Adverse effects of sunscreens in photosensitive patients.

Experimental and epidemiological evidence shows that the common photosensitive disorder of polymorphic light eruption is caused by the ultraviolet A component of sunlight. Sunscreens protect mainly against ultraviolet B; consequently they reduce sunburn and allow longer periods of exposure to the sun and to greater doses of ultraviolet A than would otherwise be possible. Patients with polymorphic light eruption who intend to obtain a tan by sunbathing should not, therefore, be treated with sunscreens which may worsen their rash, but should be advised to sunbathe without sunscreens for a shorter time.