Conventional methods of reporting perfluoroalkyl and polyfluoroalkyl substances (PFAS), commonly known as forever chemicals, may underestimate their concentrations in US drinking water by more than half, according to a study published on October 23 in the Cell Press journal Cell Reports Sustainability.
Using a computer model based on nationwide monitoring data, researchers estimated that current reporting methods capture only 44% of the average concentration of 25 PFAS in community water systems. The discrepancy arises because measurements below minimum reporting levels are frequently treated as zero, even though the chemicals may still be present in the water.
“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.”
Why PFAS Reporting Methods May Underestimate Contamination
Under the Safe Drinking Water Act, the US Environmental Protection Agency (EPA) requires public water systems to monitor 29 PFAS in drinking water. These synthetic chemicals can persist in the environment for extended periods, and previous research has linked PFAS exposure to several health concerns, 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. An analysis by the Environmental Working Group suggests that approximately 176 million Americans use water from systems where at least one type of PFAS has been detected.
However, most individual PFAS test results in the EPA dataset fall below the agency’s minimum reporting levels (MRLs). These results are recorded as “<MRL”, indicating that the measured concentration falls below the level laboratories can generally measure and report reliably.
A result below the MRL does not establish that drinking water is free of PFAS or that it is safe. Nevertheless, these measurements are often treated as zero when agencies and researchers calculate average concentrations and summarise the prevalence of contamination.
This practice can lead to an incomplete picture of PFAS levels in drinking water, particularly when a large proportion of test results fall below reporting thresholds.
Analysing More Than 1.8 Million PFAS Measurements
To investigate how below-limit measurements affect estimates of contamination, Kharel and his team analysed more than 1.8 million PFAS measurement records from the national dataset published by the EPA.
The researchers found that 98% of PFAS test results were below the minimum reporting level. Because these measurements could not be treated as precise concentrations using the conventional reporting approach, the team used computer models to estimate possible values for records listed as “<MRL”.
The models generated 500 estimates for each measurement below the reporting limit. The researchers then averaged those estimates to obtain a best prediction of the possible concentration. They repeated this calculation for all 25 PFAS that had been detected at least once in the EPA dataset.
The resulting estimates covered 9,991 community water systems and provided a way to assess how much information might be lost when below-limit measurements are treated as zero.
The analysis found that conventional reporting captured only 44% of the model-estimated average concentration of the 25 PFAS. In other words, typical reports omitted more than half of the estimated total concentration.
Cell Reports Sustainability PFAS study
The researchers also examined six PFAS for which the EPA has established legally enforceable drinking water limits, including perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). These chemicals have been linked to serious health problems.
When measurements below reporting limits were treated as zero, the national average concentration for the six chemicals was approximately 2.6 nanograms per litre of drinking water.
However, the team’s model estimated that the actual average could be around 5.2 nanograms per litre, twice the conventionally reported figure.
The results suggest that standard reporting practices may obscure a substantial portion of the estimated PFAS concentration in drinking water. However, the researchers emphasised that their calculations do not demonstrate that every measurement below the reporting limit represents a health risk.
“Our study only examines how PFAS measurements are summarized,” says Kharel. “It does not establish that every below-limit result poses a health risk.”
What the Findings Mean for Drinking Water Monitoring
The researchers stressed that their findings are based on model-generated estimates rather than direct measurements of the actual concentrations in samples that fell below reporting limits.
Even so, the approach could help water agencies, utilities and researchers interpret environmental monitoring data more comprehensively. Accounting for potential concentrations below reporting thresholds may help identify locations where additional testing could provide a clearer understanding of PFAS contamination.
“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.”
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