PFOS

Perfluorooctane sulfonate

1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecafluorooctane-1-sulfonic acid

Also known as PFOS Scotchgard

Perfluorooctanesulfonic acidPFOSPerfluorooctylsulfonic acid1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecafluorooctane-1-sulfonic acidHeptadecafluoro-1-octanesulfonic acidEftop EF 1011-Perfluorooctanesulfonic acidheptadecafluorooctane-1-sulfonic acid

An 8-carbon long-chain PFSA — IARC Group 1 carcinogen (2023); 2024 NPDWR individual MCL of 4 ppt with MCLG of zero. Legacy 3M Scotchgard and AFFF chemistry, phased out 2000–2002 under EPA Stewardship.

Chemistry

CAS 1763-23-1

Formula C8HF17O3S

Mol. weight 500.13 g/mol

Solubility 680 mg/L

Serum half-life 5.4 years

Group PFAS · long-chain-pfsa

Chemistry — what these mean

CAS: CAS Registry Number — a unique numeric ID assigned by the Chemical Abstracts Service. Every chemical has one, and it's the universal lookup key across regulatory and scientific databases.
Formula: The contaminant's molecular formula — the count of each atom in one molecule (e.g. C8HF15O2 means 8 carbon, 1 hydrogen, 15 fluorine, 2 oxygen atoms). Used to identify the compound in chemistry references.
Mol. weight: Molecular weight — the total mass of one molecule in grams per mole. Used to convert between mass-based and concentration-based measurements.
Solubility: How much of the contaminant will dissolve in water before saturating. Higher solubility means it travels easily with water; lower solubility means it tends to bind to particles or settle out.
Serum half-life: The time it takes for the body to clear half of a given dose from the blood. A multi-year half-life (typical for many PFAS) means even brief exposure accumulates over time.
Group: The chemical family this contaminant belongs to — e.g. PFAS, disinfection byproducts, heavy metals. Contaminants in the same group often share treatment options and regulatory approach.

Regulatory limits

Values in ppt (parts per trillion)

EPA MCL 4 ppt

EPA MCLG 0 ppt

Status: Enforced

Compliance by Apr 25, 2029

Detection limit 4 ppt

Regulatory limits — what these mean

EPA MCL: Maximum Contaminant Level — the highest amount of a contaminant legally allowed in drinking water, federally enforced by the EPA. Above this level, the utility must take corrective action and notify customers.
EPA MCLG: Maximum Contaminant Level Goal — the level the EPA considers safe with no known or expected health risk. Non-enforceable: it sets the health-based target, while the enforceable MCL sits above it where treatment costs make hitting the goal impractical.
Proposed status: Where the regulation sits in the EPA process — enforced (in force), proposed (awaiting finalization), under review, or withdrawn. A proposed limit is not legally binding yet, but it signals where regulation is heading.
Compliance by: The date utilities must meet a new or updated limit. Until then, the prior limit (or no limit) applies. Recent PFAS rules in particular phase in over multiple years.
Detection limit: The lowest concentration testing equipment can reliably measure. A "non-detect" result does not mean zero — it means below this threshold. Important for ultra-trace contaminants like PFAS, where the legal limit sits near what labs can actually measure.

Carcinogen classification

IARC (2023) Group 1

NTP (2016) Reasonably Anticipated to be a Human Carcinogen

Carcinogen classification — what these mean

IARC: International Agency for Research on Cancer — the WHO's cancer authority. Classifies substances into Group 1 (carcinogenic to humans), 2A (probably), 2B (possibly), 3 (not classifiable), or 4 (probably not). The most widely cited international authority on cancer risk.
NTP: National Toxicology Program, run by the US Department of Health and Human Services. Publishes the Report on Carcinogens with two categories: "Known to be a human carcinogen" and "Reasonably anticipated to be a human carcinogen."

Occurrence

UCMR 5

2023 – 2025

Occurrence — about this data

The Unregulated Contaminant Monitoring Rule — the EPA program that tests public water systems for contaminants that are not yet federally regulated. Each cycle (UCMR 1 through 6, roughly every five years) targets a different shortlist; UCMR 5 focused on PFAS. The data informs whether the EPA later regulates those contaminants.

External databases

PubChem 74483 EPA CompTox DTXSID3031864 EPA code PFOS

External databases — what these mean

PubChem 74483: PubChem CID — the unique compound ID in the NIH's PubChem chemical database. Tap the pill to open the substance record: structure, physical properties, safety data, and links to related research.
EPA CompTox: The EPA's research-side chemical registry — the CompTox Chemicals Dashboard. Each substance gets a DTXSID identifier that links it to bioactivity data, exposure estimates, and regulatory programs.
EPA code: Internal EPA code used in SDWIS (the EPA's drinking-water records system) to identify a contaminant across compliance reports and violations. Useful for cross-referencing against state monitoring data.

Group

PFAS

Regulatory status

Primary

NPDWR — federally enforced MCL

Top removal tech

Activated carbon

Primary concern: Cancer (IARC Group 1, 2023)

Affected organ systems

Liver Reproductive Immune Developmental Endocrine Thyroid Kidney

Health effects

Carcinogen Endocrine disruptor Developmental toxin Immunotoxin Hepatotoxin Reproductive toxin

Exposure routes

Drinking water Dietary Occupational In utero Breast milk

Vulnerable populations

Pregnant Infants Children

Overview

Perfluorooctane sulfonate (PFOS) is an eight-carbon perfluoroalkyl sulfonic acid and one of the most widely used per- and polyfluoroalkyl substances (PFAS) of the 20th century. From the 1950s through the early 2000s, PFOS served as the active ingredient in 3M's Scotchgard line of stain- and water-resistant treatments for textiles, carpets, and paper, and as a key component of aqueous film-forming foam (AFFF) used by military and civilian fire departments. 3M voluntarily phased out PFOS production between 2000 and 2002 under EPA's PFOA Stewardship Program. PFOS persists indefinitely in the environment, biomagnifies through aquatic food chains, and has an estimated human serum elimination half-life of roughly 5.4 years — among the longest of any PFAS measured in humans.

Pollution sources

Industry Military AFFF Manufacturing Consumer products

Sources

PFOS enters drinking water predominantly through historical use of AFFF at military bases, civilian airports, and fire-training facilities; releases from 3M and other fluorochemical manufacturing sites; and runoff from landfills containing PFOS-treated consumer goods. Communities near former Naval Air Stations, Air Force bases, and industrial AFFF storage sites have documented some of the highest PFOS groundwater concentrations on record. Because PFOS resists degradation and binds strongly to organic matter, contamination plumes can persist for decades and migrate far from the original source.

Health risks

IARC classified PFOS as a Group 1 human carcinogen in 2023, citing sufficient evidence for kidney cancer (alongside PFOA). Epidemiological studies link PFOS exposure to elevated cholesterol, thyroid disease, ulcerative colitis, reduced antibody response to childhood vaccines, decreased birth weight, and pregnancy-induced hypertension. The 2024 EPA Human Health Toxicity Assessment set the lifetime drinking-water health advisory at 0.02 ppt, below current analytical reporting limits. PFOS bioaccumulates: serum levels in the general population have declined since the 3M phaseout but remain detectable in nearly 100% of U.S. adults sampled by NHANES.

Effective treatments

Activated carbon Reverse osmosis Ion exchange

Does not remove

UV Chlorination Boil

NSF certifications

NSF/ANSI 53 Health-related contaminants NSF/ANSI 58 Reverse osmosis systems NSF/ANSI 401 Emerging compounds / incidental contaminants NSF P473 PFOA / PFOS removal

Water treatment

PFOS is effectively removed by granular activated carbon (GAC), anion exchange resin, and high-pressure membrane processes including reverse osmosis and nanofiltration. PFOS is generally easier to remove than short-chain PFAS because its longer carbon chain adsorbs more strongly to GAC media. NSF/ANSI 53 and NSF P473 certify point-of-use filters specifically for PFOS reduction; NSF/ANSI 58 covers RO systems. Conventional treatment processes — coagulation, sand filtration, disinfection, UV, and ozonation — do not remove PFOS. Boiling water concentrates rather than removes it.

Gallery

Chemistry + classification data sourced from public databases (PubChem, EPA CompTox, IARC monographs); 3D molecular models from 3d.nih.gov where available.