Richard L. McKenzie and J Ben Liley
National Institute of Water & Atmospheric Research (NIWA), Lauder, Central Otago, New Zealand 9352 NIWA Lauder, Private Bag 50061 Omakau, Central Otago 9352, New Zealand E-mail: [email protected] E-mail: [email protected]
Abstract The relatively small long-term change in ultraviolet (UV) radiation is compared with its substantial geographical variability. Action spectra published by the International Commission on Illumination (CIE) are then used to examine diurnal, seasonal, and latitudinal variations in erythemally-weighted (sunburning) UV—a health risk, and vitamin D-weighted UV—a health benefit. Vitamin D weighted UV is more strongly dependent on ozone and solar zenith angle (SZA). Consequently, its diurnal, seasonal, and geographic variability is more pronounced than for erythemally weighted UV. An algorithm is developed and used to relate vitamin D production to the widely-used UV Index. The exposure times needed to produce erythema, or sufficient vitamin D, are calculated as a function of UV Index1 (UVI), using published physiological criteria. In the summer at noon, there should be sufficient UV at mid-latitudes to photosynthesize optimal vitamin D in ~1 minute for full body exposure, whereas skin damage occurs after ~15 minutes. Further, while it should be possible to photosynthesize vitamin D in the winter at mid latitudes, the amount of skin that must be exposed is larger than on the hands and face alone. This raises the question of whether the action spectrum for vitamin D production is correct, since it has been reported that production of vitamin D is not possible in the winter at mid-latitudes. Because the benefits of UV depend on the area of skin exposed, it is preferable to expose large skin areas to sunlight for shorter times than to expose small areas for correspondingly larger times.
Keywords solar UV irradiance, ozone, erythema, vitamin D
1 UVI = 40 x UVErï, where the latter is in units of W m 2.
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