The number of Americans exposed to potentially dangerous chemicals in their drinking water has been dramatically underestimated previously, according to a new study that has found toxic PFAS chemicals at dangerous levels in dozens of US cities.

The study’s release illustrates a concerning trend just as environmental regulation in the United States have faced a full-on assault during the Trump administration, and comes just as Donald Trump himself has claimed in a speech to world leaders in Davos, Switzerland, that his country has some of the best water in the world.

“The United States has among the cleanest air and drinking water on Earth,” Mr Trump said in Switzerland, repeating a contentious claim he has made before in spite of criticism he has faced for proposed budget cuts to the Environmental Protection Agency, alongside regulatory cuts seen as undermining water and air quality in spite of America's frequent high rankings among world drinking water sources.

The chemicals analysed in the new study are included in a group known as toxic “fluorinated” chemicals, and have been linked in studies to some forms of cancer and infertility. The new analysis of water samples by the Environmental Working Group (EWG) indicates that they have been found in larger concentrations across the country — including in Miami, Philadelphia, New Orleans and the suburbs of New York City.

The study suggests that the chemicals, which are often referred to as “forever chemicals” because they do not easily break down after being released in the environment, may be a much more widespread problem than previously thought, with 43 out of 44 cities across the country testing positive for high levels of the compounds.

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“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.

Mr Cook continued, noting the work of attorney Rob Bilott, whose work to expose the effects of the chemicals in West Virginia has recently been dramatised in the Mark Ruffalo film Dark Waters: “If not for Rob Bilott’s dogged work to expose the decades of deception by DuPont, 3M and other chemical companies, the scope of PFAS pollution in our water, air and food might very well be largely secret today.”

Earlier studies by the EPA and EWG did not indicate that PFAS chemicals were quite as widespread, but the new analysis looked at 30 such chemicals, instead of the two most common ones that are commonly tested for. There are thousands of PFAS chemicals, many of which are created as by-products from manufacturing things like Teflon coating or hazmat suits.

The EPA standards for PFAS chemicals suggests a limit of 70 parts-per-trillion, though some states including New Jersey have stricter limits. Last year, the EPA delayed a decision to implement further limits, saying that it would instead test large cities for their contamination rates.

“To date, EPA has developed methods to reliably detect 29 PFAS chemicals in drinking water,” a spokesperson for the agency told BuzzFeed in an emailed statement. “Aggressively addressing PFAS will continue to be an EPA priority in 2020 and we will provide additional information on our upcoming actions as it becomes available.

Some scientists have suggested that the newest study may not illuminate an increase in chemicals, and instead simply shows a greater ability to test for the chemical compounds in drinking water.

Since Mr Trump became president in early 2017, his administration has embarked on an effort to strip environmental protections in the United States, including through the repeal of former president Barack Obama's clean water regulations that placed limits on the chemicals that can be used near streams, wetland and other bodies of water.

The rollbacks have sparked alarm from environmentalists, and have been accompanied by efforts to undermine several other environmental protections, including rules to restrict pollution from coal-powered power plants, car tailpipes and methane emissions.