EPA legal limit
4 ppt
Maximum Contaminant Level
EWG health guideline
0.004 ppt
Science-based, stricter target
IARC classification
Group 2B
Cancer research classification
Health effects
PFOS is classified as a probable human carcinogen (IARC Group 2A; NTP: reasonably anticipated to be a human carcinogen). Health associations substantially overlap those of PFOA: kidney cancer, testicular cancer, thyroid disruption, immune system suppression (including reduced vaccine efficacy in children), elevated cholesterol, pregnancy-induced hypertension, and adverse birth outcomes. PFOS bioaccumulates in blood, liver, and breast milk, with a human serum half-life of approximately 5 years — longer than PFOA — meaning cumulative body burden builds over repeated low-level exposures. The EWG health guideline of 0.004 ppt is 1,000 times stricter than the 4 ppt MCL because no safe carcinogenic threshold is established. EPA and NTP classifications draw on both epidemiological evidence and animal carcinogenicity studies conducted after 3M's phase-out.
Where it comes from
Primary contamination source: aqueous film-forming foam (AFFF) used at military installations and civilian airports for fire suppression. Decades of training exercises and emergency responses have created extensive groundwater plumes at and downwind of hundreds of U.S. bases — including significant contamination at sites studied under the Department of Defense PFAS remediation programme. Secondary sources include legacy consumer products manufactured with Scotchgard (carpets, upholstery, apparel, food packaging) produced before 3M's 2002 phase-out. Industrial sources include fluoropolymer manufacturing and metal plating operations. Like PFOA, PFOS persists in groundwater for decades after primary source removal and has been detected in surface water, sediment, and remote environments globally due to atmospheric transport and bioaccumulation through the food chain.
How it's regulated
EPA issued a Maximum Contaminant Level of 4 ppt for PFOS on April 10, 2024, as part of the Safe Drinking Water Act PFAS National Primary Drinking Water Rule — the first federal limit ever applied to PFOS. Water systems exceeding 16 ppt must notify the public; all systems above 4 ppt must achieve compliance by April 2026. The EWG health guideline is 0.004 ppt — 0.4% of the legal limit — based on carcinogen risk modelling with no established safe dose. Before 2024, the only federal benchmark was a non-enforceable EPA Lifetime Health Advisory of 70 ppt (2016), revised to 0.02 ppt in 2022, ahead of the final binding rule. UCMR5 national monitoring (2023–2025) delivered the first systematic nationwide dataset on PFOS occurrence in public water supplies. 3M has reached a multi-billion-dollar settlement with U.S. water utilities over PFOS contamination costs.
The EPA vs EWG gap
The legal limit (4 ppt) is 1000× higher than the EWG health guideline (0.004 ppt). Water can be legally compliant while still exceeding the science-based threshold.
How to filter pfos
Not all filters address pfos. Look for independently certified filters — NSF International certification means the removal claim has been independently verified.
Frequently asked questions
What is pfos?
PFOS is a long-chain, 8-carbon perfluoroalkyl sulfonate — structurally distinct from PFOA's carboxylic acid group but equally persistent and bioaccumulative. Historically used in 3M's Scotchgard fabric and surface protector, it was voluntarily phased out by 3M in 2002. EPA set a 4 ppt MCL in April 2024, and IARC classifies it as a Group 2A probable human carcinogen.
What are the health effects of pfos?
PFOS is classified as a probable human carcinogen (IARC Group 2A; NTP: reasonably anticipated to be a human carcinogen). Health associations substantially overlap those of PFOA: kidney cancer, testicular cancer, thyroid disruption, immune system suppression (including reduced vaccine efficacy in children), elevated cholesterol, pregnancy-induced hypertension, and adverse birth outcomes. PFOS bioaccumulates in blood, liver, and breast milk, with a human serum half-life of approximately 5 years — longer than PFOA — meaning cumulative body burden builds over repeated low-level exposures. The EWG health guideline of 0.004 ppt is 1,000 times stricter than the 4 ppt MCL because no safe carcinogenic threshold is established. EPA and NTP classifications draw on both epidemiological evidence and animal carcinogenicity studies conducted after 3M's phase-out.
Is pfos regulated in drinking water?
EPA issued a Maximum Contaminant Level of 4 ppt for PFOS on April 10, 2024, as part of the Safe Drinking Water Act PFAS National Primary Drinking Water Rule — the first federal limit ever applied to PFOS. Water systems exceeding 16 ppt must notify the public; all systems above 4 ppt must achieve compliance by April 2026. The EWG health guideline is 0.004 ppt — 0.4% of the legal limit — based on carcinogen risk modelling with no established safe dose. Before 2024, the only federal benchmark was a non-enforceable EPA Lifetime Health Advisory of 70 ppt (2016), revised to 0.02 ppt in 2022, ahead of the final binding rule. UCMR5 national monitoring (2023–2025) delivered the first systematic nationwide dataset on PFOS occurrence in public water supplies. 3M has reached a multi-billion-dollar settlement with U.S. water utilities over PFOS contamination costs.
Where does pfos come from?
Primary contamination source: aqueous film-forming foam (AFFF) used at military installations and civilian airports for fire suppression. Decades of training exercises and emergency responses have created extensive groundwater plumes at and downwind of hundreds of U.S. bases — including significant contamination at sites studied under the Department of Defense PFAS remediation programme. Secondary sources include legacy consumer products manufactured with Scotchgard (carpets, upholstery, apparel, food packaging) produced before 3M's 2002 phase-out. Industrial sources include fluoropolymer manufacturing and metal plating operations. Like PFOA, PFOS persists in groundwater for decades after primary source removal and has been detected in surface water, sediment, and remote environments globally due to atmospheric transport and bioaccumulation through the food chain.
How do I remove pfos from tap water?
The most effective methods for removing pfos are: Reverse osmosis (RO), NSF 58 certified RO system, Activated carbon block filter, NSF 53 certified filter. Look for NSF-certified systems — independent certification confirms removal claims have been verified.
Is pfos in your water?
Enter your ZIP code to see the measured level in your specific utility.