Perfluorooctane Sulfonic Acid (PFOS)

Perfluorooctane sulfonic acid (PFOS, CAS No. 1763-23-1) is a fully fluorinated eight-carbon sulfonic acid and one of the earliest PFAS to face global regulatory action. PFOS was widely used in firefighting foams (AFFF), metal plating, and stain-resistant treatments. It was listed under the Stockholm Convention in 2009 (initially with acceptable purposes, further restricted in 2019), restricted under the EU POPs Regulation, and designated as a hazardous substance under U.S. CERCLA.

Stockholm Convention (COP-4, 2009; COP-9, 2019); EU POPs Regulation (EU) 2019/1021; REACH Annex XVII Entry 53; CERCLA Final Rule (2024).

PFOS is one of the most tightly restricted PFAS globally. Its use is essentially banned with very limited remaining exemptions that are being progressively eliminated. Companies must ensure no PFOS is present above regulatory thresholds in products or processes. Under the EU POPs Regulation, the concentration limit is 10 mg/kg (10 ppm) in substances and mixtures. Legacy PFOS contamination from AFFF use continues to drive CERCLA remediation activity.

An electroplating facility historically used a PFOS-based mist suppressant. The facility has switched to a non-fluorinated alternative but must address legacy PFOS contamination in soil and groundwater. Under CERCLA, the facility may be liable for remediation costs, and under the EU POPs Regulation, any remaining PFOS-containing materials must be disposed of as hazardous waste.