Erythema ab igne (EAI, also known as hot water bottle rash, fire stains, laptop thigh, and toasted skin syndrome) is a skin condition caused by long-term exposure to heat (infrared radiation). Overexposure to heat causes a mild red rash that might be described as “blotchy”. Prolonged and repeated exposure causes a marked redness and colouring of the skin. Some people may complain of mild itchiness and a burning sensation, but often, unless a change in pigmentation is seen, it can go unnoticed.
It was once commonly seen in the elderly who stood or sat closely to open fires or electric heaters. Although wide use of central heating has reduced the incidence, it is still sometimes found in people exposed to heat from other sources such as heating pads, hot water bottles, and laptops. The term is derived from Latin and means “redness from fire”.
Erythema ab igne on Wikipedia.