Listen up, guys! There are plenty of rumors that tight pants can decrease your sperm count, and that keeping your cell phone in your pocket can contribute to cancer in some delicate areas. As if you didn’t have enough worry about down there, now there’s another potential threat, and this one has some actual research to back it up. According to a recent study in PNAS, exposure to second-hand smoke may actually cause mutations in sperm cells.

The main—and most dangerous—component of second-hand smoke is called "sidestream tobacco smoke," and it contains more than 4,000 chemicals. Some of the carcinogenic compounds are actually present at higher levels in sidestream smoke than in "mainstream" tobacco smoke, the smoke that is actually inhaled by smokers.

Mice were exposed to the equivalent of either 3 or 16 cigarettes’ worth of mainstream or sidestream tobacco smoke daily for two weeks. Six weeks after the exposure ended, the researchers collected sperm from the each mouse's epididymis and checked for mutations. The mutations that the scientists were looking for are called expanded simple tandem repeats (ESTRs).

In mice that had been exposed to a low level of mainstream tobacco smoke, or essentially "smoked" 3 cigarettes a day, the average frequency of ESTRs was 4 percent; for 16 cigarettes a day, the average frequency was 4.7 percent. In the mice that had been exposed daily to the lower level of secondhand smoke, the average mutation frequency was 4.6 percent. Surprisingly, the higher level of secondhand smoke only induced an ESTR frequency of 2.6 percent. The ESTR frequency in mice that were not exposed to any smoke was just over 1 percent.

These results suggest that being exposed to low levels of second-hand smoke induces just as many germ cell mutations as actually smoking nearly a pack a day. It’s odd that higher levels of sidestream smoke actually induced fewer mutations in the mouse sperm. The researchers suggest this figure may be an artifact of cell death; when toxicity inside the cell reaches a certain level, the cells will simply die, affecting the average mutation frequency.

In this study, the mutations induced by sidestream smoke occurred only in the germ cells, not in somatic cells such as bone marrow or blood. There is evidence that ESTR mutations in the germ line may be passed on to the next generation, cause genomic instability, and increase the incidence of heritable disease.

There is much more research that needs to be done to say definitively that secondhand smoke is a threat to reproduction in humans—the oddities in these results really need to be clarified before we can clearly say that about mice. However, when your sperm is on the line, it doesn’t hurt to err on the safe side and try to steer clear of secondhand smoke.

PNAS, 2011. DOI: 10.1073/pnas.1106896108 (About DOIs).