That smoking causes cancer is well known. But now, scientists have found another reason why you should not smoke. Smoking damages sperm by changing its DNA sequence.

This can result in a child inheriting the permanent genetic damage from a father who smokes.

In order to see the effects of smoking on fertility and sperm quality, researchers from the mutagenesis section of Health Canada's environmental and occupational toxicology division, with colleagues from McMaster University, studied the spermatogonial stem cells of mature mice that had been exposed to cigarette smoke for either six or 12 weeks. What the team was looking for was to see the alterations in a specific stretch of repeated portions of DNA, called Ms6-hm, which does not contain any known genes.

The smoking mice were exposed to two cigarettes per day, the equivalent based on blood levels of tobacco by-products of an average human smoker. The team then found that the rate of Ms6-hm mutations in the smoking mice were 1.4 times higher than that of non-smoking mice at six weeks, and 1.7 times that of non-smoking mice at 12 weeks. The results of the study have been published in the June 1 issue of Cancer Research.

Lead author of the study Carole Yauk said, "We were looking at male germline mutations, which are mutations in the DNA of sperm. If inherited, these mutations persist as irreversible changes in the genetic composition of off-spring." She added, "We have known that mothers who smoke can harm their foetuses, and here we show evidence that fathers can potentially damage offspring long before they may even meet their future mate."

This, according to the team, suggests that damage is related to the duration of exposure. "So the longer you smoke, the more mutations accumulate and the more likely a potential effect may arise in the offspring," Yauk said.

Males, whether they are mouse or man, generate a constant supply of new sperm from self-renewing spermatogonial stem cells. Reacting to the study, associate professor of anatomy at AIIMS Dr Rima Dada told TOI that smoking damages the DNA by mimicking tissue inflammation and increasing free radical production. "The over 200 known carcinogens present in tobacco cause testicular tissue to inflammate with the free radical level inducing the damage to the sperm," she said.