PFAS in drinking water: a plain-English guide

What PFAS are

PFAS stands for per- and polyfluoroalkyl substances. They are synthetic chemicals built around chains of carbon atoms bonded to fluorine, one of the strongest bonds in organic chemistry. That bond is what gives PFAS their useful properties (resistance to water, oil, heat, and staining) and also what makes them persist. Bacteria, sunlight, and treatment plants struggle to break them apart. The nickname “forever chemicals” comes from this persistence.

PFAS have been used since the 1940s in non-stick cookware (Teflon), stain-resistant fabrics and carpets, water-repellent clothing, food packaging (microwave popcorn bags, fast-food wrappers), cosmetics, firefighting foam used at airports and military bases, and dozens of industrial processes. The family now contains more than 12,000 named chemicals.

Why they matter for health

Decades of epidemiology, animal studies, and occupational research link PFAS exposure to:

• Higher cholesterol
• Reduced immune response to vaccines, particularly in children
• Increased risk of kidney and testicular cancer
• Thyroid disease
• Decreased birth weight
• Pregnancy-induced hypertension and preeclampsia
• Liver damage at high exposure

The strongest evidence is for the long-chain PFAS that humans have been exposed to the longest: PFOA and PFOS. Shorter-chain replacements (PFBA, PFBS, PFHxA, and others) were introduced as substitutes when major manufacturers phased out PFOA and PFOS in the 2000s. Toxicology research on the replacements is younger and the evidence base is thinner, but emerging data suggests the replacements share many of the same biological effects, and some clear the body faster while others do not.

Most American adults have detectable PFAS in their blood. The CDC’s National Health and Nutrition Examination Survey has measured population PFAS levels since 1999. Levels of the phased-out PFOA and PFOS have declined; levels of newer replacements are being tracked.

EPA’s 2024 drinking-water rule, and the 2025 rescission

In April 2024, EPA finalized the first national legally enforceable drinking-water standards (MCLs) for five PFAS, plus a Hazard Index for mixtures of four substances:

PFAS	MCL	MCLG	Status (May 2026)
PFOA	4.0 ng/L	0	In effect; retained
PFOS	4.0 ng/L	0	In effect; retained
PFHxS	10 ng/L	10	Rescission announced May 2025; still on the books pending rulemaking
PFNA	10 ng/L	10	Rescission announced May 2025; still on the books pending rulemaking
HFPO-DA (GenX)	10 ng/L	10	Rescission announced May 2025; still on the books pending rulemaking
Hazard Index of PFHxS, PFNA, HFPO-DA, PFBS	≤ 1	n/a	Rescission announced May 2025; still on the books pending rulemaking

The unit ng/L (nanograms per liter) equals parts per trillion (ppt). One ng/L is one drop in 20 Olympic swimming pools. EPA chose 4 ppt for PFOA and PFOS because the science indicates risk at any measurable level (MCLG is zero) and 4 ppt is roughly the practical limit of routine laboratory methods.

On May 14, 2025, EPA announced its intent to rescind the rules for PFHxS, PFNA, HFPO-DA, and the Hazard Index, on procedural grounds under the Safe Drinking Water Act. The PFOA and PFOS rules were explicitly retained. As of this writing, the rescission has been announced but the rules have not yet been formally withdrawn; utilities continue to monitor for all five PFAS plus the Hazard Index components.

Public water systems have until 2027 to begin compliance monitoring and until 2029 to install treatment if their water exceeds these limits.

The other PFAS your utility may have tested

Under the fifth Unregulated Contaminant Monitoring Rule (UCMR 5), every public water system serving more than 3,300 people was required to test for 29 PFAS between 2023 and 2025. Five of those are the regulated set above. The remaining 24 are unregulated, meaning EPA has not set a legally enforceable maximum level. “Unregulated” does not mean “unimportant.” It means EPA is gathering nationwide occurrence data so a future rulemaking can decide whether a limit is warranted.

Some of the unregulated PFAS your test may show:

PFBA (Perfluorobutanoic acid)

A short-chain replacement for PFOA. Used in industrial processes and as a breakdown product of other PFAS. Has a shorter half-life in the human body than PFOA (days, not years) but is widely detected. Minnesota set a state health-based value of 7,000 ng/L. No federal MCL.

PFBS (Perfluorobutanesulfonic acid)

A short-chain replacement for PFOS, marketed as a “safer alternative” when major manufacturers phased out PFOS in the early 2000s. Subsequent research has documented thyroid, kidney, and reproductive effects. Included in EPA’s Hazard Index mixture rule but does not have a standalone MCL. State reference values range from 600 ng/L (Minnesota) to 2,000 ng/L (EPA Health Advisory Level for chronic exposure).

PFHpA (Perfluoroheptanoic acid)

A seven-carbon PFAS that sits between long-chain PFOA and shorter replacements. Not currently regulated by EPA. Some state programs treat it as a proxy for general PFAS contamination because it co-occurs with PFOA in many sources. Maine includes PFHpA in its state-level combined PFAS standard (20 ng/L for six PFAS together).

PFHxA (Perfluorohexanoic acid)

A six-carbon replacement for PFOA. Clears the body faster than long-chain PFAS but is highly mobile in water and very widespread. The American Chemistry Council has actively contested broader regulation; several states (Maine, Vermont, New Jersey) include it in state-level monitoring without yet setting enforceable limits.

PFPeA (Perfluoropentanoic acid)

A five-carbon PFAS, often detected alongside PFHxA in surface water and public water systems. Less studied than PFOA or PFOS. No federal limit; limited state action.

EPA has signaled that several of these (notably PFBS and PFHxA) are candidates for future regulation as the toxicology evidence matures.

What to do if your utility shows PFAS

1. Read your utility’s most recent Consumer Confidence Report. It will list any PFAS detected and any actions the utility is taking. The link is on the water-system page above.
2. Talk to your clinician, especially if anyone in your household is pregnant, nursing, or has a young child. PFAS blood testing is available; whether it is useful in your specific situation is a medical decision.
3. Consider point-of-use filtration. The two filter technologies with the strongest published evidence for PFAS removal are:
   • Activated carbon block filters (under-sink or pitcher style), rated to NSF/ANSI 53 for PFOA/PFOS or NSF/ANSI 58 for reverse osmosis. Verify the certification list, not just the marketing.
   • Reverse osmosis (RO) systems, which remove a broader range of PFAS than carbon alone, including short-chain PFAS that pass through some carbon filters. Cheap pitcher filters that say “reduces chlorine taste” generally do not remove PFAS. Look for explicit PFAS or PFOA/PFOS certification.
4. Boiling does not remove PFAS. Boiling concentrates them by evaporating water.

This page is general information about a contaminant family and does not diagnose or treat any individual. Specific concerns about your exposure or your family’s exposure are a conversation with your physician, not a web page.

Sources

• US EPA, “Per- and Polyfluoroalkyl Substances (PFAS) Final Rule”: https://www.epa.gov/sdwa/and-polyfluoroalkyl-substances-pfas
• US EPA, “Fifth Unregulated Contaminant Monitoring Rule (UCMR 5)”: https://www.epa.gov/dwucmr
• ATSDR Toxicological Profile for Perfluoroalkyls (2021): https://www.atsdr.cdc.gov/toxprofiles/tp200.pdf
• National Academies of Sciences, “Guidance on PFAS Exposure, Testing, and Clinical Follow-Up” (2022): https://nap.nationalacademies.org/catalog/26156
• CDC NHANES PFAS biomonitoring data: https://www.cdc.gov/exposurereport/