Identification of factors associated with minimal erythema dose variations in a large-scale population study of 22 146 subjects

Background Minimal erythema dose (MED) has substantial inter- and intraindividual variations, reflecting the influence of very diverse factors. However, related studies showed little consistency probably because of their limited sample size. Objective To identify the factors associated with MED variations in a large-scale population study. Methods The MED test was performed by following the international standard procedure on 22 146 subjects. The results were analysed in adjusted multivariable linear and logistic regression models. Results This large-scale study revealed that lower MED was consistently associated with lighter skin [beta-coefficient = -0.33, 95% confidence interval (CI) -0.36 to 0.30, P = 6.41 x 10(-84)]. Females had significantly higher MED than male (beta = 0.91, 0.32-1.50, P = 2.93 x 10(-3)). Stratified analyses showed that MED was not associated with age [female: odds ratio (OR) = 0.99, 0.98-1.01; male: OR = 0.99, 0.97-1.00]. MED was lower in summer than in other seasons (spring: OR = 1.08, 1.06-1.11; autumn: OR = 1.11, 1.08-1.13; winter: OR = 1.20, 1.18-1.22). Furthermore, MED was associated with air temperature (beta = -0.36, -0.49 to 0.23, P = 4.81 x 10(-8)) and air pressure (beta = -0.64, -0.82 to 0.46, P = 8.01 x 10(-12)) in summer only while not in other seasons. Conclusions This study provides unprecedented evidence that MED is associated with skin colour, sex, season and meteorological factors, but not with age.