While there is considerable uncertainty regarding the wavelength dependence for vitamin D production, it has been suggested that there is insufficient vitamin D produced in the winter poleward of about 40° (Webb et al., 1988).1 Based on that work, we may take an upper limit threshold for insufficient vitamin D production as the daily available dose at the latitude of Boston, 42°N. This threshold is about 0.7 kJ m per day (see Fig. 2.8). Under the same conditions, erythemally weighted UV dose is approximately 0.55 kJ m per day, with a corresponding peak UVI = 1.3 at noon. Yet at the same location in summer, the erythemally weighted UV dose can exceed 5 kJ m , with corresponding peak UVI ~9. It is apparent that the only places where the minimum required vitamin D-weighted UV radiation is exceeded throughout the winter months are at latitudes between 40°S and 40 °N. On the other hand, the peak UVI at all of these latitudes can often exceed 10 (i.e., "extreme" UV) during the summer months, so at some times of the year, there is a risk of skin damage in as short a time as 15 - 20 minutes. Surprisingly, therefore, it appears that there is no region on the planet which is "just right;" where there is no risk of sunburn in summer, yet ample UV for vitamin D production in the winter. Public advice regarding personal behavior in response to UV variability needs further development, and should recognize both the benefits and the risks of UV exposure.
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