A sun allergy is an immune system reaction to sunlight, most often, an itchy red rash. The most common locations include the "V" of the neck, the back of the hands, the outside surface of the arms and the lower legs. In rare cases, the skin reaction may be more severe, producing hives or small blisters that may even spread to skin in clothed areas.
Sun allergies are triggered by changes that occur in sun-exposed skin. It is not clear why the body develops this reaction. However, the immune system recognizes some components of the sun-altered skin as "foreign," and the body activates its immune defenses against them. This produces an allergic reaction that takes the form of a rash, tiny blisters or, rarely, some other type of skin eruption.
Sun allergies occur only in certain sensitive people, and in some cases, they can be triggered by only a few brief moments of sun exposure. Scientists do not know exactly why some people develop a sun allergy and others do not. There is evidence, however, that some forms of sun allergy are inherited.
A few of the most common types of sun allergy are:
Polymorphous light eruption (PMLE) — PMLE, which usually appears as an itchy rash on sun-exposed skin, is the second most common sun-related skin problem seen by doctors, after common sunburn. It occurs in an estimated 10% to 15% of the U.S. population, affecting people of all races and ethnic backgrounds. Women are affected by PMLE more often than men, and symptoms typically begin during young adult life. In temperate climates, PMLE is usually rare in the winter, but common during the spring and summer months. In many cases, the PMLE rash returns every spring, immediately after the person begins spending more time outside. As spring turns into summer, repeated sun exposure may cause the person to become less sensitive to sunlight, and the PMLE rash either may disappear totally or gradually become less severe. Although the effects of this desensitization process, called "hardening," usually last through the end of the summer, the PMLE rash often returns at full intensity the following spring.
Actinic prurigo (hereditary PMLE) — This inherited form of PMLE occurs in people of American Indian background, including the American Indian populations of North, South and Central America. Its symptoms are usually more intense than those of classic PMLE, and they often begin earlier, during childhood or adolescence. Several generations of the same family may have a history of the problem.
Photoallergic eruption — In this form of sun allergy, a skin reaction is triggered by the effect of sunlight on a chemical that has been applied to the skin (often an ingredient in sunscreen, fragrances, cosmetics or antibiotic ointments) or ingested in a drug (often a prescription medicine). Common prescription medicines that can cause a photoallergic eruption include antibiotics (especially tetracyclines and sulfonamides), phenothiazines used to treat psychiatric illness, diuretics for high blood pressure and heart failure, and certain oral contraceptives. The U.S. Food and Drug Administration (FDA) also has linked some cases of photoallergic reaction to the nonprescription pain relievers ibuprofen (Advil, Motrin and others) and naproxen sodium (Aleve, Naprosyn and others).
Solar urticaria — This form of sun allergy produces hives (large, itchy, red bumps) on sun-exposed skin. It is a rare condition that most often affects young women.