People who work with a laptop on their knees can develop "toasted skin syndrome" an unusual-looking mottled skin condition caused by long-term heat exposure, a report said.
In one recent case, a 12-year-old boy developed a sponge-patterned skin discolouration on his left thigh after playing computer games for a few hours every day for several months.
"He recognised that the laptop got hot on the left side; however, regardless of that, he did not change its position," Swiss researchers said in an article published in the US journal Paediatrics.
Another case involved a Virginia law student who sought treatment for the mottled discolouration on her leg.
Dr Kimberley Salkey, who treated the young woman, was stumped until she learned the student spent about six hours a day working with her computer propped on her lap. The temperature underneath registered 125 degrees (52 degrees Celsius).
That case, from 2007, is one of 10 laptop-related cases reported in medical journals in the past six years.
The condition also can be caused by overuse of heating pads and other heat sources that usually aren't hot enough to cause burns. It's generally harmless but can cause permanent skin darkening.
In very rare cases, it can cause damage leading to skin cancers, said the Swiss researchers, Drs Andreas Arnold and Peter Itin from University Hospital Basel. They do not cite any skin cancer cases linked to laptop use, but suggest, to be safe, placing a carrying case or other heat shield under the laptop if you have to hold it in your lap.
Dr Salkey, an assistant dermatology professor at Eastern Virginia Medical School, said that under the microscope, the affected skin resembled skin damaged by long-term sun exposure.
Major manufacturers including Apple, Hewlett Packard and Dell warn in user manuals against placing laptops on laps or exposed skin for extended periods because of the burns risk.