WEDNESDAY, Nov. 22, 2017 -- Men who have trouble conceiving may have the air they breathe to blame, a new study by Chinese researchers suggests.
Microscopic particles in the air called particulate matter, or PM2.5, may affect the quality of sperm, which in turn can make it difficult to fertilize a woman's egg, the researchers said.
PM2.5 stands for particulate matter with a diameter 2.5 micrometers or less. That's about 3 percent of the diameter of a human hair.
"Air pollution is associated with a significant drop in normal sperm shape and size, which may result in a significant number of couples with infertility," said lead researcher Xiang Qian Lao. He is an assistant professor in the School of Public Health and Primary Care at the Chinese University of Hong Kong.
Lao cautioned, however, that this study can't prove that PM2.5 causes the damage to sperm, only that the two are associated.
"You cannot conclude it is a causal relationship in this study, but existing evidence from toxicology and other studies support that the relationship is potentially causal," he said.
Exactly how air pollution might affect sperm isn't clear, Lao said. Many components of fine particulate matter, such as heavy metals and polycyclic aromatic hydrocarbons, have been linked to sperm damage in experimental studies, he said.
The effect of air pollution on sperm is small, Lao said, but because pollution is so widespread around the globe, many men could be affected.
And, because reducing air pollution may improve the quality of sperm, "we call for global strategies to reduce air pollution for improvement of reproductive health," Lao said.
Abnormal sperm results in infertility because the sperm can't penetrate the egg, explained Dr. Tomer Singer, director of reproductive endocrinology and infertility at Lenox Hill Hospital in New York City.
"We have seen a trend in the last several decades where the concentration of the sperm, the motility and the shape of the sperm have been deteriorating," Singer said. "It's been difficult to pinpoint what has been the culprit."
This study provides strong evidence to support an association between air pollution exposure and abnormal semen, said Dr. Manish Vira, vice chair of urologic research at Northwell Health's Arthur Smith Institute for Urology in New Hyde Park, N.Y.
However, reports from the United States have not found similar results, suggesting that the negative impact may be seen only in areas with extremely poor air quality, he said.
Vira called air pollution a global health emergency and said this new study suggests that impaired fertility may be among the health consequences.
"The next step is to correlate air pollution levels with pregnancy rates to determine if the changes seen in semen translate to impaired fertility," Vira said.
For the study, Lao and his colleagues collected data on nearly 6,500 Taiwanese men 15 to 49 years old. All of the men had taken part in a medical examination program between 2001 and 2014. The program included assessing the quality of their sperm, including the total number, shape, size and movement (motility).
Exposure to PM2.5 levels was estimated from the home addresses of each man for three-month periods over two years. It takes three months for sperm to be generated, Lao said.
The researchers found an association between exposure to PM2.5 and abnormal sperm. Specifically, every 5 micrograms of PM2.5 per cubic meter of air over two years was associated with a nearly 1.3 percent drop in normal sperm shape and size.
It was also associated with a 26 percent increased risk of being in the bottom 10 percent of normal sperm size and shape, after accounting for other possible influences on sperm quality, such as smoking, drinking, age and weight.
Exposure to PM2.5, however, was also associated with a significant increase in the number of sperm. The researchers said this may be a way the body attempts to overcome the poorer quality of sperm overall.
Similar findings were seen after three months of exposure to PM2.5, the study found.
The report was published online Nov. 21 in the journal Occupational & Environmental Medicine.
The Urology Care Foundation has more on male infertility.
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