Monitored — not federally regulated
FOSA
Perfluorooctane sulfonamide
A PFOS precursor that can cross the blood-brain barrier more readily than PFOS itself. Detected in breast milk and marine wildlife. Converts to PFOS over time in the body.
01Health effects
Linked outcomes
- · Neurological effects
- · Liver toxicity
- · Developmental harm
- · Transforms to PFOS in the body
Organs affected
Brain, liver, developing fetus
Effects above are summarized from EPA, NIH/NTP, ATSDR, and IARC documentation. Not a clinical or medical claim — see our sourcing standards.
02Where it comes from
Intermediate in the production and environmental degradation of PFOS-based compounds. Found in contaminated fish and wildlife, particularly marine mammals.
03Regulatory status
Not federally regulated. Can cross the blood-brain barrier more readily than PFOS itself; bioaccumulates in fatty tissue.
04What you can do
If FOSA was detected in your water supply, two filter technologies reliably remove it at the tap:
- RO (reverse osmosis) under-sink systems — 90–99.9% removal across all PFAS chain lengths. Look for NSF/ANSI 58 certification.
- GAC (granular activated carbon) block filters — effective for long-chain PFAS; less reliable for short-chain compounds. Look for NSF/ANSI 53 with NSF P473.
See our certified-filter picks or read the in-depth PFAS removal guide.
Related compounds
Other monitored compounds
- PFBS →
Perfluorobutane sulfonic acid
Introduced as a shorter-chain PFOS replacement after 3M phased out PFOS in 2002. The shorter chain clears the body faster but is harder to filter out of water.
- PFHxA →
Perfluorohexanoic acid
One of the most frequently detected PFAS in US drinking water and a common degradation product of fluorotelomer chemistry used in food packaging and stain protection. Its short chain length makes it especially hard to remove with activated carbon filters.
- PFBA →
Perfluorobutanoic acid
Very short half-life in the body (3–4 days) compared to PFOA (years), but still persistent in the environment. Often the final breakdown product when longer-chain PFAS degrade.
- PFDA →
Perfluorodecanoic acid
A long-chain PFAS with an estimated human half-life of 7–12 years — among the most persistent compounds in the body of any monitored PFAS. Frequently detected in human blood samples even decades after primary uses ended.