Smoking can actually delete a sex chromosome that determines gender and affects fertility.

Cigarettes make you look sexy, right? Wrong. It turns out that smoking tobacco can cause men to lose their Y chromosomes, which could mean lower sperm counts.

New research from Sweden published in the journal Science only adds to the huge pile of data stacked up against tobacco smoking. It shows that men who smoke are more than three times as likely to lose their Y chromosomes as nonsmokers.

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Swedish researchers say that the loss of Y chromosomes may help explain why men are more likely to develop certain cancers associated with smoking than women.

“The strength of association between smoking and this mutation was a surprise,” said study co-author Jan Dumanski, Ph.D., a professor in the department of immunology, genetics, and pathology at Uppsala University in Sweden.

Y chromosomes are one of two types of sex chromosomes and they are found only in men. Y chromosomes are what make men men, and they affect things like the development of the testicles. Over evolutionary time, the Y chromosome has shed many of its genes, but the ones that remain are vital and are expressed in tissues throughout the body, not just the reproductive system.

Dumanski and his team analyzed data on more than 6,000 men, taking into account their age, exercise habits, cholesterol levels, education, alcohol intake, lifestyle behaviors, smoking habits, and more.

The researchers found that the Y chromosome is three times more likely to disappear from the blood cells of current smokers compared to men who have never smoked or who have kicked the habit.

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The good news is that the loss of Y chromosomes may not be permanent.

“Results indicate that … this process is reversible,” Dumanski said. That means if you’re having fertility troubles and you’ve found solace in cigarettes, it’s time to quit.

“We found that the frequency of cells with loss of the Y chromosome was not different among ex-smokers compared to men who had never smoked,” Dumanski said.

The extent of Y chromosome loss seems to be dose-dependent, meaning that the more you smoke the more likely you are to lose your sex chromosome.

“This discovery is very important for motivating smokers to quit. Smoking is simply more dangerous for men. This finding may in part explain why men in general have a shorter life span than women,” Dumanski said.

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There are numerous campaigns and resources available to teach the public about the harmful effects of tobacco smoke and to help smokers quit. However, more research still needs to be done to find out just how tobacco smoking damages DNA.

“We need to better understand the mechanism behind smoking being able to induce a large genetic damage, such as loss of an entire chromosome,” Dumanski said.