The assessments of favorable UV conditions for human health over northern Eurasia

UV radiation can have both positive and negative influence on human health. According to the classification of biological UV resources proposed by Chubarova (2007) we define favourable UV conditions as the conditions, when it is possible to get vitamin D3 at noon within an hour but when at the same time the UV index does not reach the high UV category. Different methods were used to estimate the thresholds for generating the vitamin D3 in the skin. One method was based on the approach, which has been proposed by Holick and Jenkins (2003), and another one was based on the recommendations given in the CIE 2006 publication. We compared both approaches by evaluating and comparing the year periods with the conditions favourable for vitamin D3 production. The periods were obtained through the calculation of biologically active irradiance using the TUV model with the 8 stream DISORT solver, and some other modifications described in Chubarova (2006). According to our estimates in midlatitudes the application of the second method leads to the increase in day number (approximately 18 days), when it is possible to get the vitamin D3 in clear sky conditions. It is necessary to emphasize that this difference takes place mainly due to the different thresholds of the skin exposure area recommended in these approaches, since both erythemally-weighted and vitamin D3 irradiance have similar absolute values at noon in spring and autumn, when a “jump” from unfavourable to favourable conditions and back for vitamin D3 production occurs. We have also revealed a large difference in sensitivity of erythemally-weighted and vitamin D3 irradiance to the changes in solar zenith angle, total ozone content (especially, at high solar zenith angles), and quite similar aerosol influence on both types of biologically-active irradiance. Using the updated criteria for vitamin D3 threshold from CIE 2006 we estimated the biologically active UV irradiance over northern Eurasia. The spatial and seasonal distribution of UV favourable conditions has been analyzed both for the clear sky and for the cloudy atmosphere. The calculations were based on the TOMS/OMI total ozone and effective UV reflectivity datasets. The latter one has been used for estimating the effective transmittance in cloudy conditions. The aerosol parameters necessary for computations were taken from a specially developed aerosol climatology, which has been obtained on the base of ground-based AERONET dataset, radiometric Russian datasets and satellite MODIS retrievals (collection 5) over northern Eurasia. A special attention was paid to estimating the uncertainties of MODIS AOT dataset. We found the large AOT biases in spring conditions over Siberian area. The specific features of the defined favourable UV conditions for different time periods are discussed for the various types of human skin in the clear and cloudy atmosphere.
   Acknowledgments: The work was partially sponsored by RFBR Projects #10-05-01019 and #09-05-00582.