PFAS are synthetic chemicals found in an estimated 45% of U.S. tap water, according to a U.S. Geological Survey study. Short for per- and polyfluoroalkyl substances, they’re a group of thousands of human-made compounds built around chains of carbon and fluorine atoms. The carbon-fluorine bond is one of the strongest in chemistry, which means these chemicals resist virtually every natural process that would normally break them down: heat, water, sunlight, bacteria. That durability earned them the nickname “forever chemicals.”
Why PFAS End Up in Drinking Water
PFAS were designed to repel water, oil, and grease. Since the 1950s, manufacturers have used them in nonstick cookware, waterproof clothing, food packaging, stain-resistant fabrics, and dozens of other consumer products. When these products are manufactured, used, or thrown away, PFAS can leach into soil and eventually reach groundwater.
One of the biggest single sources of water contamination is firefighting foam, known as aqueous film-forming foam (AFFF). The U.S. military is the largest global user of AFFF, and drinking water contamination has been documented near more than 300 military bases where the foam was used for fire training and firefighting. AFFF manufactured by 3M before 2001 contained hundreds of PFAS compounds, with PFAS making up 1 to 5% of the foam by weight. At one studied site, AFFF was used routinely for fire training between 1970 and 1985, and contamination persisted decades later. Industrial facilities that manufacture or use PFAS, along with wastewater treatment plants and landfills, are other common pathways into local water supplies.
Long-Chain vs. Short-Chain PFAS
Not all PFAS are identical. Scientists classify them by the length of their carbon backbone. Long-chain varieties, like PFOA and PFOS, were the original workhorses of the industry. Most have been phased out and are now called legacy PFAS. Manufacturers replaced them with short-chain versions, which were marketed as safer because they don’t accumulate in the body as readily.
That framing is somewhat misleading. Short-chain PFAS do leave the body faster. The shortest varieties have estimated half-lives measured in weeks, while long-chain compounds like PFOS can linger in your blood for 3.4 to 5.7 years. But short-chain PFAS share similar toxicity profiles, and their fluorine-carrying carbon chains still don’t break down in the environment. One study of exposed workers found that long-chain PFAS accounted for 90% of total PFAS in their blood but only 50% of the contamination in their water supply, suggesting the body accumulates long-chain varieties far more aggressively. The slow elimination rates for even shorter-chain compounds remain a concern as their use increases.
Health Effects of PFAS Exposure
PFAS exposure has been linked to a wide range of health problems. The most consistent findings across epidemiological studies include immune suppression, elevated cholesterol, liver damage, and effects on fetal growth. Researchers have also examined connections to thyroid disruption, cardiovascular disease, reproductive problems, and certain cancers. A systematic review mapping 150 different PFAS compounds found that the most commonly studied health categories were metabolic effects (37 studies), endocrine and cardiovascular effects (30 each), female reproductive effects (27), and developmental effects (26).
Because PFAS persist in the body for years, even low-level chronic exposure adds up. The estimated half-life for PFOA in humans ranges from about 1.5 to 5 years. For PFHxS, another common compound, the half-life can stretch to 8.5 years. This means that after you stop being exposed, it can take years for your blood levels to drop by half, and full clearance takes far longer.
Federal Limits on PFAS in Drinking Water
In April 2024, the EPA finalized the first-ever national drinking water standard for PFAS. The rule sets maximum contaminant levels for PFOA and PFOS at 4.0 parts per trillion each. To put that number in perspective, one part per trillion is roughly equivalent to a single drop of water in 20 Olympic-sized swimming pools. The standard reflects how toxic these compounds are at extremely low concentrations.
Public water systems are required to monitor for these chemicals and reduce levels that exceed the limits. If your water comes from a private well, however, federal standards don’t apply, and testing is your responsibility.
How PFAS Are Removed From Water
Two treatment methods stand out for effectiveness. Granular activated carbon (GAC) filters work by trapping PFAS molecules as water passes through a bed of specially processed carbon. GAC can be 100% effective for a period of time, but performance depends on the type of carbon, the depth of the filter bed, flow rate, water temperature, and the specific PFAS compounds present. The carbon eventually becomes saturated and needs to be replaced.
Reverse osmosis pushes water through a membrane with pores small enough to block PFAS molecules. It is typically more than 90% effective at removing a wide range of PFAS, including the harder-to-capture short-chain varieties. Home reverse osmosis systems installed at the point of use (usually under the kitchen sink) can meaningfully reduce PFAS in your drinking water. Standard pitcher filters and refrigerator filters vary widely in effectiveness and are generally less reliable for PFAS removal unless specifically certified for it.
How to Test Your Water
If your home is on a public water system, your utility is now required to test for PFAS and report the results. You can request a copy of your system’s Consumer Confidence Report, which lists detected contaminants.
If you have a private well or simply want independent results, you can send a sample to a state-certified drinking water laboratory. The EPA maintains a directory of certified labs organized by state. Testing typically costs between $200 and $500 depending on how many PFAS compounds are analyzed. When collecting a sample, follow the lab’s instructions carefully, as contamination from everyday products (even certain clothing) can skew results.