Five years ago, a study describing a precipitous decline in sperm counts sparked extreme concerns that humanity was on the path to extinction. Now a new study shows that sperm counts have fallen further and the rate of decline is speeding up, raising fears of a looming global fertility crisis.
The initial study, published in July 2017, revealed that sperm counts—the number of sperm in a single ejaculate—plummeted by more than 50 percent among men in North America, Europe, Australia, and New Zealand between 1973 and 2011. Since then, a team led by the same researchers has explored what has happened in the last 10 years. In a new meta-analysis, which appears today in the journal Human Reproduction Update, researchers analyzed studies of semen samples published between 2014 and 2019 and added this to their previous data. The newer studies have a more global perspective and involved semen samples from 14,233 men, including some from South and Central America, Africa, and Asia. The upshot: Not only has the decline in total sperm counts continued—reaching a drop of 62 percent—but the decline per year has doubled since 2000.
The 2017 report also revealed that sperm concentration (the number of sperm per milliliter of semen) dropped by an average of 1.6 percent per year, totaling more than a 52 percent among men in these regions over the previous four decades.
“The decline is not tapering off—it’s steep and significant,” says study co-author Shanna Swan, a reproductive and environmental epidemiologist at the Icahn School of Medicine at Mount Sinai in New York City. “Overall the drop is similar in magnitude but when we look at recent years, we see that it’s speeding up.”
Study lead author Hagai Levine, a medical epidemiologist at Hebrew University of Jerusalem’s Hadassah Braun School of Public Health, calls the results “worrisome as we were hoping that at some point the decline would be leveling off. The opposite may be true, and we may cross a tipping point when most men will be sub-fertile or when the causes of this decline will also manifest by other adverse health trends.”
Contrary to common perception, infertility impacts men and women equally, says Amy E.T. Sparks, a reproductive physiologist and director of the IVF and Andrology Laboratories at the University of Iowa Center for Advanced Reproductive Health. “I think the perception that infertility is primarily a woman’s problem may be due to the tendency for women to initially seek medical care for infertility rather than men.” In the scientific community, the prevailing view is that male and female fertility challenges are each responsible for about one-third of infertility cases; the remaining cases are due to a combination of male and female factors.
But the new data suggests a “substantial increase in the proportion of men with low sperm counts which leads to a reduced capability for fertilizing their partners,” says David M. Kristensen, a molecular toxicologist at Roskilde University and Copenhagen University Hospital in Denmark who was not involved in the study. “This is of concern for not only the families that are affected but also for societies in general, as many countries such as Italy and Japan are already suffering from shrinking populations.”
Beyond reproductive matters, there’s also a concern that reduced sperm counts are associated with a variety of health problems in men. “There is an association between semen quality and overall health—studies suggest that impaired semen quality is associated with a higher risk of testicular cancer, cardiovascular disease, and [premature] mortality,” notes Michael Eisenberg, director of male reproductive medicine and surgery and a professor of urology at the Stanford University School of Medicine, who was not involved in either meta-analysis.
“One can view the decline in sperm counts as a biomarker for male health in general,” says Kristensen.
In fact, a study in a 2018 issue of the journal Andrology found a higher risk of hospitalization among men who had lower sperm concentrations. Those with sperm concentrations below 15 million/mL—considered low—had a 53 percent greater risk of being hospitalized for any reason over the course of 36 years than those with more robust sperm concentrations between 51 and 100 million/mL. This effect persisted even after the researchers controlled for body weight, smoking, and other factors.
It’s important to note that the decline in sperm counts isn’t happening in a vacuum. Low sperm count often goes hand in hand with low testosterone levels and changes in male genital development while in the womb, says Swan, author of the book Count Down: How Our Modern World Is Threatening Sperm Counts, Altering Male and Female Reproductive Development, and Imperiling the Future of the Human Race.
In a man, the production of sperm requires a certain level of testosterone as well as the testes’ ability to regulate the temperature of the tissue in which sperm are made, Sparks explains. “Levels of testosterone have been reported to be declining during the same period of time that the sperm production rates were measured in this meta-analysis.”
It’s also important to recognize that it’s not just a matter of what a man is exposed during his lifetime that can affect his sperm quality. What an expectant mother is exposed to while she’s pregnant can affect the sperm concentrations of her male offspring: During early pregnancy—what’s called the “reproductive programming window”—certain environmental chemicals can affect women in ways that could permanently alter the reproductive development of their male babies, Swan explains. “Whatever disruption to reproductive development that occurs in utero is permanent,” she says.
By contrast, the damage done to a man’s sperm during his life—such as by smoking or being exposed to pesticides—can be reversed if the exposure to the harmful chemical stops. It takes about 75 days for sperm to mature, says Swan, which means that men essentially have regular opportunities for a do-over for their sperm quality every two and a half months.
What’s driving the decline?
Neither the 2017 nor the 2022 meta-analyses examined what is causing the drop in sperm count, but other research suggests environmental and lifestyle factors may be to blame. These include exposure to endocrine-disrupting chemicals (which mimic or interfere with the body’s hormones), smoking, and obesity. For example, a study in a 2022 issue of the journal Toxicology found that occupational exposure to pesticides was associated with sperm found in lower concentrations, sperm that were poor swimmers, and sperm with more DNA damage. And a study in a 2019 issue of the journal Human Reproduction found that men who are overweight tend to have reduced sperm concentration, lower total sperm count, and fewer motile sperm.
The fact that the sperm count decline is also occurring in countries in South and Central America, Africa, and Asia, according to the new meta-analysis, suggests that the lifestyle factors and environmental exposures that are likely to blame are present globally, Swan says.
As for what’s accelerating the sperm count decline found in the new meta-analysis, no one knows for sure. Levine suggests it may be due to “mixture effects” with chemicals—meaning that when various individual chemicals are added together in the environment, they can have a bigger, more detrimental impact by magnifying each other’s negative effects. Or, he says, the decline may result from “cumulative exposure over time.”
Given that the latest meta-analysis included data from 50 years, Swan suspects the acceleration stems from the cumulative impact of environmental chemicals over generations. Remember: While in the womb, a male fetus is exposed to the same chemicals and lifestyle factors—such as poor diet, smoking, and obesity—that his mother is exposed to while she’s pregnant. But the transmission of these exposures doesn’t stop there—the epigenetic effects of these exposures may be transmitted from one generation to the next, not just from the mother but possibly from the father too. It may be due to factors in the father’s sperm that disrupt the reproductive development of male fetuses in the womb, Levine notes.
As more generations are exposed to these environmental chemicals and harmful lifestyle factors over time, the effects may become additive.
A wake-up call
More research needs to be done to determine what is tanking sperm counts. In the meantime, men and women can try to protect their reproductive health—by consuming a healthy diet, exercising regularly, maintaining a healthy body weight, and avoiding smoking—all behaviors Eisenberg advises his male patients to adopt.
Swan also recommends reducing exposure to endocrine-disrupting chemicals by being a savvy consumer. These chemicals include: phthalates (in plastics and personal-care products such as nail polishes, shampoos, and hair sprays), bisphenol A (in hard plastics, adhesives, and the lining of some food cans), flame retardants (in furniture and carpets), perfluoroalkyl substances (in nonstick cookware and stain-resistant carpets), and pesticides (in plant-based foods and lawn-care products).
Ultimately, Levine and Swan say that local and global actions are needed to reduce or get rid of these chemicals in our environments. “We should find ways to prevent further decline and even reverse the trends,” Levine says. “We must avoid being complacent about it and fool ourselves that assisted reproduction is the solution.”