Solar urticaria. Determinations of action and inhibition spectra.

A 42-year-old woman acquired solar urticaria approximately ten minutes after exposure to sunlight. Urticaria developed from visible light emitted from a projector lamp after a similar time lag. Monochromatic rays between 400 and 500 nm induced immediate urticaria by irradiation, with four times the minimal urticarial dose. Urticaria that was induced by monochromatic rays of the projector lamp was completely inhibited by immediate reirradiation of test sites with light waves longer that 530 nm. Radiant heat exposure from an electric hair dryer at 50 degrees C had no suppressive effects on the development of urticarial lesions.