Exposure to fine particles in air pollution may be another factor that affects men’s sperm quality and their fertility, suggest researchers in Taiwan.

Although the clinical effect may be small, the findings could be important from a public health perspective due to worldwide exposure to pollution, the authors write in BMJ Occupational and Environmental Medicine.

“Particulate matter contains many toxic chemicals such as heavy metals and polycyclic aromatic hydrocarbons, which have demonstrated harmful to semen quality in laboratory and animal studies,” lead author Xiang Qian Lao told Reuters Health in an email.

“Sperm shape and size is an important parameter for fertility. Lower percentage/number of normal sperm may cause infertility,” said Lao, a researcher with the Jockey Club School of Public Health and Primary Care at The Chinese University of Hong Kong.

Although exposure to environmental chemicals has long been considered a potential contributor to infertility, little is known about the effect of air pollution, the study team writes.

They looked at both short-term and long-term exposure to the very fine particles in air pollution known as PM 2.5, which are 2.5 microns or less - about thirty times smaller than the diameter of a human hair. These fine particles can penetrate deep into the lungs and even enter the bloodstream.

This kind of particle pollution can be found both indoors and outdoors and most often comes from vehicle exhaust, the burning of wood, crops, coal or heating oil, and from emissions given off by power plants and other industries.

Cities in China and India have recently been struggling with PM 2.5 pollution levels much higher than is considered safe by the World Health Organization (WHO).

Lao’s team analyzed health exam records and health questionnaires for nearly 6,500 Taiwanese men ages 15 to 49 who participated in a medical examination program between 2001 and 2014.

Semen samples were taken and the men’s sperm quality was assessed for the total sperm number, as well as their size, shape and ability to move.

Because it takes about three months for a man to make new sperm, the researchers used each participant’s address as well as satellite data to estimate his exposure to PM 2.5 pollution over a three-month period as well as his average exposure over two years. Both long- and short-term average particle concentrations were typically within the safe range, with about half below 26 micrograms of particle pollution per cubic meter of air.

For every incremental exposure increase of 5 micrograms, however, researchers found that men’s sperm concentration increased slightly while their risk of abnormally shaped sperm rose by 18 percent with short-term exposure and 26 percent with long-term exposure.

The study wasn’t a controlled experiment designed to determine whether or how PM 2.5 pollution may affect sperm.

Still, Lao said, “Given the ubiquity of exposure to air pollution, a small effect size of PM 2.5 on sperm normal morphology may result in a significant number of couples with infertility. Thus, global strategies are necessary to minimize the impact of air pollution on reproductive health.”

The primary finding of this study is a significant decrease in the percentage of sperm that are normally shaped with increasing air pollution (PM 2.5), but the percentage of sperm that are morphologically normal is still very high, said Shanna Swan, a researcher with the Environmental Medicine and Public Health department at the Icahn School of Medicine at Mount Sinai in New York.

The change in sperm concentration is difficult to interpret, noted Swann, who wasn’t involved in the study. For one thing, the average sperm concentration in the participants was fairly low.

“Strengths of the study are its large size, its use of cutting-edge estimates of air pollution and WHO semen analysis methods,” she said in an email.

Semen quality is significantly related to fertility; men with no (or very few) moving sperm cannot fertilize an egg, she noted.