CausesBy Mayo Clinic staff
The exact cause of polymorphous light eruption isn't well understood. The rash appears in people who have developed a sensitivity to ultraviolet (UV) radiation from the sun or other sources, such as tanning beds or tanning lamps. This sensitivity results in sunlight-induced immune system activity that produces inflammation and a rash.
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. Therefore, exposure to sunlight through windows may cause a reaction in some people with photosensitivity.
Sensitivity to sunlight lessens with repeated exposure. Therefore, there are somewhat predictable features of polymorphous light eruption:
- 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 on an annual basis each spring or early summer.
- Some people gradually become less sensitive over several years and no longer experience recurring episodes.
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