What is the difference between a phototoxic reaction and a photoallergic reaction?

A phototoxic reaction (Fig. 17-1) is an exaggerated “sunburn” reaction where skin cells are damaged directly by electromagnetic radiation through the production of free radicals, toxic metabolites, heat, or by direct damage to DNA, augmented by external chemicals. It may occur within minutes to hours of exposure, though it may also be delayed for a day or two. A phototoxic reaction can be produced in anyone given a high enough dose of ultraviolet (UV) light and a phototoxic chemical. It can happen on first-time exposure without a need for sensitization.

A photoallergic reaction only occurs in sensitized individuals when electromagnetic radiation (usually ultraviolet light, but sometimes visible light) interact with an endogenous (Fig. 17-2A) or exogenous (Fig. 17-2B) chemical, converting it to an allergen that the person’s immune system recognizes as an allergen (requires more than one exposure). Photoallergic reactions typically occur 1 to 3 days after exposure (with the exception of solar urticaria, which is immediate). Phototoxic reactions tend to be well demarcated in the areas of exposure. Photoallergic reactions are also photodistributed, but often have extension of the cutaneous reaction onto covered areas or even distant sites in an autoeczematous type of eruption. Sometimes chemicals may produce both a phototoxic and photoallergic phenomenon.