The men were continuously exposed to traffic fumes
Fumes from cars and other vehicles may damage male fertility, a study suggests.
Researchers in Italy have found evidence that traffic pollution reduces the quality of sperm in young and middle-aged men.
They believe nitrogen oxide and lead in exhaust fumes may be to blame.
Their findings suggest that men who are exposed to traffic pollution every day, through work for example, are most at risk.
Dr Michele De Rosa and colleagues at the University of Naples examined the sperm of 85 men employed at motorway tollgates. On average, they were exposed to traffic fumes for six hours a day.
They compared their findings to results on tests on 85 men of the same age living in the same area, who were not exposed to traffic pollution in the same way.
They found that the tollgate workers had poorer sperm quality and, in particular, had lower sperm motility, which means they are less likely to be able to fertilise the female egg.
Our study demonstrates that continuous exposure to traffic pollutants impairs sperm quality in young and middle-aged men
Dr Michele De Rosa University of Naples
"In general, the sperm of the study group was more feeble and less active so it has a lower fertility potential," Dr De Rosa said.
The researchers concluded that exposure to exhaust fumes were to blame for the damage to the men's sperm.
The tollgate workers, which included young and middle-aged men, were exposed to substantially higher levels of nitrogen oxides, sulphur oxides, carbon monoxide and lead.
The researchers said more studies are needed to see if the sperm quality of these men improved if they were no longer exposed to traffic fumes.
Dr De Rosa said: "Our study demonstrates that continuous exposure to traffic pollutants impairs sperm quality in young and middle-aged men.
"Analysing the potential fertility of these workers after they have been removed from tollgate duty will add other important information."
He added that the findings highlighted the need for further studies to examine whether other groups of workers were being put at similar risk.
"We hope that our results will prompt clinical and epidemiological studies of male infertility in other work categories exposed to similar levels of environmental pollution," Dr De Rosa said.
'High exposure levels'
Professor Harry Moore of Sheffield University, UK, is carrying out a major study to examine the impact of chemicals in the environment on male fertility.
He suggested that only men continuously exposed to traffic fumes might be at risk.
"We have seen from other studies that there is no relationship between exposure to traffic fumes and reduced fertility in men living in urban areas compared with men living in rural areas.
"This would suggest that men would have to be exposed to high levels before it may affect their fertility," he told BBC News Online.
The study is published in the journal Human Reproduction.