Coal Tar Phototoxicity: Kinetics and Exposure Parameters.

We examined two manifestations of coal tar phototoxicity: delayed erythema and skin pain (tar smarts) by quantifying the amount (dose) of UVA and exposure conditions required to induce these phenomena in normal human skin. The minimal UVA dose required to induce delayed erythema (minimal phototoxic dose or MPD) and the minimal UVA dose required to induce an immediate smarting reaction (minimal smarting dose or MSD) were recorded in 32 subjects in a variety of settings. A log-log dose-response model described the relation between the interval of time tar was left on the skin and lowering of MPD. We examined 4 different methods of tar removal and showed that several methods using more than water alone were equally effective-judging by resultant phototoxicity. The time between tar removal and UVA irradiation is important. Even 30 mm was sufficient for the MPD to increase from 3.77 ± 1.55 to 6.1 ± 4.0 J/cm² (p. < 0.02). The smarting reaction shows a similar dependence on the time interval between tar removal and exposure. The mean MSD was less than the mean MPD at all times tested. Both manifestations of coal tar phototoxicity, reduced delayed erythema threshold and susceptibility to the smarting reaction, persisted at least 30 h after tar removal. [ABSTRACT FROM AUTHOR]