Scientists at Johns Hopkins University in Baltimore, Maryland, have discovered that air pollution can cause DNA mutations in sperm.
The findings are based on a study of mice bred in Hamilton Harbour, an industrial city in Canada. They add to ongoing concerns about the effects of air pollution on human health and fertility.
The researchers found that the mice, reared in cages kept in a shed downwind of two steel mills and a busy highway, had suffered a number of genetic changes as compared to similarly housed mice breathing filtered air.
They have revealed that DNA in the sperm of the mice in the polluted area contained 60 per cent more mutations, had more strand breaks, and had more bases that had been chemically modified through the addition of a methyl group.
The researchers say that such changes may alter gene expression and function in offspring of these mice, though they have yet not directly tested it.
"It's important to move this forward to the next step: determining whether there are any human corollaries to this," Nature magazine quoted Jonathan Samet, an epidemiologist at the Bloomberg School of Public Health, as saying.
In recent years, air pollution has been linked to respiratory and cardiovascular difficulties, developmental defects and lung cancer in humans. However, scientists' focus on the effect of pollution on sperm has been very scant.
"There has been work on the reproductive effects of pollution, but that has largely focused on outcomes of pregnancy, not on male effects," says Samet.
In some previous studies, it had been observed that the offspring of wild birds that bred near steel mills inherited more DNA mutations than their rural counterparts.
A possible reason for such mutations came to the fore when studies on mice were conducted at a later stage.
In the latest study, Carole Yauk of Health Canada in Ottawa and her colleagues monitored male mice for direct evidence of DNA damage in their sperm.
The researchers found that the experimental group started to show more signs of DNA breakage than control mice breathing filtered air, after three weeks of breathing the Hamilton air.
The DNA in the experimental group was significantly more methylated than controls at 10 weeks, while 16 weeks after the experiment began, their DNA contained more mutations at a specific site than the controls.
The researchers have yet to determine how the pollution caused the DNA damage.
"It's quite a leap to go from a lungful of air to damaging germ cells in the testes of these mice," says Christopher Somers, a biologist at the University of Regina in Saskatchewan and an author on the study.
The researchers also tested the DNA for signs of direct mutagenesis a class of chemical compounds called polycyclic aromatic hydrocarbons, which are known to cause mutations and are enriched in the Hamilton air.
However, they did not find any signs suggesting that these compounds were responsible for the damage