Clinical and histological study of coal tar phototoxicity in humans.

Coal tars of different origin were compared with regard to their capacity to cause a phototoxic reaction in human skin. The photosensitizing potencies were found to differ. Tars that were partially refined had less activity than crude coal tar. An extract of tar, liquor carbonis detergens, was the least photosensitizing. The phototoxic reaction to coal tar was found to be a two-stage process, an immediate wheal associated with sharp burning, followed by a raised, red, infiltrated lesion peaking at 24 to 48 hours. The phototoxic reaction was completely prevented when arterial blood flow was cut off. The histological findings were prominently epidermal, with strong intracellular edema sometimes leading to microvesicles. It seems a possibility that assaying phototoxic potentiality may provide a convenient measure of therapeutic efficacy of materials derived from coal tar.