Sitting with a computer on your lap will crank up the temperature of your nether regions, which could affect sperm quality.
And there is little you can do about it, according to the authors of a study published online in Fertility and Sterility, short of putting your laptop on a desk.
The researchers hooked thermometers to the scrotums of 29 young men who were balancing a laptop on their knees. They found that even with a lap pad under the computer, the men's scrotums overheated quickly.
"Millions and millions of men are using laptops now, especially those in the reproductive age range," said Dr Yefim Sheynkin, an urologist at the State University of New York at Stony Brook, who led the new study.
High scrotum temperatures
"Within 10 or 15 minutes their scrotal temperature is already above what we consider safe, but they don't feel it," he added.
So far, no studies have actually tested how laptops impact men's fertility, said Dr Sheynkin, and there is no bulletproof evidence that it would. But earlier research has shown that warming the scrotum more than one degree Celsius is enough to damage sperm.
Under normal circumstances, the testicles' position outside of the body makes sure they stay a few degrees cooler than the inside of the body, which is necessary for sperm production.
"I wouldn't say that if someone starts to use laptops they will become infertile," Dr Sheynkin said. But frequent use might contribute to reproductive problems, he said, because "the scrotum doesn't have time to cool down."
According to the American Urological Association, nearly one in six couples in the US have trouble conceiving a baby, and about half the time the man is at the root of the problem.
Both general health and lifestyle factors such as nutrition and drug use can influence reproductive health. However, Sheynkin said tight jeans and briefs are generally not considered a risk factor.
"Clothes should not significantly change scrotal temperature, because you are moving around," he said.
To hold a laptop on your knees, however, you need to sit still with your legs closed. After one hour in this position, the researchers found that men's testicle temperature had risen by up to 2.5 C.
A lap pad kept the computer cool and also made sure less heat was transmitted to the skin. But it didn't do much to cool the testicles, and might give "a false sense of security," according to Sheynkin.
Nothing can protect
"It doesn't matter what pad you use," he said. "You can put a pillow beneath your computer and it still won't protect you."
As it turned out, leg position played a far bigger role. When the men sat with their legs spread wide - made possible only by placing the computer on a large lap pad, they could keep their testicles cooler. But it still took less than 30 minutes before they began overheating.
"No matter what you do, even with the legs spread wide apart, the temperature is still going to be higher than what we call safe," said Dr Sheynkin.
Belkin International, Inc., which sells lap pads and other electronics accessories, did not wish to comment on the new findings.
Dr James F. Smith, a urologist at the University of California, San Francisco, cautioned that a clear impact of laptop use on fertility had still not been shown, and that it probably didn't play a big role.
Bad for sperm production
Still, he added that, heating up the scrotum is likely to be bad for sperm production. He often asks patients that he sees for infertility if they use a laptop and, if so, suggests that they spread their legs periodically or place the computer on a desk.
Dr. Smith said the consequences of continued overheating of the testicles - so-called scrotal hyperthermia - probably weren't permanent, but might take months to go away.
"When interested in maximising fertility potential," he advised, "minimize harmful exposures, eat a healthy diet and exercise regularly."
(Reuters Health, Frederik Joelving, November 2010)