Definition of Phototoxicity
A phototoxic substance is one which causes dangerously heightened photosensitivity (sensitivity to light), typically in the UV-A ultraviolet region. Most are effective when applied to the skin, although some drugs can induce systemic photosensitivity. Exposure to a phototoxin alone is not (necessarily) harmful, and as discussed below these substances are in fact quite common; it is the subsequent exposure to light which transforms the substance into an irritant which can cause direct tissue damage. Effectively, the phototoxin acts as an accelerant for sunburn.
Effects and Treatment
A photosensitized person, exposed to sunlight, may develop symptoms including inflammation, hyperpigmentation (darkening of the skin, often in spots or blotches), and even blistering. Treatment typically involves the use of topical steroids to suppress inflammation and swelling, and of course removal from sunlight and bright UV sources.
Examples of Phototoxins
Phototoxins are very common in everyday life; a class of phototoxic chemicals known as fucocoumarins are found in many fruits and vegetables, including limes, celery, parsley, and figs, as well as in tar. Many essential oils are phototoxic, as are many perfumes and cosmetics. Finally, some prescription drugs may induce systemic phototoxicity. A comprehensive list of phototoxic products would be impossible to compile; in general you should check up on any cosmetics you may be wearing before heading to the beach. Essential oils may present many potential health risks besides photoxicity, such as allergic and even neurotoxic effects, so do your homework even if you do not expect to spend any time in the sun. And, of course, always be familiar with all possible side effects of your prescription drugs.
For more information, see:
United States Centers for Disease Control (cdc.gov)
DermIS - Dermatology Information System (dermis.net)