New data shows that rainwater in some parts of the US contains high enough levels of potentially toxic per- and polyfluoroalkyl substances (PFAS) to possibly affect human health and may, if found in drinking water, in some cases be high enough to trigger regulatory action.

PFAS chemicals appear in an array of everyday items, such as food packaging, clothing and carpeting. Chemicals in this family are the subject of the film Dark Water, which chronicles the real-life efforts of a lawyer seeking to hold a polluting factory to account in West Virginia.

Estimates pin the number of different PFAS variants at more than 4,700 but federal regulations so far target only two of them: PFOS and PFOA. Some of these chemicals have been known to cause serious health issues such as cancer, and immune system and thyroid problems.

Previously it was known that there is widespread PFAS contamination of the nation’s lakes, rivers and groundwater reserves but until recently, researchers were largely in the dark as to whether this family of chemicals could also be ubiquitous in rain.

“There were folks not too long ago who felt the atmospheric transport route was not too important,” says Martin Shafer, principal researcher with the National Atmospheric Deposition Program (NADP), based at the University of Wisconsin-Madison. “The data belies that statement.”

During the spring and summer of this year, Shafer and his fellow researchers looked at 37 rainwater samples taken over a week from 30 different sites predominantly near the east coast, though as far afield as Alabama and Washington. They found that each sample contained at least one of the 36 different compounds being studied.

While total PFAS concentrations were generally less than 1 nanogram per liter (ng/l), the highest total concentration was nearly 5.5 ng/l in a single sample from Massachusetts. Several samples contained total PFAS levels at or about 4 ng/l.

The Environmental Protection Agency (EPA) has established a health advisory level of 70 ng/l for combined PFOS and PFOA in drinking water. But many states have either proposed or already set significantly lower drinking water standards. Wisconsin, for example, has proposed a preventative action limit of 2 ng/l for combined PFOS and PFOA.

Shafer says he suspects PFAS chemicals are entering rainwater through a variety of avenues, like direct industrial emissions and evaporation from PFAS-laden fire-fighting foams. Still, “there’s a dearth of knowledge about what’s supporting the atmospheric concentrations and ultimately deposition of PFAS”, he says.

This isn’t the only recent such study of rainwater in the US. Last year, the North Carolina division of air quality began testing near the Chemours facility, which produces the PFAS GenX, and in February of this year, detected levels of GenX in rainwater higher than 500 ng/l. The state subsequently gave the plant the go-ahead to install equipment to significantly reduce air emissions.

More than a localized problem, PFAS compounds “are transported significant distances”, explains Brooks Avery, a professor of chemistry at the University of North Carolina Wilmington, which is conducting its own study of PFAS in air and rainwater. Indeed, these compounds have appeared in the Arctic and in populations of Alaska natives.

According to Linda Birnbaum, former director of the National Institute of Environmental Health Sciences, more rainwater research is needed, as well as on “what we are inhaling and ingesti[ng] from house and office dust” contaminated by PFAS. These issues have “not been looked at until very recently”, she says.