I found out the hard way. No one mentioned the danger of sitting a laptop on my lap for hours, and I was comfortable enough to do it. When I noticed what I thought was a bruise last year, I assumed I brought it on during sleep or absentmindedly bumped into something without registering it at the time. But, weeks went by, and it didn't fade. After a bit of web scouring, I realized the leg this bruise appeared on was the one with direct exposure to the laptop's exhaust fan. The pictures and description of Erythema Ab Igne (EAI) matched my situation completely. It would go on to take about 10 months before the burn faded.
Referring to Erythema ab igne as a burn is a bit of a misnomer. It's also called “Toasted Skin Syndrome,” “Hot Water Bottle Rash Syndrome,” and “Fire Stain.” The doctors say it's actually a rash that's brought on by overexposure to a heating element. You won't necessarily “feel” a burn, but you could win yourself a permanent scar if your body is too close to certain heating components. Patches of the fire rash are blotchy and resemble bruises in pigment and pattern, but prolonged exposure to heating mechanisms can bring on patches that won't fade.
There are no known cures for this condition. You can only practice prevention, and since its an underknown affliction, many could find themselves with permanent discoloration from long car rides on heated seats, personal computers sat on laps during long research sessions at the library, and similar. Aside from discoloration, the condition isn't known to be dangerous or bring on high-risk health problems. But, it doesn't hurt to spread the word. Your niece might have the makings of a top model in 10 years. Letting her know that she can still watch youtube if she places her computer on a desk instead of her lap can help her avoid unnecessary skin damage.