Solar urticaria, also known as sun allergy, is a rare allergy to sunlight that causes hives to form on skin that’s exposed to the sun. The itchy, reddish spots or welts usually appear within minutes of sun exposure. They can last for a short time or up to hours. The cause of solar urticaria is not known. The allergy may become chronic, but the symptoms can be treated.
The main symptoms of sun allergy are reddish patches on your skin that itch, sting, and burn. If the hives cover a lot of your skin, you may have other common allergy symptoms, such as:
- low blood pressure
- difficulty breathing
The rash may be more likely to affect the areas of your skin that usually aren’t exposed to sunlight. You may not experience the rash on your hands or face, which are frequently exposed to sunlight. If you’re very sensitive to the sun, hives may also break out on areas of your skin that are covered with thin clothing.
The appearance of the rash can vary depending on individual sensitivity. Sometimes the hives can blister or become crusty. The rash doesn’t leave scars when it clears up.
The exact cause of solar urticaria is unknown. It occurs when sunlight activates the release of histamine or a similar chemical in your skin cells. The mechanism is described as an antigen-antibody reaction. That type of reaction occurs when your immune system produces antibodies to counteract the particular antigen or irritant that reacts to the sunlight. Hives are the resulting inflammatory reaction.
You may have an increased risk for solar urticaria if you:
- have a family history of the condition
- have dermatitis
- regularly use perfumes, disinfectants, dyes, or other chemicals that may trigger the condition when exposed to sunlight
- use antibiotics or other medications, including sulfa drugs, that may trigger the condition
In some cases, particular wavelengths of ultraviolet (UV) light trigger the allergic reaction. Most people with solar urticaria react to UVA or visible light.
Heat rash occurs when your pores are clogged and sweat accumulates under your clothing or under your covers. It can occur without exposure to sunlight. For example, in hot, humid weather, heat rash can occur on any part of your body that sweats, especially in the folds of your skin. Areas that may be more at risk for heat rash include:
- under your breasts
- in the groin
- in your armpits
- between your inner thighs
Solar urticaria, on the other hand, only occurs as a result of exposure to sunlight.
Heat rash can also occur in any season. Babies can get heat rash if they’re wrapped in blankets. Heat rash usually goes away on its own in a few days, while solar urticaria typically lasts only hours.
Solar urticaria is a rare allergy that occurs around the world. The median age at the time of a person’s first outbreak is 35, but it can affect you at any age. It can even affect infants. Sun allergy can occur in people of all races, though some forms of the condition may be more common among Caucasians.
Your doctor may be able to diagnose solar urticaria from a physical examination. They will look at your rash and ask you about the history of its appearance and disappearance. Solar urticaria usually breaks out within minutes of sun exposure, and it goes away fast if you get out of the sun. It doesn’t leave any scars.
Your doctor will ask you questions about your history and your reaction to sunlight. Your doctor may also need to do one or more tests to confirm a diagnosis:
- Phototesting looks at how your skin reacts to UV light from a sun lamp in different wavelengths. The wavelength your skin reacts to may help identify your particular sun allergy.
- Patch testing involves putting different substances known to trigger allergies on your skin, waiting a day, and then exposing your skin to UV radiation from a sun lamp. If your skin reacts to a particular substance, that may be what triggered the solar urticaria.
- Blood tests or skin biopsies may be used if your doctor thinks your hives may be caused by another medical condition, such as lupus or a metabolic disease.
Sometimes solar urticaria will disappear on its own.
Treatment for solar urticaria depends on the severity of your symptoms. Staying out of the sun may resolve symptoms if your reaction is mild.
In mild cases, your doctor may prescribe oral antihistamines to calm the hives or over-the-counter creams, such as aloe vera or calamine lotion.
If your reaction is more severe, your doctor may recommend other medications, such as:
- montelukast (Singulair), which is usually used to treat asthma
- hydroxychloroquine (Plaquenil), an antimalarial drug
Your doctor may also recommend phototherapy. This treatment will prepare your skin for the summer sun by regularly exposing it to ultraviolet radiation from a sunlamp in the spring. This may desensitize you, but the effects may not be long-lasting.
The British Association of Dermatologists suggests other treatments to try, including:
- cyclosporine (Sandimmune), an immunosuppressant
- omalizumab (Xolair)
- plasma exchange
- intravenous immunoglobulin
Solar urticaria may flare up only from time to time, or it may be chronic. There are few large-scale studies of treatment outcomes, but a combination of remedies seems to be most effective for treating the condition. A 2003 study of 87 cases found that about two-thirds of participants benefited from staying out of the sun, wearing dark clothing, and taking antihistamines. This same study concluded that 36 percent of people still benefited from these methods 15 years after diagnosis. For those who still had symptoms, the researchers noted, the majority managed to get good symptom control with a combination of treatments.
There are some things you can do to prevent or reduce your risk for solar urticaria.
- Limit your sun exposure, and especially stay out of the sun between 10 a.m. and 4 p.m. when the sun is strongest.
- Consider phasing in your outdoor time in the spring by gradually increasing the time you spend outdoors. That may help your skin cells adapt to the stronger summer sunlight.
- If your rash is related to a particular medication, ask your doctor if there is an alternative.
- Wear closely woven clothes with maximum coverage, such as long sleeves, long pants, or long skirts.
- Consider wearing clothing with a UPF protection factor greater than 40, which blocks UV better than sunscreens.
- Wear a broad-spectrum sunscreen on any exposed skin, and reapply regularly.
- Wear sunglasses and a hat with a broad brim when outdoors.
- Use a sun umbrella.