Solar ultra-violet radiation and vitamin D synthesis in man
The solar UVB radiation incident on a horizontal surface was measured and related to more routinely recorded meteorological variabIes in a study of the UVB climatology of the English East Midlands. Exposure of individuals in this climate was monitored and related to vitamin D status. On clear days relations were found between the logarithm of UVB intensity lƛ and airmass µ,and at 304 nm where ozone amount [O3] is the dominant atmospheric attenuating factor a2 In Iƛ/aµa was close to the ozone absorption coefficient for this wavelength. At longer wavelengths other attenuation processes have to be accounted for. Measurements of the waveband 300-316 nm were compared with irradiation over broader wavebands. On clear days the ratio of UVB to visible irradiance IB/IV was 4.16 cos z + x where z is the solar zenith angle and x is a coefficient which varies from day to day. Similar analysis for the full solar waveband IF showed a similar linearity of IB/IF with cos z for each day, but both slope and intercept changed between days. A relation between daily integrated totals of UVB and full solar radiation (300-3000 nm) was found, enabling UVB radiation to be estimated from measurements made with a standard meteorological pyranometer. The best estimates require daily figures for ozone concentration but an approximation may still be made using monthly mean concentrations or climatological averages. Diffuse UVB radiation was measured and found to be always greater than 0.5 global UVB. The shade-ring correction applicable in this region of the spectrum is ~ 0.01 greater than the geometric correction. Estimates of the anisotropy of UVB sky radiation gave the relative strength of the circumsolar region as 0.36 with an angular width of 0.78 radians. Polysulphone film was tested and found suitable for use as a personal dosimeter for solar UVB radiation. The UVB exposure of elderly long-stay hospital patients was monitored for a three month period and compared with that of a young healthy population. Plasma 25-hydroxyvitamin D concentrations were measured to assess vitamin D status and the change in plasma 25(OH)D resulting from skin irradiated with solar UVB was found to be 6.9 ± 0.4 ng J -1 for the elderly and + 7.3 ± 3.4 ng J-1 for the young volunteers suggesting little difference between the responses of the elderly and the young. The implication of these figures is that sunlight exposure: of a few hours per week is adequate to maintain a healthy vitamin D status.