Men living in areas with higher air pollution are also more likely to have a higher proportion of abnormally shaped sperm.
Researchers from the Chinese University of Hong Kong analysed sperm quality in samples from more than 6500 Taiwanese men between the ages of 15 and 49. They paired this with satellite imaging data to determine the levels of fine particulate air pollution (PM2.5) around their homes.
'We found a robust association between exposure to PM2.5 air pollution and low percentage of sperm normal morphology in reproductive-age men,' the scientists wrote in the paper, published in the journal Occupational and Environment Medicine.
If there were just 5 micrograms more fine particulates per cubic metre over 2 years around a man's home, there was a 26 percent increased risk of him being in the bottom ten percent of men for normal sperm morphology. The researchers also found an association between increased PM2.5 and higher sperm concentration.
'Although the effect estimates are small and the significance might be negligible in a clinical setting, this is an important public health challenge,' the authors wrote. '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.'
The pollution measurements were taken near the men's homes, with a fine resolution of about 1 kilometre. However, many of them would have spent a significant amount of time away from home at work, pointed out Professor Kevin McConway, a statistician at the Open University who was not involved in the study. In addition, although there was an association, this does not prove a causal link.
Professor McConway also noted that the level of air pollution in cities in Taiwan is generally higher than in cities in the UK.
'If I were young enough to worry about my fertility, I wouldn't put moving to an area with cleaner air at the top of my list of actions – though there are certainly many other health-related reasons to live in cleaner air,' Professor McConway said.
Professor Allan Pacey, an andrology researcher at the University of Sheffield, added: 'From this and other studies, I remain of the opinion that air pollution probably does have the potential to negatively influence male reproductive health.
'But the jury is still out about quite how and to what extent this impacts on male fertility, rather than measurable and small interesting changes in semen quality.'