Average levels of "forever chemicals" in U.S. drinking water may be about twice as high as standard summaries suggest, according to a new study published in Cell Reports Sustainability. The reason is a routine habit in how test results are recorded, not a new contamination event.

When a water sample contains a PFAS chemical below a laboratory's minimum reporting level, the result is usually logged as zero. Across about 1.86 million drinking water data points supplied by the Environmental Protection Agency, roughly 98% fell below that reporting level. Treating all of those as zero pulls the national average down.

For households, the study does not mean your tap water suddenly became unsafe — it is about how numbers are averaged. It means the national picture of PFAS exposure may be underestimated, which affects how regulators, utilities and researchers judge the scale of the problem.

Below the Line Is Not the Same as Zero

PFAS, or per- and polyfluoroalkyl substances, are a large family of synthetic chemicals used in nonstick coatings, stain repellents, and firefighting foam. They break down extremely slowly and have been detected in water supplies across the country.

Labs can reliably measure PFAS only above a certain concentration. Below it, the chemical may still be present, but the lab cannot put an exact number on it. "When the levels of these chemicals are below our reporting limit, it's often treated as zero," said Gehendra Kharel, an environmental scientist at Texas Christian University, according to ScienceAlert.

The research team used computer models to estimate likely values below the reporting limit for 25 PFAS, based on the pattern of results that were detected. For the six PFAS that had federal drinking water limits under the EPA's 2024 rule, the average rose to 5.2 nanograms per liter using modeled values, compared with 2.6 nanograms per liter when below-limit results were counted as zero. One nanogram per liter is equal to one part per trillion.

The team also released a tool called PFAS-CAST, so water agencies can apply the same approach to their own data.

What the Study Does and Does Not Show

The authors are clear about the limits. This is a statistical study of how measurements are summarized, not a study of health outcomes. It does not show that individual water systems are violating federal limits, and it does not establish that every below-limit reading carries a health risk.

"It does not establish that every below-limit result poses a health risk," Kharel said.

Accounting for below-limit values pushed only a few dozen additional sampling locations over EPA benchmarks for PFOA and PFOS, the two most studied PFAS. That means most of the change shows up in national averages, not in a wave of newly failing utilities. The modeled values are also estimates. They are informed by real data, but no lab directly measured those concentrations.

Research has linked some PFAS to effects including certain cancers, immune changes and developmental problems, though proving cause and effect at low exposures is difficult. The researchers called for more sensitive lab methods and broader monitoring so fewer results fall into the unmeasured zone.

Federal Rules in Flux

The study arrives during a period of regulatory change. In April 2024, the EPA set the first national enforceable limits for six PFAS in drinking water, including 4 parts per trillion each for PFOA and PFOS, with a compliance deadline of 2029.

In May 2026, the EPA proposed two changes, according to Harvard Law School's Environmental and Energy Law Program. One would rescind the limits for PFHxS, PFNA, HFPO-DA (known as GenX) and a combined Hazard Index for mixtures. The other would keep the 4 ppt limits for PFOA and PFOS but let qualifying water systems apply for a two-year extension, moving the deadline from April 26, 2029, to April 26, 2031, Jones Day reported. Comment periods closed July 20, 2026.

Nature World News could not find a final rule on either proposal as of Oct. 30. Until a final rule is issued, the 2024 limits remain the standard on the books. The study's finding that averages may be understated adds a new data point to that debate, though it does not by itself resolve whether the four rescinded limits should stand.

Checking Your Own Water

The most useful step for most households is to look up their utility's annual water quality report, often called a Consumer Confidence Report. Utilities are required to publish it, and many now include PFAS sampling results. If a result is listed as "ND" for "not detected," that means below the reporting limit, not necessarily absent.

Households on private wells face a different situation. Well water is not covered by federal drinking water rules, and owners are responsible for testing. State environmental or health agencies can point well owners to certified labs that test for PFAS.

Water filters certified to reduce PFAS, such as some activated carbon and reverse osmosis systems, can lower levels at the tap. Look for third-party certification for PFAS reduction specifically — not all filters are rated for these chemicals. Boiling water does not remove PFAS.

There is no need to panic or switch to bottled water based on this study alone. Bottled water is not guaranteed to be PFAS-free, and the study did not identify any specific system as unsafe.

The bottom line: a common recordkeeping practice may be cutting apparent PFAS averages in U.S. tap water roughly in half. The finding sharpens the national picture without showing that individual systems are breaking federal limits, and it arrives as the EPA weighs scaling back parts of its 2024 rule.

What Readers Want to Know

What did the PFAS study find? Counting below-limit test results as zero may cut apparent national PFAS averages roughly in half. For six regulated PFAS, modeled averages were 5.2 ng/L versus 2.6 ng/L under standard reporting.

Does this mean my tap water is unsafe? Not necessarily. The study did not show that individual systems exceed federal limits, and only a few dozen additional sites crossed EPA benchmarks for PFOA and PFOS.

What does "not detected" mean on my water report? It means the result was below the lab's reporting limit. The chemical may be present at a very low level that cannot be precisely measured.

What is the EPA doing about PFAS limits? In May 2026, the EPA proposed keeping limits for PFOA and PFOS with a possible extension to 2031, while rescinding limits for four other PFAS. The 2024 rule remains on the books until a final rule is issued.

How can I reduce PFAS at home? Filters certified for PFAS reduction, including some carbon and reverse osmosis systems, can help. Boiling water does not remove PFAS.

Should private well owners be concerned? Well owners are responsible for their own