Comparison of skin responses from macroscopic and microscopic UV challenges

The minimal erythema dose induced by solar-simulated radiation is a useful measure of UV sensitivity of skin. Most skin phototests have been conducted by projecting a flat field of UV radiation onto the skin in an area greater than 15 cm × 15 cm with an increment of radiation doses. In this study, we investigated the responses of human skin to solar-simulated radiation of different field sizes. Twelve human subjects of skin phototype I-IV were exposed to solar-simulated radiation (SSR) on their upper inner arm or on their lower back with a series of doses in increments of 20% in order to determine the threshold dose to induce a minimal perceptible erythema response (MED). Each dose was delivered with a liquid light guide (8 mm diameter on the back or 6 mm on the upper inner arm) and with quartz optical fibers of 200 μm diameter. The resulting skin responses were evaluated visually and investigated with a reflectance confocal microscope and imaging. The erythema response to the microscopic challenge was always diffuse with no clear boundaries extending to several times the exposed site diameter at doses greater than 2 MED. The skin returned to normal appearance from the microscopic challenge after two weeks of exposure while change in appearance for the larger areas persisted for several weeks to months. This new modality of testing provides the possibility to study skin at the microscopic level with a rapid recovery following challenge.