This paper reports on the variation of UV radiation in full sun and in shade and how this relates to the physiological production of vitamin D3 for various latitudes. Calibrated spectral measurements were used 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, 45° and vertical planes for the solar zenith angle (SZA) range of approximately 5° to 80°. For an SZA of approximately 5°, average UV 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 using shaded environments with a sky view of greater than 40% for vitamin D3 synthesis is for SZAs less than approximately 45°. Shade can be used throughout Australia during summer and winter. However, winter exposure times will vary depending on the latitude. Using shade for UVD3 exposures can reduce total UV-radiation exposure by 37% to 58% compared to full sun UVD3 exposures. This research indicates that an improved approach to optimize UV-radiation exposures for the production of vitamin D3 is to use diffuse UV radiation under shade in and around the middle of the day.
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