They’re in makeup, dental floss and menstrual products. They’re in nonstick pans and takeout food wrappers. Same with rain jackets and firefighting equipment, as well as pesticides and artificial turf on sports fields.

They’re PFAS: a class of man-made chemicals called per- and polyfluoroalkyl substances. They are also called “forever chemicals” because the bonds in their chemical compounds are so strong they don’t break down for hundreds to thousands of years, if at all.

They’re also in our water.

A new study of more than 45,000 water samples around the world found that about 31 percent of groundwater samples tested that weren’t near any obvious source of contamination had PFAS levels considered harmful to human health by the Environmental Protection Agency.

About 16 percent of surface water samples tested, which were also not near any known source, had similarly hazardous PFAS levels.

This finding “sets off alarm bells,” said Denis O’Carroll, a professor of civil and environmental engineering at the University of New South Wales and one of the authors of the study, which was published on Monday in Nature Geoscience. “Not just for PFAS, but also for all the other chemicals that we put out into the environment. We don’t necessarily know their long-term impacts to us or the ecosystem.”

High levels of exposure to some PFAS chemicals have been linked to higher cholesterol, liver and immune system damage, hypertension and pre-eclampsia during pregnancy, as well as kidney and testicular cancer.

The E.P.A. has proposed strict new drinking water limits for six types of PFAS and could announce its final rule as early as this week.

For their research, Dr. O’Carroll and his colleagues gathered nearly 300 previously published studies on PFAS in the environment. Together, these studies included 12,000 samples from surface water — streams, rivers, ponds and lakes — and 33,900 samples from groundwater wells, collected over the past 20 years. These samples don’t cover the whole planet: they are concentrated in places with more environmental researchers, like the United States, Canada, Europe, Australia and the Pacific Coast of Asia.

The samples are probably also concentrated in places where people were already concerned about PFAS contamination, Dr. O’Carroll said. He cautioned that, as a result, the findings of this new study might be skewed to show higher levels of contamination than a true global average would. There’s reason to believe, however, that there’s some level of PFAS contamination nearly everywhere on the planet, he said.

Of the countries where studies had been done, the United States and Australia had particularly high concentrations of PFAS in their water samples.

Among the available samples, the highest levels of contamination were generally found near places like airports and military bases, which routinely use PFAS-containing foam to practice fighting fires. About 60 to 70 percent of both groundwater and surface water samples near these types of facilities had PFAS levels exceeding the E.P.A. Hazard Index, which measures how hazardous mixtures of certain chemicals might be to human health, and also exceeded limits in the E.P.A.’s proposed new drinking water regulations.

This research does an admirable job of collecting the available data and highlighting the extent of global contamination from PFAS chemicals, said David Andrews, a senior scientist at the Environmental Working Group, a research and advocacy organization, who was not involved in this study.

Scientific research on the health effects of PFAS has evolved significantly in the past 10 to 20 years, he said, and what are considered safe exposure levels now are a tiny fraction of what they were a few decades ago.

The proposed E.P.A. drinking water rules, depending on their final language, will be a big step forward, he said.

Michael Regan, the E.P.A. administrator, has said his agency intends to require utilities to treat their water so that levels of some PFAS are near zero. This requirement would make the United States one of the strictest countries in terms of regulating PFAS in water.

Dr. Andrews added, however, that while treating drinking water is important, it doesn’t solve the whole problem. His own research has shown that PFAS chemicals are pervasive in wildlife, too.

“Once they’re released into the environment, it’s incredibly difficult to clean them up, if not impossible in many cases,” he said. “They can be removed from drinking water, but the ultimate solution is to not use them in the first place, especially in places where there are clear alternatives.”

For example, some outdoor clothing brands are moving away from PFAS for waterproofing their products and toward alternatives like silicones. Fast food restaurants can wrap their burgers in paper that’s been treated with heat to make it grease-resistant, or coated in a PFAS-free plastic instead. The Department of Defense is beginning to replace traditional firefighting foam with an alternative called fluorine-free foam, or F3.

In the meantime, Dr. O’Carroll said, “I’m not in any way trying to say that we should not be drinking water.” He added, “It’s more that I’m trying to say, from a societal point of view, we need to be careful what we put into the environment.”

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