Poor quality of air could be one of the reasons why there is a drop in human fertility, say researchers. Fertility issues, especially in men, have spiked over the last few decades.

Scientists from the Chinese University of Hong Kong have now found that certain type of particles in the air that are less than 2.5 microns – one millionth of a metre – in size are impacting human sperm production, reports Science Alert. While this reported effect is small, it could potentially affect a couple's chances of conceiving.

The team studied sperm donated by 6,475 subjects living in Taiwan to understand the effect of PM2.5 – particulate matter at 2.5 microns – on the health of human sperm.

Between the years 2001 and 2014, donors' sperm were studied for their health as part of a routine medical examination. Using this data, the researchers looked at the air quality around the area where each person lived based on the date when they donated their sperm.

The samples were taken over a period of two years, notes the report, three months apart to match with sperm cycles that normally range between 40 to 100 days.

They found that with every increase of five micrograms in particulate matter (PM) per cubic metre, there was an associated one percent drop in proportion of normal to abnormal sperm. "Given the ubiquity of exposure to air pollution, a small effect size of PM2.5 on sperm normal morphology may result in a significant number of couples with infertility," said Dr Xiang Qian Lao of the Chinese University of Hong Kong, one of the researchers.

The effect remained even after discounting for alcohol and smoking, say the researchers. Coming home to PM 2.5 was putting men to the bottom 10 percent of sperm concentration norms. "Although the effect estimates are small and the significance might be negligible in a clinical setting, this is an important public health challenge," said Xiang Qian Lao.

Researchers in an earlier, unrelated study found that there are a number of factors that can cause this dip in sperm count. Experiments showed that exposure to particulate matter (PM) containing heavy metals and polycyclic aromatic hydrocarbons, affected sperm count in laboratory animals. For humans, the results were found to be inconsistent.

The current study has so far established a correlation between poor air quality and drop in sperm count, but they are yet to fully find if there is any direct link between particular pollutant and its effect on sperm, points out the SA report.