It’s well known how a woman’s age can (roughly) predict her chances of giving birth to a healthy baby. Sometime around her mid- to late-30s, a woman’s chances of pregnancy complications arise, such as having a baby with a low birth weight, giving birth prematurely, and having a child with birth defects — and all of that assumes she didn’t have a miscarriage. But while all the attention is on women’s infertility, few people are talking about the 7.5 percent of American men who report seeing a doctor for their own infertility woes. And now, a new study from the National Institutes of Health (NIH) has found some contributing factors to their infertility.

It turns out that, just like women, men also become less fertile as they age, and it’s due in part to certain environmental factors and the man’s own health status. The researchers, who conducted their study in conjunction with Stanford University, found semen quality suffered among men who exerted themselves too much at work, had high blood pressure, or were taking multiple medications. Semen quality is based on the number, shape, and movement ability of sperm, among other factors that contribute to his ability to fertilize a woman’s egg.

“As men are having children later in life, the importance of diseases we once thought as separate from fertility must be re-explored,” said principal investigator Dr. Michael L. Eisenberg, director of male reproductive medicine and surgery at Stanford University, in a press release.

As per the study’s findings, it seems that the conditions men develop as they age, as well as environmental factors they’re exposed to, build over time into new issues like infertility. Previous research has found that overly intense exercise — or in this case, excessive work-related activity — produces higher levels of adrenal steroid hormones, which lead to testosterone deficiency. One of those adrenal steroids is cortisol, which is produced as a response to stress, thus making it possible this overexertion at work eventually leads to high blood pressure, and subsequently the administration of medications.

While it’s unclear how all of this is linked, the researchers did indeed find each one of them appears more often in infertile men. For the study, the researchers looked at the reproductive history, health, lifestyle, occupational activity, and sperm quality of 456 men with an average age of about 32 (mostly white and college educated) in committed relationships who had stopped using contraception. They found that 13 percent of men who reported heavy work-related activity had lower sperm counts, when compared to six percent of men who weren’t too active. Twenty-one percent of men diagnosed with high blood pressure, meanwhile, had lower sperm quality compared to 17 percent of men without high blood pressure, and when it came to those taking meds, sperm quality suffered as the number of medications rose.

Sperm counts typically range between 40 million and 300 million, according to the NIH. For this study, men with low sperm counts came in anywhere under 39 million. Previous studies have found that men’s fertility suffers as they age, too, and that their children’s risk of health problems also increases. Beyond the aforementioned factors, smoking, having an STD, cirrhosis, exposure to environmental toxins, sickle cell disease, and malnutrition have all been shown to decrease an aging man’s fertility.

“The good news is that these factors, if they are confirmed to have negative effects on male fertility, can potentially be modified by medical care or changing job-related behaviors,” said the NIH’s Dr. Germaine Buck Louis, senior author and director of the Division of Intramural Population Health Research at the Eunice Kennedy Shriver National Institute of Child Health and Human Development, in the press release. “We look forward to additional research in this area.”

Source: Eisenberg M, Chen Z, Ye A, Buck Louis GM. Relationship between physical occupational exposures and health on semen quality: data from the Longitudinal Investigation of Fertility and the Environment (LIFE) Study. Fertility and Sterility. 2015.