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To the Editor:
The comments of Dr. Pegum are interesting, but I believe another explanation is more likely. Actually our observations are the same whether the tar is placed on the skin 15 minutes or 24 hours prior to irradiation with ultraviolet light. In our studies the explanation for the failure of photosensitization lies in the fact that the artificial light source does not contain the activating wavelengths. As shown in our second paper (Arch. Derm. 84:997 [Dec.] 1961) on exposure to direct rays of the sun, all of these tar substances act as photosensitizers.