PFHxS (Perfluorohexane Sulfonic Acid)

Perfluorohexane sulfonic acid (PFHxS, CAS No. 355-46-4) is a six-carbon perfluoroalkyl sulfonic acid that was widely used as a shorter-chain alternative to PFOS. PFHxS was listed under the Stockholm Convention (Annex A, elimination) in 2022 with no acceptable purposes or specific exemptions—making it the most strictly controlled PFAS under the treaty. PFHxS is also on the REACH Candidate List as an SVHC.

Stockholm Convention (COP-10, 2022); EU POPs Regulation amendments; REACH Candidate List.

PFHxS’s Stockholm Convention listing with zero exemptions sends a strong signal that regulators view even ‘short-chain’ PFAS as warranting elimination. Companies that substituted PFOS with PFHxS now face a complete phase-out. All 186 Convention parties must ban production and use. Companies must screen products for PFHxS, its salts, and PFHxS-related compounds.

An aerospace company used PFHxS-based wetting agents in anodizing processes as a PFOS replacement. With PFHxS listed under the Stockholm Convention (no exemptions), the company must immediately switch to non-PFAS alternatives and ensure all PFHxS-containing inventories are disposed of as hazardous waste.