Photosensitivity

Photosensitivity is an unusually strong skin reaction to ultraviolet or visible light. It can arise from an underlying condition or from exposure to certain medicines or chemicals.

Phototoxicity: A light-dependent skin injury caused when a chemical absorbs light and damages nearby cells. This common drug-related pathway produces a sunburn-like reaction without requiring an immune response.
Porphyria: A group of disorders caused by abnormalities in heme production that can lead to porphyrin accumulation. Some forms cause severe light-triggered skin symptoms when porphyrins accumulate in tissue.
Photopatch testing: A diagnostic procedure that compares skin reactions to substances exposed to ultraviolet light and to unexposed controls. It helps identify photoallergic reactions to topical products or medicines.
Sunburn: An inflammatory skin injury caused by excessive ultraviolet exposure. Ordinary sunburn follows high exposure, whereas photosensitivity can occur after unusually small doses.
Photoallergy: An immune reaction in which light changes a substance into an allergen that triggers skin inflammation. This pathway explains delayed, eczema-like reactions to some medicines and topical products.
Polymorphous light eruption: A recurring itchy or burning rash that develops after ultraviolet exposure, often in spring or early summer. It is a common primary light sensitivity, typically appearing on sun-exposed skin.
Phototesting: Controlled exposure of small skin areas to measured doses of ultraviolet or visible light for diagnostic purposes. It can establish which wavelengths provoke symptoms and how much exposure triggers them.
Allergic contact dermatitis: An itchy delayed immune reaction to a substance that contacts the skin. Unlike photoallergy, it does not require light to activate the triggering substance.
Ultraviolet radiation: Electromagnetic radiation with wavelengths shorter than visible violet light. UVA and UVB wavelengths trigger many photosensitive skin reactions.
Chronic actinic dermatitis: A persistent eczematous skin disorder in which sunlight triggers severe inflammation. It illustrates how photosensitivity can become chronic and extend beyond brief sun exposure.

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