Sun allergy, also called photosensitivity, occurs when the body's immune system overreacts to sunlight exposure. Patients who are allergic to sunlight typically develop an itchy red rash on areas of the skin that were exposed to light. In rare cases, some patients may develop hives or blisters.
Patients of all skin tones (from light to dark) can develop skin allergies. It remains unclear exactly why sensitive patients develop this reaction. It is believed that light causes changes in parts of the skin that are exposed to the sun. The body then identifies this sun-altered skin as a foreign substance, the same way it would bacteria or viruses. This triggers the immune system to launch an attack, which produces symptoms of an allergic reaction.
In some cases, chemicals from products that are applied to the skin (like lotion or perfume) or chemicals in oral medications (like antibiotics or diuretics) may lead to an allergic reaction to sun exposure.
Photosensitivity is not the same as sunburn. Photosensitivity occurs when the body's immune system reacts to light. Sunburn, on the other hand, occurs when the amount of sun or ultraviolet light exposure exceeds the ability of the body's pigment called melanin to protect the skin.
Some of the most common types of sun allergies include polymorphous light eruption (PMLE), actinic prurigo (hereditary PMLE), photoallergic eruption, and solar urticaria (hives).
In some patients, the sensitivity to light gradually declines with subsequent exposures. If a cream, ointment, perfume, or other chemical is causing a sun allergy, patients should discontinue using the product and symptoms will subside. If a medication is causing symptoms, a healthcare professional may recommend discontinuing the medication or altering the dose. Once the offending chemical or medication is discontinued, the patient will not have allergic reactions to sunlight. Some patients may need to minimize their exposure to sunlight, but treatments, including antihistamines, hydrocortisone, phototherapy, and/or PUVA, may help relieve allergic symptoms.
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types of sun allergies
Actinic prurigo (hereditary PMLE): Actinic prurigo (hereditary PMLE) is an inherited sun allergy. This means the condition is passed down from parents to their children. Hereditary PMLE primarily occurs in patients of Native American ancestry. Symptoms of hereditary PMLE are generally the same as polymorphous light eruption (PMLE), but they are usually more severe. Also, symptoms usually begin during childhood or early adolescence.
Photoallergic eruption: Photoallergic eruption occurs when the sunlight interacts with a chemical that has been applied to the skin. For instance, this can occur with sunscreen, perfumes, cosmetics, or antibiotic ointments that have been applied to the skin. Photoallergic eruption can also occur if a chemical in a medication interacts with sunlight. Common prescription medications that lead to photoallergic eruptions include antibiotics (especially tetracyclines and sulfonamides), diuretics that are used to treat high blood pressure and heart failure, phenothiazines that are used to treat psychiatric illness, and certain oral contraceptives (birth control pills). Some herbs and supplements, including St. Jonh's wort, may cause photoallergic eruption. Other potential causes may include nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen (Motrin® or Advil®) and naproxen (Aleve®).
Polymorphous light eruption (PMLE): Polymorphous light eruption (PMLE) is an allergic skin rash that develops after minimal exposure to sunlight. This condition primarily affects females between the ages of 20 and 40. For unknown reasons, men rarely develop this condition. PMLE typically affects individuals during the spring and summer months. Towards the end of the summer, after the individual has been exposed to sunlight multiple times, symptoms generally become less severe. However, next spring, allergic symptoms usually return with the original intensity.
Solar urticaria (hives): Patients with solar urticaria develop hives after sun exposure. Hives are red, raised itchy welts (wheals) of varying sizes on the surface of the skin.
Sunlight is made up of ultraviolet A (UVA) and ultraviolet B (UVB) rays. Individuals may be allergic to one type of ultraviolet ray or both.
The intensity of the sun's UV rays varies, depending on the time of day. The sun's rays are the strongest from 10:00 a.m. until 2:00 p.m.
In general, the ultraviolet rays of the sun are strongest during the warmest months of the year. Also, regions of the world that are closer to the equator receive the strongest sunlight. Altitude also influences a patient's susceptibility to sunlight. The higher the altitude, the thinner the air. For instance, UV exposure is stronger at the top of mountains than in the valleys.
Also, certain surfaces reflect more of the sun's rays than others. The ability of these surfaces to reflect light influences an individual's reaction to sun exposure. For instance, most natural surfaces, such as soil, grass, and water, reflect less than 10% of ultraviolet light, according to the World Health Organization (WHO). In contrast, sand reflects 10-25% and fresh snow reflects almost 80%. Therefore, sensitive patients who are exposed to sunlight in these conditions (e.g. at the beach or ski mountain) have the greatest risk of developing an allergic reaction.
The longer patients are exposed to the sun, the more UV rays they receive and the more intense the allergic reaction.