Chemicals in the PFAS family, some of which have been linked to a range of negative health effects, have been found in drinking water in dozens of U.S. cities, according to a study released Wednesday by an environmental research and advocacy group.

PFAS, or per- and polyfluoroalkyl substances, are a group of industrial chemicals that have been used in consumer products such as nonstick cookware, waterproof fabrics and food packaging. Exposure to some of the oldest and most studied PFAS, including PFOA and PFOS, has been linked to high cholesterol, reduced immune response and an increased risk of some cancers.

Some kinds of PFAS chemicals are known as “forever chemicals” because they virtually never biodegrade.

Two samples collected in Brunswick County, North Carolina, and Quad Cities, Iowa, contained PFAS concentrations above the Environmental Protection Agency’s health advisory of 70 parts per trillion, according to the report from the Washington-D.C. based Environmental Working Group, which advocates for the regulation of toxic chemicals and pollutants

Researchers sampled drinking water at more than 40 locations in 31 states between May and December of 2019. Those samples were then analyzed by a third-party laboratory for 30 different chemicals in the PFAS family.

According to the EWG, PFAS contamination had “not been publicly reported by the Environmental Protection Agency or state environmental agencies” in 34 of the sampling locations where the chemicals were detected.

David Andrews, a senior scientist with the Environmental Working Group, said the results indicate that it’s “nearly impossible” to escape some degree of PFAS contamination in drinking water. He noted that the samples contained an average of six or seven of the 30 different PFAS for which analysts tested.

“Ultimately, we don’t know how many different PFAS compounds people are being exposed to,” Andrews said. “And I think that’s quite concerning.”

Researchers aren’t sure what specific health effects a person might experience as a result of drinking water that contains a combination of different PFAS. That’s in part because there are nearly 5,000 different chemicals in the PFAS family, and only a handful of them have been studied.

READ: The House just voted to regulate PFAS. Here’s what you need to know

Detlef Knappe, an environmental engineer with North Carolina State University who was not affiliated with the report, agrees that the study’s results suggest more PFAS chemicals are present in water systems than researchers originally thought, and that a larger number of people in the United States are exposed to the chemicals than has been estimated by previous studies.

But Knappe pointed out that PFAS concentrations in water sources can fluctuate over time. If a company dumps large amounts of contaminated water, for example, PFAS concentrations would spike until that wastewater disperses.

“A single sample can be grossly under or overestimating exposure that people have to PFAS through their drinking water,” Knappe said.

Knappe also noted that in 21 of the locations involved in this study, all of the reported concentrations of individual PFAS were estimates. That’s because even though the chemicals were detectable in each sample, they fell below the “limit of quantification.”

The limit of quantification is the number below which the exact amount of the compound can’t be precisely measured.

A 2017 data summary from the EPA analyzed drinking water samples that were collected across the country between 2013 and 2015 for six different PFAS in addition to other contaminants outside of the PFAS family.

That study used minimum reporting limits of 10 to 90 parts per trillion for the six specific compounds that researchers tested, a range the EWG argues obscured the full scope of PFAS contamination.

In 2019, the EWG proposed a maximum standard of one part per trillion for the sum all PFAS found in drinking water. While Knappe believes that standard is “a good goal,” he said it would be difficult to meet given the technical limitations of analyzing precise PFAS concentrations at very low levels.

Andrews said the results of Wednesday’s report “highlights the need for much more comprehensive testing” of PFAS in drinking water sources nationwide, and that that testing should feature lower detection limits.

“Knowing where there’s contaminated water [initiates] the process of evaluating the source water, identifying where the contamination is coming from and really stopping it,” Andrews said.

Correction: This story has been updated to reflect that in the 21 of the locations involved in this report, all of the reported concentrations of individual PFAS were estimates, not “some” as was previously stated.