PFAS

PFAS, the "forever chemicals", are near-indestructible industrial compounds that build up in groundwater, in drinking water and in people. The hazard is real — the WHO's cancer agency classes one of them as carcinogenic to humans, mainly on animal studies and mechanistic evidence from exposed people, and heavy exposure tracks with weaker vaccine responses and harm to babies — but the risk at everyday exposure levels is much less certain.

Find out whether your water supply has a PFAS problem. If it does, filter it, ideally with reverse osmosis; carbon filters remove a large but variable share.

What they are

PFAS (per- and polyfluoroalkyl substances) are carbon-fluorine compounds, used since the 1940s in non-stick cookware, water-repellent textiles and firefighting foams. The carbon-fluorine bond is so stable that they barely break down, so they accumulate in groundwater, in food chains and in human blood.^[1] The two best studied are PFOA (perfluorooctanoic acid) and PFOS (perfluorooctanesulfonic acid). In Europe, fish meat is among the largest dietary sources of exposure; see Fish and seafood.^[2]

Cancer: a hazard rating, not a risk estimate

In late 2023 the World Health Organization's cancer agency, the International Agency for Research on Cancer (IARC), put PFOA in its highest category, "carcinogenic to humans", up from "possibly carcinogenic" a decade earlier, and rated PFOS as possibly carcinogenic. That grades the hazard — whether it can cause cancer — not the risk at everyday exposures.^[3]

The human data behind it are thinner than the label suggests. In a heavily studied community living near a contaminated US chemical plant, testicular-cancer risk climbed across exposure quartiles, with a weaker gradient for kidney cancer that was not statistically significant.^[4] That is why the agency rated the human cancer evidence for PFOA only "limited" and rested its top classification on animal and mechanistic data instead. A commentary by three authors at Gradient, an industry-linked consulting firm, argues the panel overstated the case.^[5]

Immunity, babies and the heart

Vaccine response. In a Faroe Islands birth cohort, each doubling of a child's blood PFAS level went with roughly half the antibody level against tetanus and diphtheria vaccines. European regulators used this immune endpoint to set safe-intake limits.^[6]
Infants. A study of every New Hampshire birth from 2010 to 2019 compared mothers on public-supply wells downstream of PFAS-contaminated sites with those upstream, and found sharply higher infant mortality, very preterm birth and very low birth weight downstream. The authors priced the lifetime costs of those three outcomes at roughly $8 billion for each year's births across the contiguous United States — an infant-health figure extrapolated from 11 states, not a total cost of PFAS illness.^[7]
Other effects. Chronic exposure is also linked to thyroid disruption, abnormal blood lipids and reproductive harm.
Mortality. The general-population evidence is weak and partly withdrawn: the most-cited positive result, from a US national health survey, was retracted in 2025.^[8] The clearer signals come from heavily contaminated communities. In a Swedish register study of 46,553 adults in one town, those who had ever been supplied by its contaminated waterworks had about 15% higher cardiovascular mortality.^[9]

A 2022 review by the US National Academies of Sciences, Engineering, and Medicine found "sufficient evidence of an association" for weaker vaccine antibody responses, raised cholesterol, lower fetal growth and kidney cancer, and judged the cardiovascular evidence insufficient.^[10]

Much of the effect-size evidence comes from highly exposed populations that partly overlap with one another. At ordinary exposures, the dose-response is hard to detect in any one person; the population signal is consistent for immune effects, while for cancer the human evidence is limited.

Why they stay: half-lives of years

Pooled human estimates put the blood half-life at about 1.5–5 years for PFOA, 3.4–5.7 for PFOS and 2.8–8.5 for PFHxS (perfluorohexanesulfonic acid).^[11] Non-persistent chemicals such as phthalates clear in days to weeks; these take years to halve. That is why cutting intake, including filtering contaminated water, beats any attempt to flush them out.

The legal limits differ by a factor of 25

The US Environmental Protection Agency's 2024 drinking-water rule set enforceable limits of 4 nanograms per litre (ng/L, parts per trillion) each for PFOA and PFOS, with a health goal of zero, plus limits on four other PFAS. As of mid-2026 the agency has proposed keeping the 4 ng/L limits but letting water systems that apply for it push the compliance date from April 2029 to April 2031; systems granted the extension and measuring 12 ng/L or more would have to take interim measures meanwhile. It has separately proposed rescinding the limits on the four other compounds, on legal and procedural grounds rather than any new view of their health risk.^[12] Those four limits remain law for now: in January 2026 a federal appeals court refused to vacate them at the agency's request, and the case is undecided.

The European Union's limit, in force since January 2026, is 100 ng/L for the sum of 20 named PFAS, or alternatively 500 ng/L for all PFAS together, with no single-compound ceiling and no zero goal — roughly 25 times more permissive than the US limit on PFOA and PFOS individually.^[13]

What removes them from water

Reverse osmosis is highly effective against PFAS. Ultrafiltration, which keeps the minerals, lets PFAS straight through, because it removes particles rather than dissolved chemicals. Carbon pitcher filters remove a large but variable share: in one test of home filters, activated carbon removed roughly 60–70% of long-chain PFAS but only about 40% of short-chain ones, while every under-sink two-stage and reverse-osmosis filter removed nearly all of it. In four of eight whole-house carbon systems, PFAS levels in the water actually rose.^[14] The trade-offs of reverse osmosis, including what to do about the minerals it strips, are on Reverse osmosis; how PFAS fit with other household exposures is on Environmental toxins.

Getting them out of the body: possible, but not shown to help

Two removal methods have randomised-trial support, and both have only been shown to lower blood levels, not to improve health. In heavily exposed firefighters, blood or plasma donation lowered serum PFOS over a year; plasma donation was roughly three times as effective as whole blood and took about a quarter off a starting level near 11 ng/mL. The fire service that employed the participants funded the trial.^[15] Stronger are bile-acid-binding drugs (sequestrants): in adults from a contaminated area starting near 191 ng/mL, twelve weeks of cholestyramine cut serum PFOS by about 60%, and colesevelam by about 38% in a separate trial.^[16]^[17] These are prescription drugs with digestive and nutrient-absorption side effects: supervised options for the heavily exposed, not wellness routines.

Gut bacteria are an early lead. In the lab, nine species of human gut bacteria took up 25–74% of the PFAS in their growth medium within 24 hours, and mice colonised with a defined community of human gut bacteria had roughly two to eight times higher levels of one PFAS in their faeces than germ-free mice.^[18] Fibre might help the same way, by trapping PFAS secreted in bile so they leave in faeces. In a Canadian pilot, total PFAS fell in both groups but the long-chain PFAS of most concern fell significantly only with a fibre supplement,^[19] and a mouse test of oat fibre showed only non-significant trends.^[20] These are pilots and animal work, not proof that eating more fibre clears your PFAS load.

What a blood test can tell you

Serum PFAS tests are sold commercially. The National Academies' guidance uses three bands, all for the sum of seven named PFAS rather than any single compound: below 2 ng/mL, harm is not expected; 2 to under 20 ng/mL, effects are possible, especially in sensitive groups; 20 ng/mL or higher warrants additional clinical follow-up.^[21] A panel that reports only one compound cannot be compared with these bands. For most people the result doesn't change what to do.

What to do

Check your utility's water report, or test a private well, for PFAS.
If PFAS are present, filter: reverse osmosis with a remineralisation stage removes the most; a carbon filter is the cheaper, less reliable option. Don't count on an ultrafilter.
Don't read the cancer label as a personal risk number. It says PFOA can cause cancer, not how likely it is at the levels in your glass.
Don't buy a "detox" probiotic or cleanse. Eat fibre for its other benefits.
If you were heavily exposed (through work such as firefighting, or a contaminated water supply), blood testing and removal options are a matter for a doctor.