PFAS
PFOS
PFOS, or perfluorooctane sulfonic acid, is the second PFAS with its own federal drinking water limit, set at 4.0 parts per trillion, and it has been widely studied alongside PFOA.
PFOS stands for perfluorooctane sulfonic acid. It pairs with PFOA in most PFAS news because both were common and both are now regulated in drinking water.
EPA states that it made a final determination to regulate PFOA and PFOS in drinking water. Its limit table lists PFOS with a health goal of zero and an enforceable MCL of 4.0 ppt.
Like PFOA, the rule date is under review. EPA announced proposed rules in May 2026 that uphold the PFOA and PFOS limits while offering systems a path to 2031. Always check the EPA page for the latest.
If you manage a facility that used firefighting foam, plating, textiles or coatings, PFOS may matter on your site, not just in your water. Soil and groundwater tests can show whether it is there.
Use the entry on PFAS for the full picture and parts per trillion to read your lab report. The goal is to test first and decide with data.
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