Polymorphous Light Eruption (PLE or PMLE) is a skin condition triggered by exposure to sunlight (or artificial UV light).

It was first described in 1942, and results in an irritable itchy rash which appears several hours (sometimes days) following sun exposure. It often lasts several days. It can be recurrent on subsequent exposures.

PLE appears on areas of the skin newly exposed to sunlight such as the visible part of the neckline, backs of hands, arms and legs, and feet, but less commonly the face.

Source : Wikipedia

Current estimates are that it affects (at some stage) 10-15% of the (light-skinned) population. As high as 40% in some areas.

The cause is unknown (other than the UV-light trigger) . There is speculation that it may be an auto-immune malfunction, or that it's associated with micro-ognanisms on the skin's surface. It may also have links to thyroid disorders.