Skin Color, Sex, and Season Affect Minimal Erythema Dose Variations

woman with erythema
A large-scale study provides unprecedented evidence that a number of very diverse factors influence minimal erythema dose variations.

Minimal erythema dose (MED) was associated with skin color, sex, season, and meteorological factors, but not with age, according to study results published in the Journal of the European Academy of Dermatology and Venerology.

Studies have suggested that MED has substantial interindividual and intraindividual differences, reflecting the influence of diverse factors such as skin color, sex, age, season during which testing was performed and meteorological factors.  However, previous studies lacked consistency, had irreproducible results, or had opposing results. Therefore, researchers sought to investigate the factors associated with MED in a large-scale population study.

Researchers enrolled 22,146 healthy participants, between the ages of 18 and 61 years, for MED testing at the Shanghai Skin Disease Hospital. The MED test was performed following the international standard procedure, and MED was defined as the lowest irradiation dose at which skin erythema with clear borders was visible. Skin color was measured by individual topology angle on an unexposed area of the lower back. Meteorological data collected included daily temperature, relative humidity, and atmospheric pressure. The results were analyzed in adjusted multivariable linear and logistic regression models.

The stratified analysis revealed that lower MED was consistently associated with lighter skin in both men (β = -0.24, -0.28 to -0.20, P =2.59 × 10-30) and women (β = -0.33, -0.39 to -0.28, P =4.31 × 10-38), as well as in both darker and lighter skin groups. Moreover, after controlling for skin color, women had higher MED than men (β = 0.91, 0.32-1.50, P =2.93 × 10-3).  MED was not related to age in either sex.

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The researchers also discovered that MED was significantly lower in summer than in the 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), and this was not due to confounders such as sex, age, and skin color. Last, MED was associated with air pressure (β = -0.64, -0.82 to -0.46, P =8.01 × 10-12) and air temperature (β = -0.36, -0.49 to -0.23, P =4.81 × 10-8) only in the summer.

A limitation of this study includes the lack of consideration of Fitzpatrick skin types and MED. Therefore, it is unknown if these results are reproducible in broader populations.

The investigators believe that this large-scale population study provides evidence that sex, skin color, season, and meteorological factors are associated with MED. They suggest that findings be considered in the study design and interpretation of MED tests.

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Tan Y, Wang F, Fan G, et al. Identification of factors associated with minimal erythema dose (MED) variations in a large‐scale population study of 22,146 subjects [published online January 19, 2020]. J Eur Acad Dermatol Venereol. doi:10.1111/jdv.16206