Electromagnetic radiation is defined by its wavelength and frequency; for convenience, it can be classified into different regions based on its photon energy. For therapeutic purposes, dermatologists are most concerned with the ultraviolet (UV) B (290—320 nm), A-I (320—340 nm), and A-II (340-400 nm) and visible (400-800 nm) spectrum. UVB is the most erythrogenic and melanogenic. It is the major action spectrum for sunburn, tanning, skin cancer, and photoaging. The longer wavelengths of UVA are a thousand times less erythrogenic than UVB; however, they penetrate more deeply into the skin and contribute substantially to photoaging and photosensitiv-ity diseases. They also enhance UVB-induced erythema and increase the risk of skin carcinogen-esis. Visible radiation may augment the severity of some photosensitive eruptions.

Electromagnetic radiation has proven to be highly efficacious in the treatment of numerous dermatologic diseases. Phototherapy and photochemotherapy are treatment methods in which ultraviolet or visible radiation is used to induce a therapeutic response either alone or in the presence of a photosensitizing drug. Phototherapy using UVB or high-dose UVA-I is efficacious in selected dermatological diseases. To be effective, the incident radiation must be absorbed by a target or chromophore in the skin—which in phototherapy is endogenous and in photochemotherapy must be administered exogenously. Patients treated with these modalities should be monitored for concomitant use of other potential photosensitizing medications before initiation of therapy. Such drugs include phenothiazines, thiazides, sulfonamides, nonsteroidal anti-inflammatory agents, sulfonylureas, tetracyclines, and benzodiazepines.

PUVA: Psoralens and UVA

Orally administered 8-methoxypsoralen followed by UVA (PUVA) is FDA-approved for the treatment of vitiligo, psoriasis, and cutaneous T-cell lymphoma.

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