Solar UVB Radiation for the Synthesis of Vitamin D3 in Four Skin Types - Model Propagation Studies

The study presents model computations of the wavelength of absorption of radiation energy in the ultraviolet (UV) band in the epidermis cells keratinocytes for four skin types: White-skin, Pale Yellow-skin, Brown-skin and Black-skin. Keratinocytes cells are idealized as spherical dielectric microcavities, with central melanin granule and a layer of keratin molecules surrounding the cell. Refractive indices of epidermis tissue, melanin and keratin are obtained from the published data. The size distribution of keratinocytes in the epidermis is obtained with the conjecture that the differentiated keratinocytes grow exponentially in the process of cell differentiation. The mole concept is used to calculate the size of melanin granules, one each for four skin types, using the published data of melanin content of the four skin types. The thickness of keratin layer around the cell keratinocyte does not occur in the model computations. In White-skin, the model solar UVB radiation wavelength range of 296 - 300 nm is close to the maximal range of 295 - 300 nm, of the range 290 - [~]317 nm, of the synthesis of vitamin D3 by the compound 7-dehydrocholesterol in skin cells. In Pale Yellow-skin, Brown-skin and Black-skin, the model solar UVB/(limited UVA) radiation wavelength ranges for the synthesis of vitamin D3 are 290 - 295 nm, 290 - 297 nm and 307 - 314 nm, and 313 - [~]317 nm, respectively. The solar UVB radiation synthesizes vitamin D3 to the maximum in White-skin and the least in Black-skin. The synthesis of vitamin D3 is to a lesser extent in Pale Yellow-skin compared to White-skin. The solar UVB radiation synthesizes ample levels of vitamin D3 in the pigmented Brown-skin. The solar UVB radiation is more effective to synthesize vitamin D3 in Pale Yellow-skin compared to Brown-skin, since the Brown-skin is prone to pigmentation. The UVA radiation wavelength ranges that cause tanning of skin are analyzed. In Brown-skin, in particular, the wavelength range is 315 - 342 nm. Also, the physical mechanism of the UVA-caused tanning is similar to that of the UVB-caused tanning of skin. The UVA radiation phototherapies toward tanning of skin may be applied in the wavelength range of 340 - 370 nm. A sunscreen gel, in Brown-skin, in particular, may have the formulation to absorb the effective solar UVB and UVA radiation of wavelengths of 290 - 310 nm and 315 - 340 nm, respectively. The UVB radiation phototherapies to synthesize vitamin D3, especially for individuals allergic to medicinal vitamin D3, have varied model wavelength ranges for the four skin types. The study emphasizes the specific role of UV radiation toward synthesis of vitamin D3 and therapeutic use in skin tanning for different skin types. The preparation of a sunscreen gel for skin protection may vary with skin type.