PFOS (perfluorooctane sulfonate) is a synthetic chemical that contaminates drinking water supplies across the United States, affecting an estimated 158 million people. It belongs to a larger family of chemicals called PFAS, often nicknamed “forever chemicals” because they don’t break down naturally in the environment. PFOS is one of the most well-studied and most commonly detected PFAS compounds in tap water.
Why PFOS Doesn’t Break Down
PFOS is built around a chain of eight carbon atoms, each bonded to fluorine. The carbon-fluorine bond is one of the strongest in organic chemistry, which is why PFOS resists heat, water, oil, and biological degradation. It doesn’t decompose in sunlight, doesn’t get broken down by soil bacteria, and passes through conventional water treatment largely intact. This persistence is what earned PFAS the “forever chemical” label.
That same durability made PFOS commercially useful for decades. It was a key ingredient in stain-resistant coatings for furniture, carpet, and paper products, as well as in industrial processes. Although production has mostly stopped over the last 15 years and PFOS is now listed under the Stockholm Convention on Persistent Organic Chemicals, the compound lingers in soil, groundwater, and surface water from past use.
How PFOS Gets Into Drinking Water
The single biggest source of PFOS in groundwater is aqueous film-forming foam, known as AFFF. This firefighting foam was designed to smother fuel and chemical fires, and it was used extensively at military bases, airports, oil refineries, and fire training facilities. When sprayed, AFFF soaks into the ground and carries PFOS directly into the water table. Decades of repeated use at these sites created contamination plumes that can stretch for miles.
Industrial manufacturing is the other major pathway. Factories that produced PFOS or used it in coatings, electronics, or metal plating discharged it into waterways or let it seep into surrounding soil. Even after a facility shuts down, PFOS remains in the sediment and continues leaching into nearby water sources. During fires, products containing PFAS can also break down and release PFOS into the air, soil, and water.
How Widespread the Contamination Is
EPA testing data show that more than 158 million Americans have detectable PFAS in their drinking water. That’s nearly half the country’s population, and the number is likely an undercount since only 57 percent of water systems had reported full results at the time of the data release. Among those living in states without their own PFAS regulations, roughly 53 million people were exposed to levels above current federal limits.
Contamination tends to cluster near military installations, airports, and industrial zones where AFFF or PFOS-containing products were used heavily. But because PFOS travels easily through groundwater, communities miles downstream from the original source can also be affected.
What PFOS Does in Your Body
Once you ingest PFOS through drinking water, your body absorbs it efficiently but eliminates it extremely slowly. The average half-life of PFOS in human blood is about 5.4 years, meaning it takes over five years for your body to clear just half of a single dose. This happens because your kidneys reabsorb PFOS instead of filtering it out into urine, a quirk of human biology that makes our elimination rate far slower than most other species.
That slow clearance means PFOS accumulates with repeated exposure. Even low daily doses from tap water build up over months and years. PFOS exposure has been linked to effects across multiple body systems: liver damage, elevated cholesterol, immune suppression (including reduced vaccine response), reproductive problems, and developmental effects in children. The National Cancer Institute notes associations with hepatic, cardiovascular, immune, reproductive, and developmental harm in both human and animal studies.
Current Regulatory Limits
The EPA has set a legally enforceable maximum contaminant level for PFOS in drinking water, and water systems across the country are now required to test for it and treat it if levels exceed that threshold. The World Health Organization has proposed a provisional guideline of 100 nanograms per liter for PFOS and its close relative PFOA, but WHO itself has acknowledged that this value is based on what’s technically achievable rather than what’s fully health-protective. Multiple agencies, including Health Canada and the U.S. EPA, have set stricter limits.
The WHO’s own background document notes that its provisional number “should not be compared to health-based values established by other agencies” and encourages countries to adopt stricter standards when possible. In practice, the U.S. standard is among the most stringent in the world.
How Water Systems Remove PFOS
Three treatment technologies are proven to remove PFOS from drinking water effectively. Each works differently, and municipal systems often use them in combination.
- Granular activated carbon (GAC) adsorbs PFOS onto the surface of carbon particles. It can achieve 100 percent removal for a period of time, and it works especially well on longer-chain compounds like PFOS. Effectiveness depends on bed depth, flow rate, water temperature, and how much other organic matter is present. The carbon eventually becomes saturated and needs to be replaced or regenerated.
- Ion exchange resins use specially designed beads that swap harmless ions for PFOS molecules. Like GAC, ion exchange can remove 100 percent of PFOS for a time before the resin needs regeneration. It tends to last longer than carbon before breakthrough occurs.
- Reverse osmosis pushes water through a membrane with pores small enough to block PFOS and most other PFAS. It’s typically more than 90 percent effective across a wide range of PFAS, including shorter-chain compounds that GAC and ion exchange struggle with. The tradeoff is higher energy use and water waste.
Testing Your Water at Home
Public water systems are required to test for PFOS using EPA Method 533 or Method 537.1. These laboratory techniques use advanced mass spectrometry to detect 29 different PFAS compounds at extremely low concentrations in both groundwater and surface water sources. Your water utility’s results should be available in annual consumer confidence reports or through the EPA’s online databases.
If you’re on a private well, you’ll need to arrange testing yourself through a certified lab. Standard home water test kits don’t measure PFAS. Look for a lab that specifically offers EPA Method 533 or 537.1 analysis.
Choosing a Home Water Filter
Not all water filters remove PFOS. Basic pitcher filters and faucet-mounted models vary widely in their ability to handle PFAS. The most reliable way to verify a filter’s performance is to check for third-party certification. Look on the product label or packaging for certification under NSF/ANSI 53 (for carbon-based filters) or NSF/ANSI 58 (for reverse osmosis systems), specifically for PFAS reduction. These certifications mean the filter has been independently tested and verified to reduce PFAS to below specified thresholds.
Under-sink reverse osmosis systems are generally the most effective home option, capturing over 90 percent of PFAS including both long-chain and short-chain varieties. Whole-house carbon filtration systems can also work well for PFOS specifically, but they require regular filter replacement to maintain performance. Whichever system you choose, following the manufacturer’s replacement schedule matters. A saturated filter stops removing PFOS entirely.