“Virtually all Americans are exposed to these compounds,” said Xindi Hu, the study’s lead author. “They never break down. Once they are released into the environment, they are there.”

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As part of the study, which was published Tuesday in Environmental Science & Technology Letters, the researchers examined concentrations of six types of PFAS chemicals in drinking water supplies around the country. The data came from more than 36,000 samples collected by the Environmental Protection Agency between 2013 and 2015.

They also looked at sites where the chemicals are commonly found — industrial plants that use them in manufacturing, military bases and civilian airports where fire-fighting foam is used and wastewater treatment plants.

What they found: 194 of 4,864 water supplies across nearly three dozen states had detectable levels of the chemicals. Sixty-six of those water supplies, serving about six million people, had at least one sample that exceeded the EPA’s recommended safety limit of 70 parts per trillion for two types of chemicals — perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA).

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“It’s a big problem in a lot of communities,” said Richard Clapp, professor emeritus at Boston University’s school of public health. “It’s happening in a lot of places.”

From Decatur, Ala., to Merrimack, N.H., residents have been wrestling with high levels of the potentially harmful chemicals, and public officials have been scrambling to figure out how to prevent them from contaminating drinking water supplies.

The federal government does not currently regulate PFAS chemicals. But they are on the EPA’s list of “unregulated contaminants” that the agency monitors, with the goal of restricting those that most endanger public health. Partly because the rules that it must follow are complicated and contentious, officials have failed to successfully regulate any new contaminant in two decades.

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Only once since the 1990s has the EPA come close to imposing a new standard — for perchlorate, a chemical that sometimes occurs naturally but also is found in explosives, road flares and rocket fuel. It has turned up in the drinking water of over 16 million people.

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Joel Beauvais, who leads the EPA’s Office of Water, told the Post earlier this year that the system mandated by Congress demands the agency move deliberately. “It’s a rather intensive process to get one of these drinking-water regulations across the finish line,” he said.

There are reasons for that, Beauvais said at the time. A substance may occur in only a very small number of drinking-water systems or might occur only in extremely low levels. Before the EPA imposes new limitations on the nation’s water utilities, it has to prove that there is a meaningful opportunity to improve public health. “These are very consequential regulations,” Beauvais said. “They are consequential from a health perspective. They are consequential from an economic perspective.”

One of the agency’s approaches is to issue health advisories that can prompt state and local officials to take action or at least notify residents about contaminants. In May, it issued advisories for PFOS and PFOA, urging utilities around the country to follow more stringent guidelines than the EPA previously had recommended.

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In the wake of that advisory, at least one Alabama community declared its tap water unfit to drink and told residents to avoid it until officials could install a temporary, high-powered filter for the water supply. Some communities in New Hampshire received bottled water while authorities considered ways to address high levels of the contaminants in nearby groundwater. A company in upstate New York agreed to install carbon filters in private homes where high levels of the chemicals had been detected.

Clapp said that as evidence has mounted about the potential health risks posed by PFAS compounds and how ubiquitous they are, few people would argue that they should remain unregulated.

“We’re definitely overdue,” he said. “It’s not a question of whether, but rather at what level should they be regulated.”

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Separately on Tuesday, another Harvard-led study, published in Environmental Health Perspectives, examined the effect of PFAS exposure in about 600 adolescents from the Faroe Islands off the coast of Denmark. Individuals exposed to the substances at a young age displayed lower-than-expected levels of antibodies to tetanus and diphtheria after being immunized, raising the prospect that the chemical exposure could be reducing the effectiveness of childhood vaccines.