As though it weren't enough to worry about the air we breathe, both in and out of doors, some people must also worry about allergies to sunlight. A dermatologist once observed that one of the greatest mysteries is the development in an otherwise healthy person of an allergy to something as essential to life on earth as the sun's rays. Nevertheless, it does occur. In some instances a topically applied substance or an internal medication is responsible for making the individual more sun-sensitive. But in other cases the sun's ultraviolet radiation alone is enough to trigger hives or other allergic reactions.

Scientists refer to all adverse reactions to the sun as photosensitivity reactions. Those reactions that are sunburnlike, whether they are caused directly by the ultraviolet rays or the combination of the radiation and some chemical applied to the skin or ingested, are known as phototoxic reactions. Ordinary sunburn is the most common example of a phototoxic reaction. Those that involve the immune system on an allergic basis are known as photoallergic reactions.

Photoallergies occur much less commonly than phototoxic reactions, although as a rule they require less radiation energy to trigger them. These reactions may be either of the immediate hypersensitivity kind or the delayed allergic kind. Solar urticaria and polymorphous light eruption are two photoallergic conditions that result from ultraviolet radiation alone. Photoallergic contact dermatitis and systemic photoallergy, on the other hand, result from the interaction of ultraviolet radiation and a topical or ingested chemical or medication.