US fertility experts warned teenage boys and young men to consider limiting the time that they use laptop computers positioned on their laps, as long-term use may affect their fertility.

The increasing popularity of laptop computers (LC), coupled with existing evidence that elevated scrotal temperature can result in sperm damage, prompted researchers from the State University of New York at Stony Brook to undertake the first study into the effect of heat from LC on scrotal temperature.

The findings are reported in Europe's leading reproductive medicine journal Human Reproduction. They show that using an LC on the lap increased the left scrotal temperature by a median 2.6?C and the right by a median 2.8?C. Several previous studies have shown that increases in testicular or scrotal temperatures of between 1?C and 2.9?C are associated with a sustained and considerable negative effect on spermatogenesis and fertility.

Lead researcher Dr Yefim Sheynkin, Associate Professor of Urology and Director, Male Infertility and Microsurgery at the University, said: "By 2005, there will be 60 million laptop computers in use in the USA and a predicted 150 million worldwide. Continued improvements in power, size and price of LC have favored their increased use in younger people and laptop sales now exceed those of desktop computers."

With the exception of an anecdotal report of genital burns, the effect of portable computers on scrotal temperature when they are used on the lap was not known, he said.

"Laptops can reach internal operating temperatures of over 70?C. They are frequently positioned close to the scrotum, and as well as being capable of producing direct local heat, they require the user to sit with his thighs close together to balance the machine, which traps the scrotum between the thighs."

The researchers worked with 29 healthy volunteers aged 21 to 35, measuring scrotal temperatures with and without laptops. Two one-hour sessions of scrotal temperature measurements were performed on different days in the same room with a median room temperature of 22.28?C. The men were dressed in the same casual clothing for each session and sessions with and without LC were conducted at the same time of the day. Body temperature was taken by mouth beforehand and each volunteer spent 15 minutes standing in the room to adjust to room temperature before being seated. A non-working LC was placed on the lap so that the volunteer could adopt the right position to balance the laptop, then removed, and the seating position held for one hour, with scrotal temperature being measured every three minutes. The same procedure was repeated for one hour, with the same baselines controls, but this time with a working laptop. The temperature of the bottom surface of the LC was also measured at intervals.
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