A study released Wednesday by an environmental watchdog group found heightened levels of potentially toxic chemicals in tap water supplies serving dozens of major American cities.

The report, published by the Environmental Working Group, found that 20 cities and regions nationwide – including Washington, D.C., Philadelphia, Miami and Louisville, Kentucky – contained PFAS levels of at least 10 parts per trillion. Forty-three areas, including New York City, Nashville, Las Vegas and Sacramento, had detectable PFAS at least 1 part per trillion.

Only one city, Meridian, Mississippi, which uses well water 700 feet below the surface, found no PFAS, while Tuscaloosa, Alabama and Seattle had levels lower than the 1 part per trillion limit advised by the EWG.

PFAS, per- and polyfluoroalkyl substances also known as "forever chemicals," have been linked to reproductive and developmental, liver and kidney, and immunological effects, as well as high cholesterol and obesity.

EWG's work expanded on data from an EPA program that ended in 2015, analyzing water samples using an EPA-approved independent laboratory for a larger set of PFAS compounds.

PFAS compounds remain in food packaging, cookware and other consumer products.

They are also still used in firefighting foam, though Wisconsin, New York, Washington, and Colorado are beginning to switch to non-PFAS foam, either due to action by state legislators or by self-regulation by individual fire departments. It must also be phased out in fire departments nationwide by 2024.

There's more to know, experts say:'Cancer-linked' chemicals in Chipotle, Sweetgreen packaging?

PFAS:White House, CDC feuding over study of toxic chemicals in drinking water

The nature of PFAS is "so stable," said Diana Aga, a professor of chemistry at the University at Buffalo, that it remains present in the natural environment long after products containing PFAS are phased out of use.

She's alarmed by the finding. "That's really scary, and I believe it," she told USA TODAY. "It's not unreasonable that they saw (these PFAS levels)."

“We don’t know how long these communities have been drinking PFAS-contaminated water, but we do know that these chemicals have been used and discharged all across the country for years,” said EWG President and co-founder Ken Cook in a statement.

"EWG endorses a health-based drinking water guideline of 1 (part per trillion) for all PFAS to be protective of children’s health," said co-author Sydney Evans, a science analyst with the EWG, in an emails to USA TODAY, citing research that suggests vaccines may be less effective on children with high levels of PFAS.

The EPA currently has a non-enforceable limit of PFAS levels at 70 parts per trillion, but has not yet established a "maximum contaminant level" that can be enforced by federal and local governments.

Aga, who is unaffiliated with the study, explained that most research into PFAS' effects are just associations, making it difficult to establish stricter regulation at a broader level.

"There may be positive associations between high levels of PFAS in the blood, but there's no direct evidence," she said to USA TODAY. "It's hard to make regulation when there's no direct link."

The highest concentrations, which exceeded the EPA's guidelines, were found in Brunswick County in North Carolina and in the Quad Cities region of Iowa and Illinois at rates surpassing 100 parts per trillion.

A bill that, if passed, would require the EPA to enforce a maximum contaminant level has passed the House is currently in committee in the Senate.

Five states – Massachusetts, Minnesota, Michigan, New Jersey and Vermont – have established maximum contaminant levels, while six others, including Washington, California and New York, have proposed regulations.

Notably, many states' maximum contaminant levels, including Minnesota's current regulations and New York's proposed limits, are far stricter than the EPA guidelines, hovering around the 10 to 20 parts per trillion mark.

An EPA spokesperson told USA TODAY that efforts to address PFAS are "active and ongoing," citing an action plan that would take "important steps" in the detecting and cutting down on PFAS.

The EPA announced last year new methods that increased the number of PFAS chemicals monitored in drinking water to 29.

"For a while, there was no acceptable method (to detect PFAS), and it's still limited," Aga said.

Per Reuters, a drafted report from an office of the U.S. Department of Health and Human Services in 2018 argued that the 70 parts per trillion threshold should be up to 10 times lower than what is recommended. The White House and the EPA allegedly tried to stop the publication of the report, Reuters reported.

"The more we learn, the lower these safety limits tend to go," EWG President Cook told reporters.

Follow Joshua Bote on Twitter: @joshua_bote