Sperm counts are dropping among men across the world, according to research published in the academic journal Human Reproduction Update.
Researchers called their findings a "significant public health concern."
They note that sperm count is not only an indicator of human fertility.
"It also is an indicator of men's health, with low levels being associated with increased risk of chronic disease, testicular cancer and a decreased lifespan," the researchers said in a press release.
Professor Hagai Levine of Hebrew University of Jerusalem’s Hadassah Braun School of Public Health led the study.
He believes he knows what is causing the lower sperm counts. He said it has to do with “lifestyle choices and chemicals in the environment."
The study notes that sperm count declines have accelerated in developed parts of the world, including North America, Europe and Australia.
“Our findings serve as a canary in a coal mine. We have a serious problem on our hands that, if not mitigated, could threaten mankind’s survival," Levine warns.
The study's authors are calling for global action to promote healthier environments.