dermatologic disorders
Photosensitivity
Chemical photosensitivity
Over 100 substances, ingested or applied topically, are known to predispose to cutaneous reactions after sun exposure. A limited number are responsible for most reactions (see table). Reactions are divided into phototoxicity and photoallergy. Phototesting can help confirm the diagnosis. Treatment for chemical photosensitivity is topical corticosteroids and avoidance of the causative substance. light-absorbing compounds directly generate free radicals and inflammatory mediators, causing tissue damage manifesting as pain and erythema (like Sunburn ). This reaction does not require prior sun exposure and can appear in any person, although reaction severity is highly variable. Typical causes of phototoxic reactions include topical (eg, perfumes, is a type IV (cell-mediated) Overview of Allergic and Atopic Disorders . Light absorption causes structural changes in the drug or substance, allowing it to bind to tissue protein and function as a hapten, making the complex allergenic. Prior exposure to the allergen is required. The reaction is usually eczematous, with erythema, scaling, pruritus, and sometimes vesicles. Typical causes of photoallergic reactions include aftershave lotions, sunscreens, and sulfonamides. Photoallergy occurs less often than phototoxicity, and the reaction may extend to non–sun-exposed skin. 