Photoepicutaneous testing: influence of the vehicle, occlusion time and concentration of the test substances on the results.

The effects of various vehicles, the occlusion time and the concentration of the test substances on the phototoxic reactions were studied using the Finn Chamber method in photoepicutaneous testing, with methoxsalen and coal tar as test substances. Petrolatum proved to be a suitable base for methoxsalen and carbowax for coal tar. The optimal concentration of coal tar was 5% and that of methoxalen 0.03-0.05%. The optimal occlusion time for methoxsalen was 1-2 hours and for coal tar 24 hours. It was concluded that in order to obtain the most reliable results, these parameters should be determined separately for every photosensitizing compound to be tested.