You hear "PFAS" used as a single thing. It is not. It is a family of thousands of chemicals, and the regulatory and filtration story is different for each one. The water filter you bought may handle PFOA + PFOS but be silent on GenX. The news article you read about your local water may be talking about PFNA and not PFOA at all.
This is the field guide to the four PFAS chemicals that actually matter for drinking water in 2026: PFOA, PFOS, GenX, and PFNA. Plus a quick word on PFHxS, which is the fifth name on the EPA's regulated list.
PFOA — perfluorooctanoic acid
The original villain of the PFAS conversation. PFOA was used as a manufacturing surfactant in the production of PTFE (Teflon). Not as the coating itself — that is PTFE — but as a processing aid. It was the chemistry that DuPont was sued over in West Virginia in the early 2000s, and the chemistry phased out of US manufacturing under the EPA Stewardship Program (2010 commitment, 2015 deadline).
PFOA is no longer made in the US, but it persists in the environment. Soil, groundwater, and drinking water sources contaminated decades ago still contain it. The 2024 EPA rule sets the maximum contaminant level at 4 parts per trillion — one of the lowest drinking-water thresholds the EPA has ever set for any chemical.
PFOS — perfluorooctanesulfonic acid
PFOA's sibling. PFOS was used in firefighting foam (the kind sprayed on military bases and airports), stain-resistant treatments (Scotchgard was a major historical user), and as a surfactant in industrial processes. 3M voluntarily ceased PFOS production in 2002 — earlier than the PFOA phaseout — but the legacy contamination is widespread, particularly near military bases and fire-training facilities.
PFOS is regulated alongside PFOA at 4 parts per trillion under the EPA's 2024 rule.
GenX — HFPO-DA, the PFOA replacement that became a problem
When PFOA was phased out, the chemical industry needed a replacement processing aid for fluoropolymer manufacturing. The replacement was HFPO-DA, marketed as GenX. It was sold as the "shorter chain" alternative — fewer fluorinated carbons in the molecule, theoretically less persistent in the body.
Then the testing came in. GenX is itself a PFAS. It bioaccumulates. It contaminates water near the plants that produce it (the Cape Fear River near Wilmington, NC, became the GenX poster case). The EPA's 2024 rule regulates GenX at 10 parts per trillion as part of a Hazard Index alongside PFHxS and PFNA.
The pattern with GenX is the recurring PFAS pattern: replace one fluorinated chemical with another and discover the same problems on a slightly different timeline.
PFNA — perfluorononanoic acid
Another long-chain PFAS, similar to PFOA but with one more fluorinated carbon. Historically used in some fluoropolymer manufacturing and in stain-resistant coatings. Less consumer-product visibility than PFOA but real environmental presence. Regulated at 10 parts per trillion in the 2024 EPA rule.
PFHxS — perfluorohexanesulfonic acid
The fifth name in the regulated list. Shorter-chain than PFOS but in the same family. Also regulated at 10 parts per trillion in the Hazard Index group.
What your filter has to do
The EPA's 2024 final rule covers six compounds: PFOA, PFOS, PFHxS, PFNA, GenX (HFPO-DA), and a sixth via the Hazard Index. Different filters handle different subsets:
NSF/ANSI P473 certified filters handle PFOA and PFOS specifically. The protocol does not cover GenX or PFNA. So a P473 cert is necessary but not sufficient for the full EPA regulated list. Examples: Aquasana AQ-5300+, Brita Elite, Clearly Filtered.
Reverse osmosis filters handle the full PFAS class because they filter by molecule size rather than chemical affinity. Anything that does not pass through the membrane (most PFAS molecules are too large) is removed. Example: AquaTru.
Activated carbon block filters can handle PFOA and PFOS if certified to P473, but the GenX story is more variable. Some carbon filters reduce GenX, some do not — it depends on the carbon type and contact time.
The practical answer
If you want a filter that confidently handles the entire EPA-regulated PFAS list, reverse osmosis is the most reliable. P473-certified filters are excellent for the PFOA + PFOS pair specifically, but GenX and PFNA are not covered by that protocol.
If you are buying a filter today and you live near known industrial PFAS contamination (military base, fluoropolymer plant, fire-training facility), prioritize RO. If you live in an area with general background-level contamination and want a sensible improvement over tap, P473 is good enough.
For brand picks across both filtration types, see our water cornerstone and countertop guide.
