US fertility experts are warning teenage boys and young men to consider limiting the time that they use notebook computers positioned on their laps, as long-term use may affect their fertility.
The increasing popularity of notebook computers (NC), 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 NC on scrotal temperature.
The findings are reported in Europe's leading reproductive medicine journal Human Reproduction. They show that using an NC 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 notebook computers in use in the USA and a predicted 150 million worldwide. Continued improvements in power, size and price of NC have favoured their increased use in younger people and notebook 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.
"notebooks 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 notebooks. 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 NC 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 NC was placed on the lap so that the volunteer could adopt the right position to balance the notebook, 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 notebook. The temperature of the bottom surface of the NC was also measured at intervals.
"We found that scrotal temperatures rose by 2.1°C when the men sat with their thighs together, which is necessary to keep NC on the lap. But, the rise was significantly higher when the NC were used – 2.8°C on the right side and 2.6°C on the left," said Dr Sheynkin. “ It shows that scrotal hyperthermia is produced by both special body posture and local heating effect of NC.”
The median surface temperature of Pentium 4 computers used increased from nearly 31°C at the start of the experiment to nearly 40°C after one hour.
Dr Sheynkin said: "The body needs to maintain a proper testicular temperature for normal sperm production and development (spermatogenesis). Portable computers in a notebook position produce scrotal hyperthermia by both the direct heating effect of the computer and the sitting position necessary to balance the computer. The magnitude of scrotal hyperthermia associated with abnormal spermatogenesis is unclear. But, previous studies suggest that 1°C above the baseline is the possible minimal thermal gradient capable of inhibiting spermatogenesis and sperm concentration may be decreased by 40% per 1°C increment of median daytime scrotal temperature.
"We don't know the exact frequency and time of heat exposure capable of producing reversible or irreversible changes in spermatogenesis. Studies have shown significant but reversible changes after short-term heating. However, NC produce significant repetitive transient scrotal hyperthermia for years, and insufficient recovery time between heat exposures may cause irreversible or partially reversible changes in male reproductive function."
Dr Sheynkin said his team now planned further studies to evaluate the heating effect of NC on testicular function and sperm parameters. For now, he did not know an exact time for safe use. However, their study showed that within the first 15 minutes of use scrotal temperatures increased by 1°C, so it did not take long to reach a point that may affect testicular function. Also, frequent use may cause intermittent temperature rises, which could significantly increase a single heating effect.
"Until further studies provide more information on this type of thermal exposure", he said, "teenage boys and young men may consider limiting their use of NC on their laps, as long-term use may have a detrimental effect on their reproductive health."
Dr Sheynkin added that two NC brands were tested randomly to avoid criticism that brands may differ.
"All notebook computers generate significant heat due to the increasing power requirements of computer chips. New notebooks with higher power requirements may produce even more heat. So far, computer fans and 'heat sinks' are not sufficient. It's possible that external protective devices could somewhat help, but it is essential to confirm their protective effect in a clinical study to prevent commercial advertising and use of inefficient and useless products."