Effect of Repeated Low-Dose UVR Exposure on Skin Inflammation Threshold, Skin Biomarkers, and Vitamin D in Healthy Adults

UVR can cause photoaging, skin burns, and skin cancer. The skin counters these effects through photoadaptation, which involves melanin production, skin thickening, and immune responses. This study investigated the effects of repeated, suberythemal low-dose UVR on the skin's inflammation threshold, assessed as minimal erythema dose, across different skin phototypes. In addition, epidermal and stratum corneum thickness, melanin index, erythema index, skin and blood levels of immunological biomarkers, and blood vitamin D3 25(OH) were measured. Over 9 weeks, 31 subjects were exposed to UVR (0.8 standard erythema dose) on the lower back 3 times per week. Results showed a 50% increase in minimal erythema dose, with a stronger effect in lighter phototypes. Increase in minimal erythema dose was paralleled by an increase in erythema index. Melanin index and stratum corneum thickness increased significantly by 12 and 34%, respectively, with melanin having a stronger effect on the minimal erythema dose increase. Vitamin D levels rose by 21%, adjusting for seasonal decline. Immunological markers of T helper 1/T helper 2 response and vascular markers declined, indicating local immunosuppression, although no systemic changes in immunological markers were observed. These findings suggest that even low UVR doses are sufficient to induce photoadaptation. Further research is needed to evaluate the long-term benefits and risks of this exposure.