The Evaluation of Noninvasive Measurements of Erythema as a Potential Surrogate for DNA Damage in Repetitively UV-exposed Human Skin. 
Erythema (i.e. visible redness) and DNA damage caused by ultraviolet radiation (UVR) in human skin have similar action spectra and show good correlation after a single exposure to UVR.  We explored the potential to use instrumental assessments of erythema as a surrogate for DNA damage after repeated exposures to UVR.  We exposed 40 human subjects to three different exposure schedules using two different UVR sources.  Cyclobutane-pyrimidine dimers (CPDs) in skin biopsies were measured by immunofluorescence, and erythema was assessed by both the Erythemal Index (EI) and the Oxy-hemoglobin (Oxy-Hb) content.  Surprisingly, the skin with the highest cumulative dose ended up with the lowest level of DNA damage, and with the least erythema, as assessed by Oxy-Hb (but not EI) 24 h after the last UV exposure.  Although the level of CPDs, on average, paralleled Oxy-Hb (R2 = 0.80-0.94, P = 0.03-0.11), the correlation did not hold for the pooled individual measurements (R2 = 0.009, P = 0.37) due to potential individual differences in UV-induced photoadaptation.  We suggest that the methodology may be optimized to improve the correlation between DNA damage level and erythema to enable noninvasive risk assessment based on erythema/Oxy-Hb content for individual human subjects.