Hot laptop computers may be a factor in the decline of male fertility, according to newly published research from the State University of New York.

The study of 29 male volunteers found that using a Pentium 4 laptop for an hour raised scrotal temperatures by 2.8 degrees centigrade on the right side and 2.6 degrees on the left. Previous studies have shown that temperature rises of between one and 2.9 degrees can harm fertility.

The main reason for the temperature rise was found to be sitting with the legs together in order to balance the laptop PC. But the report found that some laptops can reach 70 degrees centigrade after prolonged use, which can add significantly to the temperature around the genital area.

"We found that scrotal temperatures rose by 2.1 degrees centigrade when the men sat with their thighs together. But the rise was significantly higher when the laptop computers were used," said Dr Yefim Sheynkin, director of male infertility and microsurgery at the State University of New York.

"Until further studies provide more information on this type of thermal exposure, teenaged boys and young men may consider limiting their use of laptop computers on their laps, as long-term use may have a detrimental effect on their reproductive health."

The study speculated that laptop use may be a factor in the 15 to 20 per cent of couples who experience fertility problems each year.

Research from the University of Aberdeen showed that sperm counts fell by nearly 30 per cent between 1989 and 2002. The reasons are unclear, but drug use, alcohol, smoking, pesticides and chemicals have all been cited as possible causes.

The State University of New York also reported that there is anecdotal evidence of scrotal burns from laptops.

There are over 100 million laptop users worldwide and it is one of the fastest growing areas of computer sales. The report refused to name the brand of laptops used in the research.

A PDF of the research findings can be downloaded here.