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 appear in roughly 45% 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 roughly 6,000 public water systems. Of those 29, six are now subject to federal limits under the EPA's 2024 National Primary Drinking Water Regulation.
As of March 2026, the EPA is reconsidering limits on four of those six compounds – PFHxS, PFNA, PFBS, and HFPO-DA – following industry lawsuits and the new administration's regulatory review. PFOA and PFOS limits remain in effect at 4 parts per trillion (ppt) each.
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 eventually paying $671 million to settle a class action lawsuit with 3,500 residents.
According to EPA UCMR5 data analyzed by the Environmental Working Group, PFOA was detected in 17.3% of tested water systems between 2023-2025. The highest concentrations were in industrial areas with historical fluorochemical manufacturing – Michigan, New Jersey, and West Virginia.
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 based on animal studies showing kidney and testicular tumors. 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, effective 2024. This is one of the two PFAS compounds with limits that remain in effect as of March 2026.
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 24.1% of tested systems, making it the most commonly detected PFAS in US drinking water. Military bases and airports where firefighting foam was used show the highest levels. According to USGS research published in 2024, PFOS concentrations near former Air Force bases in California and Alaska exceeded 100 ppt – 25 times the new federal limit.
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. A 2020 study in Environmental Health Perspectives found that children with elevated PFOS exposure had 50% lower antibody responses to vaccines. 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, effective 2024. This limit remains in effect.
PFNA is a nine-carbon PFAS that replaced PFOA in some manufacturing after the 2006 PFOA phase-out. It shows up in non-stick cookware, water-resistant clothing, and food packaging.
EPA UCMR5 data detected PFNA in 9.2% 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. A 2018 CDC study found PFNA in the blood of 98% of Americans tested, though at lower concentrations than PFOA or PFOS. The compound crosses the placenta and has been measured in umbilical cord blood at levels similar to maternal blood. Half-life: 2-4 years.
Regulatory status: The EPA proposed a 10 ppt limit in 2024 but is reconsidering it as of March 2026. No federal enforceable limit is currently in effect.
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 11.8% 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: The EPA's 2024 toxicity assessment found PFHxS affects thyroid function, cholesterol levels, and immune response. The compound is particularly problematic for pregnant women – it crosses the placenta readily and has been detected in breast milk at higher 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 proposed a 9 ppt limit in 2024 but is reconsidering it as of March 2026. No federal enforceable limit is currently in effect.
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 7.4% of systems, often at higher concentrations than longer-chain PFAS because it's viewed as less regulated and therefore used more freely.
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, but a 2023 study in Environmental Science & Technology found PFBS in 85% of US adults tested.
Regulatory status: The EPA proposed a 2,000 ppt limit in 2024 – 500 times higher than the PFOA/PFOS limit – but is reconsidering even this lenient standard as of March 2026. No federal enforceable limit is currently in effect.
GenX is Chemours's trade name for a group of PFAS compounds it began using in 2009 as a PFOA replacement at its Fayetteville, North Carolina plant. The company claimed GenX was safer because it clears the body faster. Then in 2017, journalists discovered GenX contaminating the Cape Fear River at levels up to 6,000 ppt, and the "safer alternative" story collapsed.
EPA UCMR5 data shows GenX in 3.1% of systems nationally, but concentrated heavily in North Carolina, Delaware, and West Virginia – states with active fluorochemical manufacturing.
Sources: Fluoropolymer manufacturing (Teflon alternatives), chrome plating, electronics production.
Health effects: GenX targets the liver. A 2020 EPA toxicity study found liver damage in rats at exposure levels comparable to contaminated drinking water near the Fayetteville plant. The compound also affects kidney function and causes developmental problems in animal studies. Half-life in humans: roughly 2-3 months, faster than PFOA but not fast enough to prevent accumulation from daily exposure.
Regulatory status: The EPA proposed a 10 ppt limit in 2024 but is reconsidering it as of March 2026. North Carolina set a state limit of 140 ppt in 2019. No federal enforceable limit is currently in effect.
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 4.7% 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. A 2019 study in Environmental Health found PFBA in 62% of US blood samples tested, indicating widespread exposure. 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, the remaining 22 were rarely detected or found at very low concentrations:
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 March 2026 reconsideration removing limits on four of those compounds, mixture calculations are in limbo. Water systems are only required to address PFOA and PFOS for now.
According to UCMR5 data, 62% 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.
If any of the regulated compounds (PFOA or PFOS) exceed 4 ppt, your water system must notify customers and reduce contamination. For the four compounds under reconsideration (PFNA, PFHxS, PFBS, GenX), there's currently no federal enforcement action, 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 March 2026, the agency announced it was reconsidering four of the six limits following industry challenges and political pressure.
What happens next depends on litigation, industry negotiations, and the next administration's stance on PFAS. PFOA and PFOS limits remain in effect. 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. Some contamination is legal again, at least temporarily.
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 proposed 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 PFAS in US drinking water, appearing in 24.1% of public water systems tested under EPA's UCMR5 program from 2023-2025. PFOA is second at 17.3% detection. 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 proposed a limit for PFBS that's 500 times more lenient than for PFOA, 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 Chemours introduced in 2009 as a supposedly safer replacement for PFOA. Then in 2017, reporters discovered GenX contaminating North Carolina's Cape Fear River at levels up to 6,000 parts per trillion. 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 62% of contaminated water systems have multiple PFAS compounds present, with a median of three different compounds per system. 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 enforcement of mixture limits is currently uncertain.
Which PFAS compounds does the EPA regulate? As of March 2026, the EPA actively enforces limits on two PFAS compounds: PFOA and PFOS, both at 4 parts per trillion. The agency proposed limits on four additional compounds in 2024 (PFNA, PFHxS, PFBS, and GenX) but is reconsidering those standards following industry lawsuits. 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.