Because of its unique aspects, photoallergic contact dermatitis deserves special mention. In this condition, the sun's ultraviolet rays convert an otherwise harmless chemical into an allergen, and persons with photocontact allergies generally show up at the dermatologist's office complaining that they have developed an "allergy to the sun." It is, however, an allergy to a specific chemical substance applied to the skin that has been promoted by the sun; by themselves, neither the sun nor the chemical is usually capable of provoking the dermatitis. Photocontact allergens include topical antibacterials, such as the halo-genated salicylanalides, cosmetics, fragrances, and even sunscreen ingredients.

Confirming a diagnosis of photoallergic contact dermatitis requires the same steps as those involved in diagnosing ordinary allergic contact dermatitis, with one major exception: Instead of employing simple patch tests to isolate the allergen(s), photopatch tests must be performed. The patches of test materials are usually removed somewhere between one and three days after they are applied, and then the skin sites underneath are irradiated briefly with ultraviolet A (UVA) light. Following this, they are covered once again and then examined two days later for the presence of dermatitis.

As in the case of ordinary contact allergies, simply discontinuing the use of the offending substance will put an end to the problem. When this is not possible, the use of high SPF sunscreen or sunblock agents is advisable. But if the problem happens to be the sunscreen itself, a suitable substitute must be sought. For example, if you are found to be allergic to PABA or PABA derivatives, which are very common sunscreen agents, you should look for non-PABA-containing sunscreens, of which quite a number are currently available. If you still have a problem, you might try one of the several chemical-free sunblock agents that contain miconized titanium dioxide, such as Neutrogena's Chemical-Free Sunblocker.

Once under way, severe photoallergic reactions need to be quietedwith topical corticosteroid creams, oral antihistamines, and short courses of oral corticosteroids.