PFOS (Perfluorooctane Sulfonate)

PFOS (perfluorooctane sulfonate) is one of the most widespread PFAS "forever chemicals" found in US drinking water. It was the key ingredient in 3M's Scotchgard fabric protector and was widely used in aqueous film-forming foam (AFFF) for firefighting at military bases and airports.

The EPA set a Maximum Contaminant Level of 4 parts per trillion (ppt) for PFOS in 2024 — the same strict limit as PFOA. PFOS is often found alongside PFOA in contaminated water supplies. The IARC classified PFOS as a Group 2B possible carcinogen in 2023.

Why Is PFOS Dangerous?

PFOS is extremely persistent in the human body, with an estimated half-life of 5.4 years — meaning it takes over 5 years to eliminate half of what you've absorbed. This is one of the longest half-lives of any PFAS compound, making chronic exposure particularly concerning.

Key health concerns include:

• Elevated cholesterol levels — one of the most consistent findings across population studies
• Thyroid disease and hormone disruption
• Immune system suppression and reduced vaccine response (documented in children by the Faroe Islands studies)
• Increased risk of kidney and testicular cancer
• Reproductive effects including decreased fertility and pregnancy-induced hypertension
• Liver damage and non-alcoholic fatty liver disease

Where Does PFOS Come From?

PFOS was manufactured primarily by 3M from the 1950s until 2002, when the company voluntarily phased it out. The primary sources of PFOS contamination in drinking water today are:

• Military bases: The Department of Defense identified over 700 installations with PFAS contamination from AFFF firefighting foam
• Airports: AFFF was standard for airport fire response for decades
• Industrial sites: Facilities that manufactured or used PFOS
• Landfills: PFOS-containing consumer products continue to leach into groundwater

PFOS vs. PFOA and Other PFAS

PFOS and PFOA are the two most studied PFAS chemicals and have the strictest EPA limits (4 ppt each). PFOS has a longer half-life in the body (5.4 years vs. 3.5 for PFOA). Other related PFAS include PFHxS (a PFOS replacement with an even longer half-life), PFNA, and GenX.

How to Remove PFOS From Your Water

• Reverse Osmosis: Most effective method, removing 90-99% of PFOS. Under-sink systems cost $150-$500.
• Granular Activated Carbon: Effective when NSF-certified for PFAS, removes 90%+. Replace filters on schedule.
• Ion Exchange Resins: Effective in whole-house treatment systems ($1,000-$3,000+).

Look for filters certified to NSF/ANSI 53 or NSF P473 for PFAS removal. Independent testing organizations like the NSF and Water Quality Association maintain databases of certified products.