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 United States drinking water by as much as half, finds a study publishing October 23 in the Cell Press journal 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, it's 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 EPA 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 dataset fall below the EPA's minimum reporting levels (MRLs) and thus are recorded as "
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 dataset published by the EPA. They found that 98% of PFAS test results were below the MRL.
Using computer models based on the dataset, the researchers then estimated possible concentrations for PFAS records listed as "
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 EPA 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 counting below-limit results as zero, the national average concentration is about 2.6 nanograms per liter in drinking water for the six chemicals. But the team's model estimates that, in reality, the average could be 5.2 nanograms per liter
"Our study only examines how PFAS measurements are summarized," says Kharel. "It does not establish that every below-limit result poses a health risk."
Although these values are model-generated predictions rather than measurements of actual concentrations below reporting limits, Kharel says the findings could help provide a more complete picture of PFAS levels in US drinking water and improve how environmental monitoring data are interpreted.
"When a water quality report says 'not detected,' that means below the reporting limit, not zero," says Kharel. "Our method gives water agencies and utilities a way to account for that difference, so they can see where more testing would help and be clearer about what they do and do not know."
