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How to Detect a Sun Allergy

by Dominique Vanier, B.Sc.H., M.Env.Sc., on 25 May 2016, Allergies
sun allergy

Is it normal to develop itchy patches on sun-exposed skin?

While an itchy, red rash may seem like a harmless reaction after spending too much time in the sun, these patches can also be a sign of a sun allergy…

A sun allergy, also known as photosensitivity, is an abnormal sensitivity to sunlight triggered by the immune system. Signs and symptoms of a sun allergy include: redness; itchy red bumps or welts; pain; hives and blisters; scaling; and occasionally blue-gray discoloured skin patches.

Certain factors make some more at risk than others for developing a sun allergy. For example, the most common type of sun allergy – polymorphic light eruption – predominantly affects those with fair skinned, and impacts women more often than men. Additionally, some individuals can be predisposed to sun-induced skin reactions when taking certain medications or having another skin condition.

Most common causes of sun allergy

Causes of a sun allergy vary, and can range from a delayed hypersensitivity reaction to an underlying systemic diseases. The following are the most common causes of photosensitivity:

  • Polymorphic Light Eruption (PMLE). PMLE is the most common cause of photosensitivity, often presenting as a sudden rash within 30 minutes to 24 hours after exposure to the sun. PMLE typically appears in the spring or after first exposure to the sun, such as during a vacation, with symptoms generally subsiding as sun exposure continues. The cause of PMLE is unknown, however it is thought to involve a delayed Type 4 hypersensitivity reaction.
  • Solar Urticaria. Like PMLE, solar urticaria is also thought to be an immune-mediated sensitivity reaction, where ultraviolet (UV) exposure may lead to the formation of antigens in those affected. As in typical allergic reactions, the body recognizes the antigen as an “invader” and mounts an attack. Solar urticaria is a benign but chronic condition that results in urticaria (hives) shortly after UV exposure.
  • Phototoxicity is a type of chemical photosensitivity that can result from swallowing or applying an irritating substance to the skin. Over 100 substances, including various drugs, perfumes, and even some plants may lead to redness and inflammation on sun-exposed areas. These reactions tend to occur when the skin has been exposed for only a short period of time.
  • Similar to phototoxicity, a photoallergy is an allergic reaction to an oral or topical substance. However, symptoms are usually delayed, appearing 24 to 72 hours later, and can affect non sun-exposed skin areas.
  • Systemic Disease. Some underlying diseases, namely Systemic Lupus Erythematosus (SLE) and Porphyria, can involve photosensitivity. Exposure to sun can also exacerbate symptoms associated with these diseases.

What to do to prevent sun allergy?

Since the etiology of a sun allergy is not always known, prevention is key. It is important to avoid sudden and prolonged exposure to the sun, and sometimes exposure to sun altogether. Additionally, wearing long-sleeved clothing and hats can help protect the skin from UV radiation. The Canadian Dermatology Association recommends using a sunscreen with a SPF of at least 30 and applying frequently during the day to minimize damage from UV radiation.

An individual with a sun allergy should seek medical advice from their healthcare practitioner to determine the cause and triggering factors of their sun allergy. A sun allergy can be serious and can be better managed with the help of your doctor.



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