detections, the median was 6.8 ppt and the 90th percentile was 17.3 ppt.
880 systems had a result at or above the reference on the page's stated comparison basis. For contaminants with a limit, that basis is the yearly average at the highest sampling location, not the highest single sample.
Systems with the highest recorded PFOS results
| System | State | Population served | Recorded value | Records / periods |
|---|---|---|---|---|
| Nashville, Town of NC0464020 | North Carolina | 5,900 | 345 ppt | 7 |
| Running Springs Water District CA3610062 | California | 5,268 | 131 ppt | 6 |
| Emmaus Borough Public Water PA3390032 | Pennsylvania | 14,000 | 127 ppt | 7 |
| Yeshiva Farm Settlement NY5903150 | New York | 300 | 94.5 ppt | 4 |
| Orangewood Water System FL6511311 | Florida | 6,464 | 78.1 ppt | 10 |
| Gwa Northern Water System GU0000006 | Guam | 139,207 | 67.5 ppt | 37 |
| Potrero Elementary School CA3700963 | California | 300 | 57.1 ppt | 2 |
| Tempe City of AZ0407100 | Arizona | 165,000 | 55.2 ppt | 10 |
| Liberty Utilities - Bellflower-Norwalk CA1910211 | California | 72,624 | 55.0 ppt | 8 |
| Pa American Audubon PA1460055 | Pennsylvania | 8,200 | 54.1 ppt | 13 |
| Somers Town House/Hall NY5906659 | New York | 32 | 53.3 ppt | 2 |
| City of Oxford MS0360011 | Mississippi | 28,500 | 52.9 ppt | 2 |
| Montebello Land & Water Co. CA1910091 | California | 26,554 | 46.5 ppt | 6 |
| Fkaa J. Robert Dean W.T.P. FL4134357 | Florida | 86,000 | 46.2 ppt | 2 |
| North Miami, City of FL4130977 | Florida | 89,349 | 44.5 ppt | 4 |
| Town of Thurmont MD0100023 | Maryland | 6,200 | 44.5 ppt | 8 |
| Lake Panasoffkee Water Assn 2wps FL6600990 | Florida | 4,642 | 44.1 ppt | 4 |
| Barrow Utilities & Elec. Coop., INC. (Bu AK2320078 | Alaska | 4,900 | 44.1 ppt | 4 |
| Augusta-Richmond Co Ws GA2450000 | Georgia | 204,000 | 43.6 ppt | 4 |
| Liberty Water Rio Rico AZ0412011 | Arizona | 17,960 | 40.5 ppt | 6 |
| Ocala, City of (2 Wtps) FL3420922 | Florida | 66,449 | 39.0 ppt | 2 |
| City of Sacramento Main CA3410020 | California | 527,979 | 38.8 ppt | 6 |
| Downey - City, Water Dept. CA1910034 | California | 110,556 | 38.0 ppt | 10 |
| Palm Springs, Village of FL4501058 | Florida | 32,467 | 38.0 ppt | 3 |
| Irondale Water System AL0000751 | Alabama | 10,098 | 37.5 ppt | 3 |
Filters and testing
NSF/ANSI 53 or NSF/ANSI 58 with a PFOA/PFOS reduction claim
A standard number by itself is not a contaminant claim. Check the product's current certification listing and performance data sheet for PFOS or the specific contaminant you want to reduce. A utility-level record is not a test of your own faucet.