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To the Editor:
Everett, Daffer, and Coffey's experiments on coal tar and ultraviolet light (Archives of Dermatology 84:473 [Sept.] 1961) finding that tar actually screened ultraviolet light from the skin are interesting but not unexpected. If a photosensitizing substance is placed on the skin and ultraviolet light given immediately the photochemical reaction will take place outside the body and will produce no biological effect; it will, moreover, screen the body from the activating light. If, however, the photosensitizing substance is applied some time before, e.g., the previous day, and the residue wiped off with a translucent substance, then light of the active wavelength can be expected to penetrate into the skin and the photochemical reaction will take place inside the body, resulting in a biological effect.This paradoxical screening/activating effect probably holds good for most photosensitizing substances, and it explains why sulfonamide ointments were used with success in Spain as
Pegum JS. COMMENTS ON PHOTOSENSITIZATION. Arch Dermatol. 1962;85(4):542–543. doi:10.1001/archderm.1962.01590040106025
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