In-utero exposure to common air pollutants may lower semen quality and increase the risk of reproductive system disease in men, new research finds.

The peer-reviewed Rutgers University study looked at whether exposure to particulate matter called 2.5 (PM2.5) and nitrogen oxide may shorten the distance between the anus and genitals, or the anogenital distance, in developing fetuses and newborns.

Crucially, anogenital distance is a marker of reproductive health related to hormone levels, lower semen quality, fertility and reproductive disorders, and the research identified a likely link between it and exposure to the pollutants.

“When we see shorter anogenital lengths, it’s telling us there is lower testosterone activity in the womb … and it may have implications for fertility and reproductive health down the road,” said Emily Barrett, a biostatistics and epidemiology professor at the Rutgers School of Public Health, and the study’s lead author.

The findings come amid growing concern over global drops in semen quality, which have so far been tied to exposure to other toxins like PFAS and phthalates. Sperm concentration levels have dropped by 51% in recent decades, and the Rutgers study is among the first “to suggest that the air around is contributing to that, as well”, Barrett added.

PM2.5 is among the most common and well-studied air pollutants, and is linked to cancer and respiratory and circulatory disease. Among common sources are diesel exhaust, heavy industry emissions and wildfires, and the Environmental Protection Agency is poised to lower ambient air limits as evidence of its toxicity at smaller exposures becomes clearer.

Nitrogen oxide is a common toxin linked to cardiopulmonary disease, decreased lung function growth in children, asthma and other respiratory ailments. Among common sources are heavy industry like power plants, and traffic.

In animal studies, anogenital distance is used to determine developmental toxicity of pollutants – reduced distances are a sign that a toxic exposure is interfering with fetal testosterone production.

Researchers suspected that the same might hold true in humans and pulled anogenital distance data from The Infant Development and Environment Study (TIDES), an ongoing study of about 700 pregnant women and their children launched in 2010 in Minneapolis; Rochester, New York; San Francisco and Seattle. It tracks anogenital distance at birth in children, and at one year for boys.

The study compared TIDES data with air pollution levels in the residential neighborhoods where the study’s participants lived. Researchers identified a link between elevated PM2.5 exposure during the “male programming window” at the first trimester’s conclusion and anogenital distance.

The male fetus typically develops testosterone during this period, and that affects anogenital distance at birth.

“Testosterone is really important for the development of the male reproductive system, and anything that disrupts that normal testosterone surge during gestation has the potential to then have a cascade of effects that impacts all future reproductive development,” Barrett said.

The researchers also found a link between PM2.5 exposure during “mini puberty”, a period in early infancy when hormone production is high, and shorter anogenital distance in males at one year old.

PM2.5 may also carry other toxins, like cadmium and lead, that interfere with hormone production. Though the study didn’t include women, those women with longer anogenital distances are at higher risk of polycystic ovary syndrome, Barrett said.

The best way for pregnant people to protect their fetuses is to follow air quality advisories and stay indoors when pollution levels are high. N95 masks can be used outdoors, and furnace filters with a rating of MERV 13 are effective at reducing indoor air pollution.

Policymakers and regulators should also be doing more to rein in pollution and not leaving it to individuals to protect themselves, Barrett added.

“This is a public health issue that impacts all of us and there should really be a nationwide and worldwide effort to reduce air pollution,” she said.