People who work with a laptop on their knees can develop "toasted skin syndrome", an unusual-looking mottled condition caused by long-term heat exposure, a report said today.

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 today 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 They do not cite any skin cancer cases linked to laptop use, but suggest, to be safe, placing a carrying case or other heat shield under the laptop if you have to hold it in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.

A medical report several years ago found that men who used laptops on their laps had elevated scrotum temperatures. If prolonged, that kind of heat can decrease sperm production, which can potentially lead to infertility. Whether laptop use itself can cause that kind of harm has not been confirmed.

In the past, "toasted skin syndrome" has occurred in workers whose jobs require being close to a heat source, including bakers and glass blowers, and, before central heating, in people who huddled near pot-bellied stoves to stay warm.

Dr Anthony Mancini, dermatology chief at Children's Memorial Hospital in Chicago, said he had treated a boy who developed the condition from using a heat pad "hours at a time" to soothe a thigh injured playing football. Dr Mancini said he'd also seen a case caused by a hot water bottle.

He noted that chronic, prolonged skin inflammation could potentially increase chances for squamous cell skin cancer, which is more aggressive than the most common skin cancer. But he added it was unlikely computer use would lead to cancer since it was so easy to avoid prolonged close skin contact with laptops.