Polymorphous light eruption

Polymorphous light eruption, also known as polymorphic light eruption, is an itchy rash caused by sun exposure in people who have developed a sensitivity to sunlight (photosensitivity). The rash usually appears as red, tiny bumps or slightly raised patches of skin.

Polymorphous light eruption occurs most often during spring and early summer when a person's exposure to sunlight increases. Repeat episodes are less likely as the summer progresses, but polymorphous light eruption often recurs each year after the first incident.

Although polymorphous light eruption usually goes away on its own without treatment, medications may be needed to treat severe or persistent cases. Measures to protect the skin from sun exposure or light therapy may help prevent recurring episodes of polymorphous light eruption.

Symptoms Causes Risk factors

The term "eruption" refers to the rash, which usually appears within minutes to hours — or sometimes within a couple of days — after exposure to sunlight. The rash usually appears on areas of the body that tend to be covered during winter but exposed in summer: the upper chest, front of the neck and the arms.

Characteristics of the rash may include:

  • Dense clusters of small bumps
  • Raised rough patches
  • Redness
  • Itching or burning
  • Blistering and swelling (less common)

Rarely people may have other signs or symptoms, such as fever, chills, headache or nausea. These conditions may be the result of an associated sunburn rather than polymorphous light eruption.

When to see a doctor

See your doctor if:

  • You have any rash with no obvious cause, such as a known allergy or known exposure to poison ivy

A number of conditions — including some serious diseases — can cause skin rashes with similar appearances. It's important to get a prompt diagnosis and appropriate treatment.

Seek immediate medical care if your rash is:

  • Widespread
  • Painful
  • Accompanied by fever

The exact cause of polymorphous light eruption isn't well-understood. The rash appears in people who have developed a 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 referred to as photosensitivity. Photosensitivity results in sunlight-induced immune system activity that produces inflammation and 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. Therefore, exposure to sunlight through windows may cause a reaction in some people with photosensitivity.


Sensitivity to sunlight lessens with repeated exposure in polymorphous light eruption. 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 eventually may no longer experience recurring episodes.

Although anyone can develop polymorphous light eruption, several factors are associated with an increased risk of the condition:

  • Women are more likely to develop the disorder.
  • The first episode most often appears during the teenage years or 20s.
  • People with fair skin or those living in northern regions are more likely to develop the disorder.
  • A family history of polymorphous light eruption among some people with the condition suggests a possible genetic risk factor.
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