Photocontact dermatitis is the result of an interaction between a harmful substance present in the skin and ultraviolet radiation. In other words, no dermatitis evolves from the chemical alone (e.g. if the subject stays indoors) but UV exposure is also required. Photocontact dermatitis is therefore always localised to light-exposed skin, viz. on face, ears, dorsal aspects of hands, and other areas not protected by clothing. The radiation responsible is within the longwave UV (320-400 nm, UVA).
The interaction between chemical in the skin and UVA can lead to a direct damaging effect on the skin tissue by the disposal of accumulated energy or by the formation of new photoproducts. The result is a phototoxic reaction, hitting everybody if the two requirements are fulfilled: sufficient amounts of photosensitizer in the skin and of UVA. (Even in Northern Europe there is enough UVA on sunny days the year for these reactions to occur.) Common photosensitizers are the psoralens, found in ubiquitous plants of the Umbelliferae family (wild parsnip, giant hogweed); the ensuing disease is often called phytophotodermatitis. The psoralens are actually used in dermatological therapy (PUVA).
The pathogenetic mechanism may also involve an immunological component in addition to the skin absorption of a photosensitizer and exposure to UVA. In a photoallergic reaction a sensitization similar to contact allergy is also required and therefore, only a few subjects in the population suffer. Common causal agents are Musk ambrette (in after shave-products), salicylic anilids (in soaps and detergents), p-aminobenzoic acid (in sunscreens), fragrances and balsams (for skin care).
Testing for Photocontact dermatitis
A photoallergic reaction can be diagnosed by photopatch testing similar to regular patch testing but with (and without) the exposure to artificial UVA. This is not possible in phototoxic reactions since everybody reacts under proper circumstances; instead, research interest has to be focused on the potency of chemicals to be activated by UVA.