The skin is a carefully designed interface between our bodies and the outside world. It is infection-proof when intact, nearly waterproof, and filled with protective mechanisms. Sunlight threatens the health of the skin. Normal skin is highly variable in its ability to resist sun damage. Natural skin pigmentation is its main protection. The term photosensitivity refers to any increase beyond what is considered normal variation.

There are over three dozen diseases, two dozen drugs, a variety of herbal preparations, and several perfume and cosmetic components that can cause photosensitivity. There are also several different types of reaction to sunlight—phototoxicity, photoallergy, and polymorphous light eruption. In addition, prolonged exposure to sunlight, even in normal skin, leads to skin aging and cancer. These effects are accelerated in patients who have photosensitivity.

• Phototoxicity is a severely exaggerated reaction to sunlight caused by a new chemical in the skin. The primary symptom is sunburn, which is rapid and can be severe enough to blister (a second degree burn). The chemicals associated with phototoxicity are usually drugs. The list includes several common antibiotics—quinolones, sulfonamides, and tetracyclines; diuretics (water pills); major tranquilizers; oral diabetes medication; and cancer medicines. There are also some dermatologic drugs, both topical and oral, that can sensitize skin.
• Photoallergy produces an intense itching rash on exposure to sunlight. Patients develop chronic skin changes (lichen simplex) as a result of scratching. Some of the agents that cause phototoxicity can also cause photoallergy. Some cosmetic and perfume ingredients, including a compound that was formerly used in sunscreens—para-amino benzoic acid (PABA)—can do this. Most sunscreen preparations in the early 2000s, however, no longer include PABA.
• Polymorphous light eruption (PLE) resembles photoallergy in its production of intensely itching rashes in sunlight. However, this condition lessens with continued light exposure, and so is seen mostly in the spring. Also, there does not seem to be an identifiable chemical involved. PLE is most likely to develop in fair-skinned individuals. It is estimated to affect about 10% of the United States population compared to 21% of the Swedish population. The female: male ratio is 2.5: 1, but it is thought that the imbalance may be due to the fact that women are more likely than men to seek treatment for PLE.
• There is a form of inherited PLE that affects Native Americans. The inheritance pattern is autosomal dominant.

Diseases of several kinds increase skin sensitivity:

• A hereditary disease called xeroderma pigmentosum includes a defect in repair mechanisms that greatly accelerates skin damage from sunlight.
• A family of metabolic diseases called porphyrias produce chemicals (porphyrins) that absorb sunlight in the skin and thereby cause damage.
• Albinos lack skin pigment through a genetic defect and are thus very sensitive to light.
• Malnutrition, specifically a deficiency of niacin known as pellagra, sensitizes the skin.
• Several diseases like acne, systemic lupus erythematosus, rosacea, and herpes simplex (fever blisters) decrease the resistance of the skin to sun damage. Rosacea is sometimes described as a photoaggravated skin disorder because its symptoms increase in severity when patients are exposed to sunlight.
• Photosensitivity is increasingly recognized as a common development in HIV-positive patients. Risk factors for photosensitivity in this group include African American ethnicity and treatment with highly active antiretroviral therapy (HAART).

The pattern of appearance on the skin, a history of drug or chemical exposure, and the timing of the symptoms often suggests a diagnosis. A skin biopsy may be needed for further clarification.

Removal of the offending drug or chemical is primary. Direct sunlight exposure should be limited. Some people must avoid sunlight altogether, while others can tolerate some direct sunlight with the aid of sunscreens.

A sunscreen with an SPF of 15 or greater protects most skin from damage. Such protective garments as hats and long-sleeved shirts are highly recommended in addition.