There are over 12,000 PFAS compounds, but the EPA's UCMR5 testing program tracks only 29. Of those, six are now regulated under federal drinking water limits: PFOA, PFOS, PFNA, PFHxS, PFBS, and HFPO-DA (GenX). These six account for most documented health risks, and in EPA's UCMR5 testing at least one PFAS was detected in about a third of US public water systems. This guide breaks down each compound – what it is, where it comes from, and what the science says about exposure.
When the EPA says there are "over 12,000 PFAS compounds," they're not exaggerating. Chemical manufacturers have created thousands of variations on the same basic structure: carbon chains studded with fluorine atoms that don't break down in the environment. That's why we call them forever chemicals.
But here's what matters for your drinking water: only 29 of those 12,000 compounds are tested for in the EPA's UCMR5 monitoring program, which ran from 2023-2025 and tested about 10,000 public water systems. Of those 29, six are covered by the EPA's April 2024 National Primary Drinking Water Regulation: five with individual limits and PFBS through a mixture Hazard Index.
In May 2025 the EPA announced it would rescind the limits for PFHxS, PFNA and HFPO-DA and the Hazard Index (which is how PFBS is covered), and in May 2026 it formally proposed doing so. The PFOA and PFOS limits of 4 parts per trillion (ppt) each stay, with compliance due in 2029 (2031 under a companion EPA proposal).
This guide covers the compounds you'll actually encounter in water quality reports. For each one, you'll get the full chemical name, common sources, documented health effects, current regulatory status, and how often it shows up in US water supplies.
PFOA is the most studied PFAS compound. DuPont used it for decades to make Teflon at its West Virginia plant, contaminating the Ohio River and local water supplies. In 2017 DuPont and Chemours agreed to pay $671 million to settle about 3,500 personal-injury lawsuits from residents.
In KYE's analysis of EPA UCMR5 data, PFOA was detected in 12.3% of tested water systems between 2023-2025. Some of the highest readings came from systems near historical fluorochemical sites, including Lubeck and Parkersburg near DuPont's former Washington Works plant in West Virginia (up to about 180 ppt), plus systems in Pennsylvania and New Jersey.
Sources: Teflon manufacturing, stain-resistant fabrics, food packaging with grease-resistant coatings, firefighting foam (AFFF) used at military bases and airports.
Health effects: The EPA classifies PFOA as a likely human carcinogen, citing human evidence for kidney and testicular cancer along with animal tumor data. Human epidemiological studies link PFOA exposure to high cholesterol, thyroid disease, pregnancy-induced hypertension, and ulcerative colitis. The compound has a half-life of 2-4 years in human blood, meaning it takes that long for your body to eliminate half of what you've absorbed.
Regulatory status: EPA limit of 4 ppt, finalized in 2024 with compliance due in 2029 (2031 under EPA's May 2026 proposal). It is one of the two PFAS limits the EPA plans to keep.
PFOS was the primary ingredient in 3M's Scotchgard fabric protector until the company voluntarily phased it out in 2002. But "phased out" doesn't mean gone – PFOS persists in soil, groundwater, and human blood decades after use stopped.
The EPA's UCMR5 data shows PFOS in 12.8% of tested systems, the most common of the two regulated PFAS (PFOA: 12.3%), though short-chain compounds like PFPeA (19.4%) and PFBA (18%) turn up more often. Areas near military bases and airports where firefighting foam was used are a common source of high levels.
Sources: Firefighting foam (AFFF), Scotchgard and similar fabric treatments, chrome plating operations, carpet treatments, food packaging.
Health effects: Similar to PFOA but with stronger evidence for immune system suppression. Research on Faroese children published in JAMA in 2012 linked a doubling of PFOS exposure to about 49% lower diphtheria vaccine antibody levels at age 7. Other documented effects include liver damage, thyroid disruption, and developmental delays. Half-life in human blood: 3-5 years.
Regulatory status: EPA limit of 4 ppt, finalized in 2024 with compliance due in 2029 (2031 under EPA's May 2026 proposal). The EPA plans to keep this limit.
PFNA is a nine-carbon PFAS used mainly as a processing aid in making PVDF fluoropolymers. It was phased out alongside PFOA under EPA's 2010/2015 PFOA Stewardship Program, but it persists in the environment.
EPA UCMR5 data detected PFNA in 0.8% of tested systems. Concentrations tend to be lower than PFOA or PFOS – typically 2-10 ppt where present – but the compound is highly persistent and accumulates in human tissue.
Sources: Non-stick cookware coatings, stain-resistant textiles, food packaging, industrial wastewater from fluorochemical plants.
Health effects: Animal studies link PFNA to liver toxicity, developmental problems, and immune suppression. CDC's national biomonitoring (NHANES) detects PFNA in the blood of nearly all Americans tested, though at lower concentrations than PFOA or PFOS. The compound crosses the placenta and has been measured in umbilical cord blood. Half-life: 2-4 years.
Regulatory status: The EPA finalized a 10 ppt limit in April 2024, then in May 2026 proposed rescinding it. Compliance was not due until 2029, so no federal PFNA limit is being enforced today.
PFHxS is a six-carbon compound used in chrome plating, firefighting foam, and stain-resistant treatments. It's structurally similar to PFOS but with a shorter carbon chain, which affects how it moves through the environment and human body.
Detected in 9.8% of systems in UCMR5 testing. PFHxS often co-occurs with PFOS because both were used in 3M's firefighting foam formulations.
Sources: Firefighting foam (AFFF), chrome plating facilities, stain-resistant fabric treatments, metal finishing operations.
Health effects: EPA's toxicity assessment links PFHxS to effects on thyroid function, cholesterol levels, and immune response. The compound is a particular concern in pregnancy: it crosses the placenta and has been detected in breast milk, though at lower concentrations than in maternal blood. Half-life: 5-8 years, making it one of the most persistent PFAS in human tissue.
Regulatory status: The EPA finalized a 10 ppt limit in April 2024, then in May 2026 proposed rescinding it. Compliance was not due until 2029, so no federal PFHxS limit is being enforced today.
PFBS is a four-carbon PFAS marketed as a "safer" alternative to PFOS because its shorter chain means faster elimination from the human body. 3M switched to PFBS in many products after phasing out PFOS in 2002.
But "safer" is relative. According to EPA UCMR5 data, PFBS was detected in 15.9% of systems, more often than PFOA and PFOS.
Sources: Chrome plating, firefighting foam (newer AFFF formulations), photography, hydraulic fluids, mist suppressants.
Health effects: PFBS has a much shorter half-life than PFOA or PFOS – roughly 30 days in humans instead of years. That sounds better until you realize continuous low-level exposure from drinking water means the compound never fully clears your system. Animal studies show thyroid effects and changes to cholesterol metabolism. Human health data is limited compared to longer-chain PFAS.
Regulatory status: PFBS never got its own limit. The 2024 rule counted it only inside the mixture Hazard Index, using a health-based value of 2,000 ppt (500 times the PFOA/PFOS limit), and in May 2026 the EPA proposed rescinding that Hazard Index. No federal PFBS limit is being enforced.
GenX is the trade name for a PFAS technology DuPont (later spun off as Chemours) introduced in 2009 as a PFOA replacement, made at its Fayetteville, North Carolina plant. The company claimed GenX was safer because it clears the body faster. Then NC State researchers reported in 2016 GenX in the Cape Fear River at up to 4,560 ppt, local reporting broke the story in 2017, and the "safer alternative" story collapsed.
In KYE's analysis of EPA UCMR5 data, GenX turned up in 0.4% of systems nationally (45 systems), concentrated in the Carolinas and the Ohio River valley.
Sources: Fluoropolymer manufacturing (Teflon alternatives), chrome plating, electronics production.
Health effects: GenX targets the liver. EPA's 2021 toxicity assessment identified liver effects in rodents as the most sensitive health endpoint. The compound also affects kidney function and causes developmental problems in animal studies. It clears the human body faster than PFOA, but daily exposure from drinking water keeps levels topped up.
Regulatory status: The EPA finalized a 10 ppt limit in April 2024, then in May 2026 proposed rescinding it. North Carolina set a non-enforceable health goal of 140 ppt in 2017. No federal GenX limit is being enforced today.
PFBA is a four-carbon compound used as a PFOA replacement in various industrial applications. Like PFBS, it's marketed as a short-chain "safer" alternative because it leaves the body faster.
Detected in 18% of systems in UCMR5 testing, usually at low concentrations (under 20 ppt). PFBA often appears alongside other PFAS as a mixture component rather than the primary contaminant.
Sources: Fluoropolymer production, ski wax, metal plating, electronics manufacturing.
Health effects: Less studied than longer-chain PFAS, but early research suggests liver and kidney effects similar to other short-chain compounds. Half-life: roughly 3-4 days, but continuous exposure from water means steady-state concentrations build up over time.
Regulatory status: Not currently regulated at the federal level. No proposed EPA limit.
The EPA's UCMR5 program tested for 29 PFAS compounds total. Beyond the seven covered above, a few of the remaining 22 are common, while most were rarely detected (KYE analysis of EPA UCMR5 data):
The full list matters for regulatory compliance and lab testing, but for understanding your personal exposure, focus on the six compounds with proposed or active EPA limits: PFOA, PFOS, PFNA, PFHxS, PFBS, and GenX.
Your water doesn't contain one PFAS. It contains several, usually at low levels individually but adding up to higher total exposure.
The EPA's 2024 rule addresses this with a Hazard Index for mixtures: (PFNA concentration ÷ PFNA limit) + (PFHxS concentration ÷ PFHxS limit) + (GenX concentration ÷ GenX limit) + (PFBS concentration ÷ PFBS limit). If the sum exceeds 1.0, the water violates the standard.
But with the EPA's May 2026 proposal to rescind the Hazard Index, mixture calculations are in limbo. If that proposal is finalized, water systems will only have to address PFOA and PFOS.
According to KYE's analysis of UCMR5 data, 68% of systems with detectable PFAS had multiple compounds present. The median was three different PFAS per contaminated system. Check your water quality by zip code to see which compounds show up in your area.
If you're on a public water system, your utility tested for these compounds between 2023-2025 under UCMR5. You can request results from your water provider or look them up in our database.
Once compliance begins (2029, or 2031 under EPA's May 2026 proposal), a water system whose running annual average of PFOA or PFOS exceeds 4 ppt must notify customers and reduce contamination. For the four compounds the EPA proposes to drop (PFNA, PFHxS, PFBS, GenX), there's no federal enforcement, though some states have their own limits.
If you're on a private well, you're not covered by EPA testing. You can pay for PFAS testing yourself – expect $300-600 for a full panel – or use our tool to see if nearby public systems have detected PFAS, which suggests possible groundwater contamination in your area.
For filtering PFAS at home, reverse osmosis and activated carbon systems remove 90%+ of most compounds. We cover filter options and effectiveness in our water filtration guide.
The EPA spent years developing these standards. Then in May 2025, while water utility groups were challenging the rule in court, the agency announced it would rescind four of the six limits, and in May 2026 it formally proposed doing so.
What happens next depends on that rulemaking, the litigation, and future administrations' stance on PFAS. The EPA plans to keep the PFOA and PFOS limits. The other four are in regulatory limbo.
For water utilities, that creates a planning problem – many already invested in treatment systems to meet the proposed limits. For families, it means checking your water quality now and not assuming federal limits will protect you. Four of these compounds may end up with no federal limit at all.
We track EPA policy changes and update our database as regulations shift. Get your detailed water quality PDF report to see current contamination levels and how they compare to both the 2024 EPA limits and stricter state standards where applicable.
What is the most common type of PFAS in drinking water? PFOS (perfluorooctane sulfonate) is the most frequently detected of the regulated PFAS in US drinking water, appearing in 12.8% of public water systems tested under EPA's UCMR5 program from 2023-2025. PFOA is second at 12.3% detection. Unregulated short-chain PFAS such as PFPeA (19.4%) and PFBA (18%) turn up more often. Both compounds were widely used in firefighting foam and industrial applications for decades before being phased out.
Are short-chain PFAS like PFBS safer than PFOA and PFOS? Short-chain PFAS like PFBS leave your body faster – weeks instead of years – but that doesn't make them safe. Continuous exposure from contaminated water means they build up to steady-state concentrations. Animal studies show short-chain PFAS still affect the liver, thyroid, and kidneys. The EPA's 2024 rule used a health-based value for PFBS that's 500 times more lenient than the PFOA limit, which tells you something about the uncertainty around its risks.
What is GenX and why is it controversial? GenX (HFPO-DA) is a PFAS compound DuPont (whose chemicals business became Chemours) introduced in 2009 as a supposedly safer replacement for PFOA. Researchers then found GenX in North Carolina's Cape Fear River at up to 4,560 parts per trillion, and the contamination became public in 2017. Studies now show GenX causes liver damage and developmental problems in animals. It's controversial because chemical companies marketed it as safe without adequate testing, repeating the same pattern that led to PFOA contamination.
How many different PFAS compounds can be in my water at once? EPA UCMR5 data shows that 68% of water systems with detected PFAS have more than one compound present. Your water might contain PFOA, PFOS, and PFNA together, each at levels below the individual limit but adding up to higher total exposure. That's why the EPA created a Hazard Index formula to account for mixtures, though in May 2026 the EPA proposed rescinding the Hazard Index and the PFNA, PFHxS and GenX limits, keeping only PFOA and PFOS.
Which PFAS compounds does the EPA regulate? The EPA's 2024 rule sets limits of 4 parts per trillion for PFOA and PFOS, with compliance due in 2029 (2031 under a May 2026 proposal). The same rule set limits for PFNA, PFHxS and GenX and a mixture Hazard Index that includes PFBS, but in May 2026 the EPA proposed rescinding those four. Some states have stricter limits than the federal government – check your state environmental agency for local regulations.
You can Check your water quality by zip code to see if your area is affected. If you're looking for solutions, Find water filters that remove PFAS certified for removal. For more details on what's in your tap water, Get a detailed water quality report for your area.
Check your water now. Enter your zip code at KnowYourExposure.com to see what contaminants have been detected in your local water supply – including PFAS, lead, and other regulated compounds.