Precise determination of the erythema response of human skin to ultraviolet radiation and quantification of effects of protectors

Background: We describe highly reproducible methods for quantifying the erythema response of precisely selected areas (spots) of human skin to graded doses of ultraviolet radiation (UVR). These methods have permitted evaluation of the efficacy of protectors, such as sulforaphane from crucifers, that defend cells through induction of cytoprotective (phase 2) genes. Methods: Spots on the back were precisely located by opaque, adhesive, vinyl templates provided with 16 circular, 2.0 cm diameter occludable windows. Doses (100-800 mJ/cm²) of narrow-band (311 nm) UVR were administered, and the erythema index (a*) was measured with a chromometer on treated and control areas, before and after radiation. Results: Daily variations in basal a* values of each spot were much smaller than the differences of a* values among spots of one individual, or those of corresponding spots among different individuals. The increments in erythema responses to UVR (Δa*) were similar despite large variations of basal a* of spots. The most appropriate measure of UVR-evoked erythema is therefore the Δa* value for each spot, which is an independent observational entity. Δa* was proportional to UVR dose, and independent of spot location. To evaluate effectiveness of protectors against UVR damage we paired horizontally adjacent spots for treatment and controls. Vertical or random spot pairing did not provide significantly higher consistency. Protective efficacy against UVR erythema is appropriately expressed as percent reduction in Δa* values upon treatment with inducers. Conclusions: The protection of skin against UVR damage can be quantified precisely from changes in erythema index (Δa*) obtained with a chromometer.