Male computer users could lose a little more than their data if they are not careful with the way they position their laptops.

Teenagers and young men should keep their laptops off their laps because they could damage fertility, according to an expert.

Laptops, which reach high internal operating temperatures, can heat up the scrotum which could affect the quality and quantity of men's sperm.

"The increase in scrotal temperature is significant enough to cause changes in sperm parameters," said Dr Yefim Sheynkin, an associate professor of urology at the State University of New York.

"It is very difficult to predict how long the computer can be used safely. It may not be at all, if the testicular temperature goes up high within a very short period of time," he said.

Adolescents and young men who use laptops several times a day over many years face the greatest risk.

"Long-term use may have a detrimental effect on their reproductive health," he said.

Sheynkin and his team studied the impact of using a laptop on 29 healthy volunteers between the age of 21-35 by measuring scrotal temperature before and after they used a computer on their lap.

The research is reported in the journal Human Reproduction.

Even without turning the laptop on, the scrotal temperature rose by 2.1 degrees Celsius.

"It shows that scrotal hyperthermia is produced by both special body posture and the local heating effect of laptop computers," Sheynkin said.