Men’s sperm counts have plummeted by up to 60% over the last 40 years in Western countries and by nearly 30% since 2001 in China. Experts lack firm answers regarding the cause of the sperm deficit but suspect that behaviors such as smoking or exposures to hormone-disrupting compounds in plastics or pesticides are to blame. A handful of papers have questioned whether air pollution could also affect semen quality. Now, a new study links sulfur dioxide emitted from burning fossil fuels to depressed sperm count and concentration in Chinese men.

“Infertility is a global public health issue affecting at least 50 million couples worldwide,” says Yuewei Liu, an environmental epidemiologist at Hubei Provincial Center for Disease Control & Prevention. Data suggest that poor semen quality accounts for 90% of male infertility. Impaired semen clearly interferes with conception, but it is also often an indicator of other health problems.

While most research has focused on risky behaviors and commercial chemicals as potential causes, experts have recently suggested that air pollutants might damage sperm quality. However, studies on air pollution and semen quality have been inconsistent due to inaccurate measures of an individual’s exposure to pollutants and insufficient sample sizes.

So Liu and his team decided to study semen samples collected from 1,759 men in Wuhan, China. They had all visited Tongji Hospital from 2013 to 2015 seeking help to conceive a child with their partners. The researchers measured sperm concentration, total sperm, and total motile sperm in each sample, controlling for factors that might affect semen quality such as age and smoking. Then the scientists drew on government data from nine air quality monitoring stations in Wuhan—a transportation hub and manufacturing powerhouse—to estimate exposure to air pollutants such as sulfur dioxide, nitrogen dioxide, carbon monoxide, and ozone. Liu employed a model that analyzed the location of the monitoring stations in relationship to each man’s home to predict individual daily pollutant exposures. Because human sperm develops over 90 days, the researchers calculated pollution exposures for the 90 days prior to semen collection so they could look at key periods of sperm development.

When Liu and the team used a statistical test to rate semen quality against increasing air pollution, they found no impact from NO 2 , CO or O 3 . However, for each 10 µg/m3 increase in SO 2 exposure during the first stage of sperm development, sperm concentration dropped by 6.5%, total sperm count by 11.3%, and total motile sperm by 13.2%, Liu says. Levels of SO 2 during the later stages of sperm development did not appear to impact sperm quality. The annual mean SO 2 concentrations in Wuhan during the study period ranged from a high of 33 µg/m3 in 2013 to 18 µg/m3 in 2015. In the U.S., annual mean SO 2 concentration was less than 5 µg/m3 in 2013.

“Our results indicate for the first time that SO 2 exposure may lower semen quality by affecting the earliest stage of sperm development, 70 to 90 days before ejaculation,” Liu says. He speculates that SO 2 could impair sperm by triggering oxidative stress and damage to DNA. “Given the limited evidence from epidemiological and in vivo studies, further studies are needed to confirm the association of NO 2 , CO and O 3 with semen quality,” Liu adds. He recommends caution in generalizing the findings to other populations since the men were all from one city in China.

“Even though the study was limited to one city, this paper adds evidence to the existing literature showing a downward trend in sperm concentration and count with increasing exposure to air pollution,” says Bénédicte Jacquemin, an epidemiologist at the health research institute ISGlobal in Barcelona. The study’s findings are more applicable to countries such as China and India where SO 2 pollution is severe, she notes. “Exposure to SO 2 might not be the cause of decline in sperm quality in North America and Europe, as regulations have lowered SO 2 levels for a couple of decades now.”

This article is reproduced with permission from C&EN (© American Chemical Society). The article was first published on October 25, 2017.