Causes

The exact cause of polymorphous light eruption isn't well-understood. The rash appears in people who have developed sensitivity to components of sunlight, and in particular ultraviolet (UV) radiation from the sun or other sources, such as tanning beds or tanning lamps. This sensitivity is called photosensitivity. It results in immune system activity that causes a rash.

UV radiation

UV radiation is a wavelength of sunlight in a range too short for the human eye to see. UV light that reaches the earth is divided into two wavelength bands � ultraviolet A (UVA) and ultraviolet B (UVB).

A person with photosensitivity can react to both types of UV radiation. Although UVB doesn't penetrate glass, UVA does. UVA may even penetrate through most sunscreens. So exposure to sunlight through windows or even sunscreen-protected skin may cause a reaction in some people with photosensitivity.

Photosensitivity

Sensitivity to sunlight lessens with repeated exposure in polymorphous light eruption. Features of polymorphous light eruption are somewhat predictable:

• An episode is most likely to occur after the first one or two exposures to sunlight after a long period of no exposure. This usually means that an episode occurs during the spring or early summer or during a winter vacation in a sunnier location.
• Episodes are less likely to occur as the summer progresses.
• After the first episode of polymorphous light eruption, additional episodes are likely to recur each spring or early summer.
• Some people gradually become less sensitive over several years and eventually no longer experience the annual rash.