Past increases in the UV that humans are exposed to have been small. In contrast, there are huge geographical and seasonal differences in UV, which have more important implications for health than the small trends. Highest UV intensities occur in the tropics, but latitude for latitude, the peak UV intensities are relatively much higher in the Southern Hemisphere.
There are even larger geographical and seasonal variabilities for beneficial UV. In the summer, the UVVitD is approximately twice UVEry, but the two are approximately equal for mid-latitude winter conditions. At high latitudes in the winter hemisphere, UVEry becomes larger than UVVitD. The results are similar for both clear-sky calculations and for all sky conditions.
Despite these departures from proportionality at larger SZA, vitamin D weighted UV can still be estimated from knowledge of erythemally weighted UV (or UVI). An algorithm to estimate vitamin D production from UVI shows that the production of vitamin D from sunlight is dominated by the midday period when UV intensities are at a maximum. When the sun is high in the sky, such as near noon at mid to low latitudes in the summer, sufficient vitamin D can be produced from a few minutes of sun exposure to the face and hands. But the exposure time should be limited to less than approximately 15 minutes to avoid erythema. A better strategy would be to expose a larger fraction of the body for a shorter time period, preferably when the sun is lower in the sky. When the UVI is 3, skin damage occurs after approximately one hour, but sufficient vitamin D can still be produced in a few minutes.
There should be sufficient UV radiation available in the mid latitude winter to produce vitamin D. However, under those conditions it is necessary to expose larger areas than hands and face alone. Because of the low temperatures, this proviso sets a practical limit on our ability to maintain adequate levels of vitamin D in the winter; a situation which is exacerbated by our modern lifestyle in which periods spent outdoors are greatly diminished. The situation could be improved by promoting physical outdoor activities, such as jogging, during the midday period in winter when the peak UVI is low. During the winter at mid latitudes, most people will probably require supplementation of vitamin D from other sources. These could be dietary (e.g., increased consumption of oily fish, fortification of foods, or from vitamin D supplements), or from exposure to higher UV intensities from holidays abroad, or from artificial sources. The latter options carry risk of overexposure.
Further work is needed to resolve the inconsistency in the literature between the action spectrum of vitamin D and statements that have been made about the production of vitamin D in summer and winter. In particular, if the rate of production stated for the summer is correct, then it should be possible to produce vitamin D at mid latitudes in the winter, contrary to the current advice.
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