Ultraviolet radiation (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, sub-erythemal low-dose UVR on the skin's inflammation threshold, assessed as minimal erythema dose (MED), across different skin phototypes (FP). Additionally, epidermal and stratum corneum (SC) thickness, melanin index (MI), erythema index (EI), skin and blood levels of immunological biomarkers and blood 25(OH)D3 were measured. Over nine-weeks, 31 subjects were exposed to UVR (0.8 standard erythema dose), on the lower back three times per week. Results showed a 50% increase in MED, with a stronger effect in lighter phototypes. Increase in MED was paralleled by an increase in EI. MI and SC thickness increased significantly 12% and 34%, respectively, with melanin having a stronger effect on the MED increase. Vitamin D levels rose by 21%, adjusting for seasonal decline. Immunological markers of Th1/Th2 response and vascular markers declined, indicating local immunosuppression, though 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.