Per- and polyfluoroalkyl substances, or PFAS, are a class of synthetic compounds used in a variety of industrial processes and found in dozens of household items. The newest research suggests these chemicals are also accumulating in drinking water sources in cities across the United States.

Previous studies have linked PFAS with a variety of health problems, including cancer and high cholesterol. Research suggests PFAS compounds can disrupt the development of a fetus and depress the efficacy of vaccines.

"These compounds don't break down," Evans said. "Once they're there, they're there forever. That's why we call them 'forever chemicals.' And they're toxic at very low levels over long periods of time."

Previous efforts to test for PFAS in water samples were limited in their scope. Earlier efforts also weren't able to identify PFAS compounds at very low levels. The latest tests were able to identify PFAS compounds at concentrations as low as 1 part per trillion, the equivalent of a drop or two of water in a body the size of three Olympic swimming pools.

Under pressure from the EPA, the two most infamous PFAS compounds, PFOA and PFOS, were phased out of production by major chemical companies like Dupont, but research suggests many other PFAS compounds -- including compounds designed to be safe replacements for PFOA and PFOS -- are similarly toxic.

"The more we look at the newer ones, we're seeing the same kinds of toxicity," Evans said.

There are a variety of factors, including their longevity, that make PFAS especially problematic.

"Most pollutants stick to sediment or dissolve in oils, but because PFAS are water soluble, they spread out and travel great distances," Rick Rediske, professor at the Annis Water Resources Institute in Michigan, told UPI.

"The most significant factor, from a toxicology standpoint, is that certain ones stick to proteins in our blood," Rediske said. "DDT and pesticides go in to our fat. Lead goes into our bones. Mercury goes into muscle. Because PFAS are carried around in our blood and aren't discarded, they naturally concentrate over time. And they attach to the proteins that carry antibodies, cholesterol and hormones, that's why you get so many different health effects caused by these compounds."

The latest EWG study tested for 30 different PFAS compounds, but there are currently 600 PFAS compounds currently being used by various industries. Most of these aren't well studied, so their potential toxicities aren't understood.

Part of the problem is that researchers working to study the safety of these compounds and their prevalence in the environment can't keep up with their production by industrial sources, who are not required to report the development of new PFAS compounds and are reluctant to share trade secrets.

In Europe, PFAS compounds are banned -- not so in the U.S.

"There are no federal limitations on their use, on their discharge or on their presence in drinking water," Evans said.

Evans, Rediske and other concerned scientists want to see the chemicals regulated as a class.

"If we can classify these as hazardous substances and mandate that their use and discharge are reported, then we will at least have a starting point of knowing what to test for," Evans said.

While PFAS compounds are found in dozens of products, Rediske said the prevalence of the most toxic of the known PFAS compounds in drinking water are there as a result of the use of flame-retardant foams -- particularly those used to put out aviation fires.

Previous testing has found extremely high levels of PFAS in water samples collected near U.S. Navy and Air Force bases.

"People tend not to care about cleaning up after there is an aviation accident," Rediske said. "They're just trying to put out the fire and save people's lives."

Some municipalities are currently working to remove PFAS from their local water systems using granulated activated charcoal, but such filtration systems are currently mandated -- and they're not cheap or easily scalable.

"It's very effective, but it doesn't work on some of the smaller compounds," Rediske said. "There still are PFAS compounds that are in the water that are smaller and thought to be less toxic but we really don't know."

Evans said in addition to pushing for the EPA to regulate PFAS and regulate the compounds as an entire class, she and her colleagues will continue to advocate for more comprehensive testing. While the latest survey was the most comprehensive to date, she said the technologies used to test for PFAS continue to improve.

"The laboratory technologies are expanding, and the testing needs to keep up with that," Evans said.