To the Editor.—
Kaye et al1 described the efficacy of opaque photoprotective agents for protection against UV-A and visible radiation, and Sayre et al2 elegantly described the optical properties of physical sunscreens. The former report is of special interest, because it appears clinical investigators are now recognizing a point we stressed for many years3: A broad UV-A screen will almost certainly be colored. A screen for visible radiation must, by definition, be colored, preferably a pleasing brown. Physical sunscreens suffer from the same defect as many chemical sunscreens: they rub off or are removed by aqueous fluids.Browning agents such as dihydroxyacetone (DHA) form a brown color in the stratum corneum that absorbs blue light with overlap into the adjoining UV-A region.3 Thus, DHA covers the window of vulnerability left by chemical sunscreens, ie, long-wavelength UV-A radiation and blue light. Furthermore, the chemically bound color produced
Johnson JA. Durable Protection Against Long-Wavelength UV-A Radiation and Blue Light. Arch Dermatol. 1992;128(3):409. doi:10.1001/archderm.1992.01680130131020
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