This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Common ways of reporting perfluoroalkyl and polyfluoroalkyl substances (PFAS)—the man-made, so-called forever chemicals that pose environmental and health threats—could underestimate the concentration of PFAS in U.S. drinking water by as much as half, according to a study being published Oct. 23 in Cell Reports Sustainability. Using a model based on data from across the country, the researchers estimate that by counting PFAS levels that fall below minimum reporting levels as zero, current reports capture only 44% of the average concentration of 25 PFAS in community water systems.
"When the levels of these chemicals are below our reporting limit, they're often treated as zero when we calculate how widespread they are," says Gehendra Kharel, first author of the study at Texas Christian University in Fort Worth. "We set out to understand how treating those results as zero changes the picture of PFAS occurrence in drinking water." Under the Safe Drinking Water Act, the Environmental Protection Agency (EPA) requires public water systems to monitor the levels of 29 PFAS in drinking water. These chemicals can persist in the environment over a long time, and previous research has linked PFAS exposure to many health issues, including cancer, liver damage and developmental problems in children.
In January 2026, the published a near-complete release of monitoring data from water systems across the country. While an analysis by the Environmental Working Group suggests that some 176 million Americans use water from systems where at least one type of PFAS was detected, most individual PFAS test results in the data set fall below the EPA's minimum reporting levels (MRLs) and thus are recorded as "< MRL." The MRL generally means a concentration that most laboratories can reliably measure and report, but it doesn't suggest that the water tested is PFAS-free or that it's safe, Kharel says. But these data points are often treated as zero when agencies and researchers summarize average PFAS levels in drinking water.
To investigate the PFAS values below the MRL, Kharel's team analyzed more than 1.8 million PFAS measurement records in the national data set published by the EPA. They found that 98% of PFAS test results were below the MRL. Using computer models based on the data set, the researchers then estimated possible concentrations for PFAS records listed as "< MRL." The models generated 500 estimates of what each below-limit value could be.
The team then averaged those estimates to come up with a best prediction. They performed this calculation for all 25 PFAS that had been detected at least once in the EPA's data set. They found that treating below-limit results as zero captured only 44% of the average concentration of the 25 PFAS that the models estimated across 9,991 community water systems.
The typical reports left out more than half of the estimated total. The has set legally enforceable limits covering six PFAS in drinking water, including perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), that have been linked to serious health problems. When below-limit results are counted as zero, the national average concentration is about 2.6 nanograms per liter in drinking water for the six chemicals.
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