PFOA and PFOS are both “forever chemicals,” but they’re not interchangeable terms. They come from different industries, have slightly different chemistry, and contaminated different parts of the country through different pathways. The EPA now regulates both at the same limit, 4 parts per trillion, for good reason.
What PFOA Is
PFOA stands for perfluorooctanoic acid. It’s an 8-carbon PFAS compound with a carboxylate group at one end. It was used primarily in the manufacturing of Teflon and other fluoropolymers. PFOA doesn’t become the non-stick coating itself, it acts as a processing aid during manufacturing at high heat.
DuPont used PFOA at its Washington Works facility in Parkersburg, West Virginia for decades. Internal documents showed the company knew about health concerns by the 1960s and did not disclose them. The resulting litigation, which involved tens of thousands of plaintiffs and eventually led to the film “Dark Waters,” is the most documented case of PFOA water contamination in the US.
DuPont phased out PFOA production by 2015 under EPA pressure. The company’s successor, Chemours, developed GenX (HFPO-DA) as a replacement. GenX is also regulated under the 2024 rule.
What PFOS Is
PFOS stands for perfluorooctane sulfonate. It’s also an 8-carbon compound, but with a sulfonate group instead of a carboxylate. That structural difference is why it’s classified separately even though the two compounds behave similarly in the body and the environment.
PFOS was the active ingredient in Scotchgard, 3M’s stain and water repellent for fabrics and carpets. It was also the key ingredient in AFFF, the aqueous film-forming foam used in military aviation, airports, and industrial firefighting operations. When AFFF is applied to a fire, it soaks into the ground and eventually reaches groundwater.
3M voluntarily phased out PFOS production in 2002 after EPA raised concerns. The US military continued using AFFF until 2024. PFOS plumes from military bases, including those in Colorado Springs, Tucson, and dozens of other locations, are among the most significant PFAS contamination sites in the country.
How They’re the Same
Both are long-chain PFAS. Both bioaccumulate in blood and tissue over time because the body can’t break down the carbon-fluorine bond. Both are extremely persistent in the environment, hence the “forever chemicals” label. Neither degrades meaningfully under normal environmental conditions.
Studies have associated both with the same health concerns: increased risk of kidney and testicular cancer, thyroid hormone disruption, reduced immune response (including reduced vaccine efficacy in children), and developmental effects. The evidence base comes from epidemiological studies of communities with known high exposure, not from controlled human experiments. Associational, not proven causal.
How They Differ
The practical difference is in their sources. If your water utility is near a Teflon or fluoropolymer manufacturing plant, PFOA contamination is the more likely finding. If you’re near a military base, airport, or industrial facility that used AFFF, PFOS is the more likely finding.
In terms of the regulatory framework, they’re treated equivalently. Both have an MCLG of zero. The MCLG is the health goal. A zero MCLG means the EPA has concluded that no exposure is without some level of associated risk. The MCL of 4 ppt reflects the most protective level that current treatment technology can reliably achieve.
The Shift to Shorter-Chain Replacements
When PFOA and PFOS were phased out, manufacturers shifted to shorter-chain PFAS, including PFBS, PFHxS, and GenX. The assumption was that shorter chains wouldn’t bioaccumulate as readily.
Research has complicated that assumption. Shorter-chain PFAS do appear to clear the body faster. But they’re also manufactured in much higher volumes, may be more water-soluble, and travel farther from contamination sites. The EPA’s 2024 rule covers PFNA, PFHxS, HFPO-DA (GenX), PFBA, and PFBS through the individual and mixture MCLs.
Testing for Both
If you’re testing your water for PFAS, make sure the test includes PFOA and PFOS at minimum. EPA Method 537.1 covers about 18 PFAS compounds including both. EPA Method 533 covers over 25 PFAS compounds, including shorter-chain compounds not covered under Method 537.1. The right test depends on what compounds are relevant in your area.
The EWG Tap Water Database shows which specific PFAS compounds your utility has detected. For private wells, mail-in water tests from certified labs are required. See best mail-in water tests for panel options that include PFAS.
For information on what actually removes both compounds, read how to remove PFAS from drinking water. And for background on the federal limits that now apply to both compounds, the EPA PFAS rule explainer covers the compliance timeline and what it means for your utility. More on PFAS generally.