Erythemal and vitamin D weighted solar UV dose-rates and doses estimated from measurements in mainland France and on Réunion Island

Making the sunshine vitamin - How much sun exposure is needed to maintain 25-hydroxy vitamin D concentration?

Our objective was to calculate the time in the sun necessary to maintain existing 25-hydroxyvitamin D (25(OH)D) concentration at locations across Australia and New Zealand. We used a microsimulation model to estimate changes in monthly 25(OH)D concentration using data on standard erythemal dose, solar zenith angle, and climatological ozone. We estimated the number of standard vitamin D doses per 10-min interval and used a dose-response equation to determine the average time in the sun to maintain existing 25(OH)D concentration according to month and time of day. Across all locations in summer, 5-10 min outdoors between 8 a.m. and 4 p.m. on most days of the week, with 35% of the body surface area exposed, is sufficient to maintain existing 25(OH)D concentration. In winter, at mid-to-high latitudes, time outdoors during the middle of the day is required. In winter, with 10% of the body surface area exposed, greater than 45 min in the middle of the day is required in most locations to maintain existing 25(OH)D concentration. These data can be used to inform guidelines regarding maintaining vitamin D via sun exposure and may help health practitioners identify patients who may be vitamin D deficient.

Measurements of biologically effective solar radiation using erythemal weighted broadband meters

In this paper, we describe conversion factors (CF) for the calculation of biologically effective irradiances (BEI) from erythemal irradiance for three effects: photosynthesis of previtamin D3, psoriasis healing, and inactivation of the SARS-Cov-2 virions. CFs were empirically derived from measurements of spectral solar ultraviolet (UV) radiation during all sky conditions at four mid-latitude sites in the Northern Hemisphere, namely Aosta, Belsk, Reading, and San Diego. These CFs were found to depend on solar zenith angle (SZA) and total column ozone, but are largely independent (within ± 5% for SZA < 60°) of local conditions such as surface albedo, visibility and other local atmospheric patterns. The values of these empirical CFs are consistent with analytical CFs derived with radiative transfer calculations (model FastRT) for clear-sky and overcast conditions. To validate these analytical CFs, one-hour radiant exposures for the three biological effects were calculated from erythemal measurements at Reading between 2012 and 2021 and compared with similar exposures calculated directly from the spectral UV measurements. The two datasets agreed within 10% for SZA < 65°, demonstrating the utility of the conversion method. These results suggest that the proposed analytical CFs can be used with confidence to estimate radiant exposures for the three biological effects from measurements of the UV Index at any northern mid-latitude site.