For adults and children alike, summer is the season to take in the mystery of the outdoors, its unexpected pleasures and hidden thrills.

But summer has its share of unpleasant discoveries, too — as dermatologists know all too well. For this is also the season of the mystery rash. “This time of year, a lot of people come in with rashes and have no idea what happened,” said Dr. Deborah S. Sarnoff, a dermatologist in New York.

In truth, many of these rashes are not at all mysterious. Very often, Dr. Sarnoff and other dermatologists find, they result from a photosensitivity reaction, a combination of the sun’s UVA radiation and exposure to a drug, perfume or another substance.

Many commonly used drugs can cause such a reaction, including antibiotics like the tetracyclines (doxycycline is one), ciprofloxacin and the sulfa drugs (Bactrim, for example); the diuretic hydrochlorothiazide; and over-the-counter anti-inflammatory drugs, like ibuprofen and naproxen.

People with the photosensitive reactions “may have been on the drug for a long time, so they don’t put two and two together,” said Dr. Sarnoff, who is senior vice president of the Skin Cancer Foundation.

The full list of substances that can touch off a photosensitive reaction is very long and includes, ironically, sunscreens that contain benzophenones, the retinoids used to treat acne and sun-induced wrinkles, and fragrances like musk and coumarins.

The guilty substances change with the times, as compounds go in and out of fashion. But the problem never vanishes. New irritants frequently appear, keeping dermatologists on their toes and consumers mystified.

Most photosensitivity reactions result from exposure to UVA radiation, the so-called tanning rays that have been linked to premature aging of the skin and to melanoma, the most deadly form of skin cancer.

UVA radiation can pass through glass, so a reaction can occur after a car ride or sitting indoors near a window, further mystifying affected individuals. And though the sun is the most common source of UVA radiation, it is not the only source. This type of UV radiation is found in tanning booths and, in small amounts, is emitted by fluorescent bulbs.

Some people also react to the wavelengths that make up visible light.

FULL ARTICLE: http://www.nytimes.com/2011/07/26/health/26brody.html?_r=3&ref=health