Adequate vitamin D3 skin synthesis versus erythema risk in the Northern Hemisphere midlatitudes

Health-optimum-exposure index (HOEI) is proposed to assess if the prescribed amount of vitamin D3 (target value) could be synthesized in the human skin without erythema appearance. It is defined as the ratio between the vitamin D3 quantity received during the maximum allowed outdoor exposure without erythema risk and the target value. Sunbathing is safe for HOEI>1 and 1/HOEI represents a part of minimal erythema dose (MED) necessary to obtain the target value. We examine the following targets: a vitamin D3 quantity equivalent to 1000 IU vitamin D3 taken orally, and an optimal vitamin D3 quantity defined by Krzyścin et al. (2016). The biologically weighted (previtamin D3 and erythemal) doses from the Northern Hemisphere midlatitudinal stations are analyzed to find HOEI dependence on personal and meteorological factors. HOEI depends mostly on the exposed skin area, person's age, and sun elevation at noon but not on the Fitzpatrick skin phototype. We found that only young adults (<21 yr) could safely obtain vitamin D3 quantity, which is equivalent to 1000 IU taken orally, almost throughout the whole year. Duration of such exposures appears <1 h only in the warm subperiods of the year (April-September) for a person with minimal erythema dose of 330 J m−2. Exposing larger part of the body (~30%) enables the oldest persons (>59 yr) to reach 1000 IU target during warm days in spring and summer. The optimal daily vitamin D3 quantity could only be synthesized only by young adults for about 40-60% of days in the May-August period if they expose at least 1/3 part of their body surface area. Vitamin D3 supplementation seems to be necessary over the whole year for the oldest persons with daily dosage of ~2000 IU but reduced to ~1000 IU in summer for sunseekers exposing significant part of the body.