Passive smoking may cause mutations in the DNA of sperm, according to a study in mice. The finding suggests that men exposed to second-hand smoke could pass on any resulting genetic abnormalities to their children.
Men who smoke are known to be at higher risk of developing abnormalities in their sperm, including reduced motility and increased DNA damage. "Recently, the International Agency for Research on Cancer concluded that there is enough evidence to link paternal smoking in humans with increased risk of childhood cancer, suggesting that tobacco smoking causes heritable germ cell mutation in humans," wrote Francesco Marchetti of the Lawrence Berkeley National Laboratory in California, who led the new research, in a paper in the Proceedings of the National Academy of Sciences.
Allan Pacey, a fertility expert at the University of Sheffield, said: "What we don't know, and what we overlook, is the influence of passive smoking. I guess it's no surprise that passive smoking causes the same kind of damage, because you're just inhaling the same stuff, albeit at different levels."
In Marchetti's study, 32 mice were exposed to the total amount of smoke generated by between three and 16 cigarettes, for varying times up to 90 minutes per day for two weeks. The different conditions modelled the effects of low and high doses of direct smoking and passive smoking. The scientists then examined the sperm of these mice six weeks later, looking at a region of their DNA that doesn't code for proteins.
The frequency of mutations in the sperm of control mice – which were not exposed to cigarette smoke – was around 1.3%-1.5%. In those mice simulating direct smoking, the average mutation rate was 4% and 4.7% for low and high doses. For those mice simulating passive smoking, mutation rates were 4.6% and 2.6% for low and high doses respectively.
The researchers wrote that while it wasn't clear whether the same relationship would apply to regions of the genome that code for proteins, "our data suggest that paternal exposure to second-hand smoke may have reproductive consequences that go beyond the passive smoker."
Marchetti said that the findings provided "compelling evidence in support of the argument that passive smoking should be regarded as a germ cell mutagen in humans". Male exposure to second-hand smoke was likely to have harmful consequences for reproduction similar to those from first-hand smoke.
Pacey said that, in human terms, the level of "passive smoking" the mice were exposed to in the study was high. "Most people's experience of passive smoking is maybe on a Friday night entering and exiting a pub. And it might be the odd whiff when somebody exhales when you're walking down the street."
He said that while the long-term consequences for human health were uncertain, the advice for would-be fathers was clear. "If you're trying to conceive, stopping smoking is good advice and removing yourself from the influences of passive smoking is good advice," he said. "The advice to any man who wants to be a father is to stop smoking at least three months before he tries."
