Thickness.‡

equivalent to the thickness of 0.01 mm of pure ozone at standard conditions of temperature [273.15K] and pressure [1013.25 Pa]); FWHM, full width half maximum of a spectrometer’s spectral response function; HPLC, high performance liquid chromatography; UV-B, ultraviolet radiation of wavelengths in the range 280-315 nanometers; 25(OH)D, blood serum circulating 25-hydroxyvitamin D; 7-DHC, 7-dehydrocholesterol = provitamin D. ‡ Published in the journal website (www.ASPjournal.com) on xx-xxx-xxx. 2 Vitamin D production in human skin occurs only when incident UV radiation exceeds a certain threshold. From simulations of UV irradiances worldwide and throughout the year, we have studied the dependency of the extent and duration of dermal vitamin D production in terms of latitude, time, total ozone, clouds, aerosols, surface reflectivity and altitude. For clear atmospheric conditions, no dermal vitamin D production occurs at 51 degrees latitude and higher during some periods of the year. At 70 degrees latitude, vitamin D synthesis can be absent for five months. Clouds, aerosols and thick ozone events reduce the duration of vitamin D synthesis considerably, and can suppress vitamin D synthesis completely even at the equator. A web page allowing the computation of the duration of dermal vitamin D production worldwide throughout the year, for various atmospheric and surface conditions, is available o