Latitudinal variations over Australia of the solar UV exposures for vitamin D3 in shade compared to full sun

Abstract

This paper reports on the variation of UV radiation in full sun and in shade and how this relates to the physiologic production of vitamin D3 for various latitudes. Calibrated spectral measurements were employed to measure the UV radiation in the shade of numerous shade environments for varying solar zenith angles and seasons. This was for exposures to the horizontal, 45o and vertical planes for the solar zenith angle (SZA) range of approximately 5o to 80o. For a SZA of approximately 5o, average ultraviolet radiation irradiances required for vitamin D3 production (UVD3) were 0.67 W/m2 and 0.20 W/m2 for global and for shade with a sky view of greater than 40% respectively. The best time to expose the human body to UV radiation while utilising shaded environments with a sky view of greater than 40% for vitamin D3 synthesis is for SZAs less than approximately 45o. Shade can be used throughout Australia during summer and winter. However, winter exposure times will vary depending on the latitude. Utilising shade for UVD3 exposures can reduce total UV exposure by 37% to 58% compared to full sun UVD3 exposures. This research indicates that an improved approach to optimise UV exposures for the production of vitamin D3 is to utilise diffuse UV under shade in and around the middle of the day.