The research was conducted in Veneto, Italy, one of several areas of the world where industrial use of PFAS has caused drinking water contamination and led to the chemicals accumulating in people’s blood. The mid-Ohio Valley, where a DuPont plant released the chemicals into the Ohio River, is another.

Researchers have already documented that PFOA and PFOS, two compounds in a class known as PFAS , reduce the fertility of male mice, rats, and rabbits. The new study, published by the Journal of Clinical Endocrinology and Metabolism, shows that young men exposed to the chemicals have a range of problems with their reproductive systems — and also lays out, for the first time, how these chemicals interfere with hormones inside the cell.

There’s a new reason to be concerned about toxic chemicals used in nonstick pans, waterproof products, and firefighting foam: PFOA and PFOS impair male reproductive health, according to a study released in early November.

The researchers compared male high school students who had been exposed to high levels of PFOA and PFOS in Veneto to young men who hadn’t been exposed and found that those in the exposed group had shorter penises, lower sperm counts, lower sperm mobility, and a reduction in “anogenital distance,” a measure that scientists see as a marker of reproductive health. The percentage of normally shaped sperm in the exposed group was just over half that in the control group.

The chemicals “have a substantial impact on human male health as they directly interfere with hormonal pathways potentially leading to male infertility,” the scientists concluded in the article, which was peer reviewed and published on November 6.

After observing the abnormalities in the young men, whose average age was 18, the researchers probed further to see how the chemicals might be affecting their reproductive development. Experiments they conducted in the laboratory show that PFOA and PFOS can bind with testosterone receptors inside the cell and disrupt the normal function of the hormone.

“The structures of testosterone and PFAS are very, very similar,” said Andrea Di Nisio, a biologist at the University of Padova and one of the paper’s 12 authors. “And we said, ‘They look the same, so maybe PFAS act like testosterone in the cell.’ And they did.”

The study was small, comparing just 212 exposed young men with 171 controls in this part of northern Italy. Yet with communities around the world regularly discovering that their water is contaminated with PFAS chemicals, the research has international implications for people who have been exposed to the contaminants over the past decades.

“As the first report on water contamination of PFCs goes back to 1977, the magnitude of the problem is alarming as it affects an entire generation of young individuals, from 1978 onwards,” the researchers wrote.

Indeed, the study suggests one possible answer to a phenomenon that’s been puzzling researchers for the past few years: a disturbing global decline in sperm counts. Over the past four decades, sperm counts in the U.S., Australia, Europe, and New Zealand have fallen more than 50 percent, according to a 2017 study conducted by researchers at the Mount Sinai School of Medicine and Hebrew University School of Public Health. The worldwide trend is largely seen as a mystery, though there is a growing recognition that environmental contaminants play a role.

“We have several other candidates” in addition to PFOS and PFOA, said Pete Myers, the founder and chief scientist of Environmental Health Sciences, who has written about the harms other chemicals, particularly the plastic additives phthalates and BPA, pose to men’s reproductive health. “The difference with these two PFAS chemicals is that they’re vastly more persistent.” PFOS and PFOA can stay in the human body for years and persist in the environment indefinitely.

The exposures to all of these environmental pollutants can have a cumulative effect, according to Myers. “Having PFOA and PFOS in there with phthalates and BPA, it adds up,” he said.

PFOA and PFOS have been linked to numerous other health problems, including reduced immune function, obesity, kidney cancer, testicular cancer, and increased cholesterol levels.

For the reproductive effects seen in Veneto, at least part of the harm likely occurred before the young men were born, according to Di Nisio, who has begun to think about how to prevent future developmental exposures to PFAS.

“We’re left with the question of how to remove PFAS from blood,” said Di Nisio. Because the chemicals are transferred from mothers to babies during pregnancy, “we have to find a way to eliminate them.” If we don’t, he said, the ongoing chemical exposures will set up even more boys for what may be a lifetime of reproductive difficulties.