Study: U.S. tap water PFAS may average twice official figures

Average levels of PFAS in U.S. drinking water may be about twice as high as official figures show, according to a study in Cell Reports Sustainability, because results below laboratory reporting limits are usually counted as zero. ScienceAlert reported the findings on Oct. 29.

Researchers analyzed about 1.86 million drinking water data points supplied by the Environmental Protection Agency. About 98 percent fell below the minimum reporting level, the lowest concentration a lab will report. Standard summaries treat those results as zero, ScienceAlert reported.

The team used computer models to estimate likely values below that level for 25 PFAS, based on the pattern of the results that were detected. For the six PFAS with federal limits under the EPA’s 2024 drinking water rule, the modeled average was 5.2 nanograms per liter, compared with 2.6 nanograms per liter when below-limit results were counted as zero. One nanogram per liter equals one part per trillion. The researchers say standard reports may understate the average by 56 percent.

“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 Nature World News.

The findings do not mean that water systems are violating federal limits. Accounting for the below-limit values pushed only a few dozen additional sampling locations over EPA benchmarks for PFOA and PFOS. The values are model estimates, not direct lab measurements, and the study does not measure health outcomes. Kharel said: “It does not establish that every below-limit result poses a health risk.”

The team released a tool, the PFAS Censoring-Aware Screening Tool, or PFAS-CAST, so water agencies and other research groups can apply the same approach to their own data. Kharel said the method gives water agencies and utilities a way to account for the difference. The researchers recommend more sensitive testing, which would remove the need to round down, and more extensive monitoring of water systems. They wrote that the approach could help direct limited resources to places where better evidence would have the greatest benefit.

The study arrives while the federal limits are being rewritten. The EPA’s 2024 rule set enforceable limits of 4 parts per trillion for PFOA and PFOS, along with limits for PFHxS, PFNA, GenX (HFPO-DA) and a combined hazard index for mixtures. In May 2026 the agency proposed two rules. One would keep the PFOA and PFOS limits but let qualifying systems request a two-year extension, moving the compliance date from April 26, 2029, to April 26, 2031. The other would rescind the limits for PFHxS, PFNA, GenX and the hazard index, according to Nature World News.

The comment periods on both proposals closed July 20, 2026. As of Oct. 30, Nature World News found no final rule on either, and the 2024 limits remain in effect until the EPA issues one.

PFAS are a large family of synthetic chemicals. ScienceAlert noted that they have been linked to faster aging, cancer spread, weaker bones and multiple sclerosis, but that cause and effect are difficult to prove. Neither ScienceAlert’s nor Nature World News’s report included a response from the EPA to the study.