PFOS (perfluorooctane sulfonic acid) is, alongside PFOA, one of the two longest-studied “forever chemical” PFAS compounds. EPA finalized a legal limit of 4.0 parts per trillion for PFOS in April 2024, with an MCLG of zero. Systems have until 2029 to comply; EPA’s May 2025 plan proposes an optional two-year compliance extension for PFOA/PFOS specifically, but the 2024 rule remains in force as of this writing.
What actually removes PFOS
- Reverse osmosis (NSF/ANSI 58) — a tight membrane that rejects PFOS along with most other PFAS compounds.
- Activated carbon (NSF/ANSI 53) — granular activated carbon specifically certified for PFOS reduction (NSF’s older P473 protocol is now folded into NSF/ANSI 53). Confirm PFOS is named on the certification sheet.
- Anion exchange (NSF/ANSI 53) — resin media that binds PFAS compounds, common in point-of-entry systems.
What doesn’t work
A carbon pitcher or faucet filter certified only under NSF/ANSI 42 (chlorine taste and odor) has not been tested against PFOS — a general “activated carbon” claim on the packaging isn’t the same as a PFOS-specific certification. Boiling does not remove PFOS; because water evaporates and PFOS doesn’t, boiling can concentrate it slightly rather than eliminate it. Standard water softeners (cation exchange for hardness) are not certified for PFAS removal.
How to choose
- Confirm PFOS is explicitly listed on the filter’s NSF/ANSI 53 or 58 certification — PFOS and PFOA are often tested and certified together, but not always, and some filters are only rated for one.
- PFOS and PFOA frequently co-occur (both were used in firefighting foam and industrial applications), so a filter certified for both offers broader protection than one certified for only a single compound.
- Track the manufacturer’s recommended cartridge-replacement schedule closely — PFAS-rated media loses capacity once saturated, often before any change in taste or odor would tip you off.
Test before you buy
PFOS contamination tends to cluster around specific sources (firefighting foam training sites, certain industrial facilities, landfill leachate), so confirming your own water’s PFOS level with a certified lab test — or checking your public utility’s UCMR5 results — is a better starting point than assuming a filter is needed.