From 1973 to 2018, sperm concentration globally declined by more than half, according to a new analysis by an international team of researchers, published in the Human Reproduction Update journal on Tuesday. Meanwhile, total sperm counts declined 62.3% during the same period.

“Overall, we’re seeing a significant worldwide decline in sperm counts of over 50% in the past 46 years, a decline that has accelerated in recent years,” the lead researcher and professor at the Hebrew University of Jerusalem’s Hadassah Braun School of Public Health, Hagai Levine, said in a statement. 

A 2017 paper, from the same researchers, found that a decline in sperm counts was accelerating in North America, Europe, Australia, and New Zealand based on samples collected between 1973 and 2011.  

The more recent study, to build on previous findings, looked at South America, Asia, and Africa to understand the decline in sperm counts across the world. Using samples from over 57,000 men across 53 countries, combined with statistics used in prior studies, researchers found that men in those regions have also experienced a significant decline in sperm count as those in the previous study. 

Researchers found that sperm concentration has fallen 1.16% each year since 1972. However, when researchers looked at data from after 2000, they found the annual decline was 2.64%.

“Our findings serve as a canary in a coal mine,” Levine said. “We have a serious problem on our hands that, if not mitigated, could threaten mankind’s survival. We urgently call for global action to promote healthier environments for all species and reduce exposures and behaviors that threaten our reproductive health.”

The analysis does not examine the causes behind the decline in sperm concentration and count, but Levine said that “lifestyle choices and chemicals in the environment are adversely affecting this fetal development.”

The study comes on the same day the world’s population hit 8 billion people, according to the UN’s estimate. But all signs point to a slowing of the overall population growth rate. It took 12 years for the world’s population to increase from 7 billion to 8 billion, but it’s expected to take another 15 years for it to reach 9 billion. 

Meanwhile, the UN says fertility has fallen in several countries in recent decades. And the “cumulative effect of lower fertility, if maintained over several decades, could be a more substantial deceleration of global population growth in the second half of the century,” the UN’s director of the population division said in a statement. 

In the analysis, researchers note that sperm count is an “imperfect proxy for fertility,” but sperm concentration and count are closely linked to fertility chances. 

“On a population level, the drop in mean [sperm concentration] from 104 to 49 million/ml that we report here implies a substantial increase in the proportion of men with delayed time to conception,” researchers wrote in the study. “Thus, [sperm concentration] provides the most stable and reliable measurement for comparisons within and among populations and over time.”