UV-related web tools

VitD-ez - Duration of Vitamin D Synthesis in Human Skin

Vitamin D production in human skin occurs only when UV radiation exceeds a threshold. From simulations of UV irradiances, the VitD-ez www page computes the daily duration of dermal vitamin D production at midday when UV radiation exceeds the required threshold. The VitD-ez www page is a simplified version of the more complex VitD web page ( The simplified VitD-ez www page is convenient for users unfamiliar to radiative transfer modelling, because the model input is limited and more intuitive.

The sky conditions in the simple VitD-ez www page are somewhat different than the complex VitD web page. "Cloudless" is a very clear, pristine atmosphere with an aerosol turbidity coefficient (Ångström β) of 0.02. "Scattered clouds" in VitD-ez correspond indeed to scattered clouds in the VitD complex version, but with a cloud fraction of 30%. "Broken clouds" in the VitD-ez version are not "broken clouds with radiation enhancement" as in the VitD version, but are scattered clouds like the previous with a cloud fraction of 70%. "Overcast" in Vit D-ez is the same as in the complex VitD version. For all cloudy scenarios in VitD-ez, the cloud liquid water column is set to 240 g m-2.

Ozone layer thicknesses of "Thin", "Medium" and "Thick" refer to ozone columns of 200, 350 and 500 DU, respectively. DU = Dobson unit (1 DU = 1matm-cm, equivalent to the thickness of 0.01 mm of pure ozone at standard conditions of temperature [273.15K] and pressure [1013.25 Pa]).

All other values should be self-explanatory, and agree with the more complex VitD page.

Author: Ola Engelsen
Norwegian Institute for Air Research
N-9296 Tromsø

Copyright © 2005 Ola Engelsen

Last modified September 20th 2006.

The current model has undergone internal checking and validation. If you have any questions or comments on this service and how it could be improved for your needs, please contact the author.

If you use this program and publish the results, I would appreciate a lot if you cite it:
Ola Engelsen, Magritt Brustad, Lage Aksnes and Eiliv Lund (2005) Daily Duration of Vitamin D Synthesis in Human Skin with Relation to Latitude, Total Ozone, Altitude, Ground Cover, Aerosols and Cloud Thickness, Photochemistry and Photobiology, 81(6), 1287-1290.
A reprint is freely available at

The underlying simulations are done using the following model:
Engelsen O. and Kylling A., Fast simulation tool for ultraviolet radiation at the Earth's surface. Optical Engineering, 44 (4), 041012 (2005).
This model interface has mostly the same input options as the VitD web tool.

VitD changelog


Based on the work by Webb et al. [1988] we established a biological effective UV dose rate (BED) for photoconversion of 7-dehydrocholesterol (7-DHC) to previtamin D3 in skin [Brustad et al. 2004]. Webb et al. [1988] found no detectable photoconversion of 7-DHC to previtamin D3 for mid-February and only a small production of previtamin D3 at mid-March in Boston. For mid-February within a half hour of the solar noon at the maximum intensity of the day, they measured surface irradiances of 0.024, 1.0 and 10 mW m-2 nm-1 with an Optronics 742 spectroradiometer at wavelengths of 300, 306 and 316 nm, respectively. The BED was established by adding the measured surface irradiances weighted by the relative efficiencies for converting 7-DHC to previtamin D3. The weights are 0.92, 0.45 and 0 for 300, 306 and 316 nm, respectively [Webb et al. 1988]. Below the threshold BEDthreshold=0.024*0.92+1.0*0.45=0.472 we assumed that no photoconversion to previtamin D3 took place. This program computes the time period when UV radiation exceeds this threshold for the user specified time, location, and radiative regime (i.e. atmospheric and surface conditions).


The resulting output is three numbers in a single row at the bottom: start time (British time), stop time (British time) and duration (hours)


Webb, A. R. , L. Kline and M. F. Holick (1988) Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. J. Clin. Endocrinol. Metab. 67, 373-378.

Brustad, M., E. Alsaker, O. Engelsen, L. Aksnes, and E. Lund (2004) Vitamin D status in middle-aged women at 65-71 °N in relation to dietary intake and exposure to ultraviolet radiation. Publ. Health Nutr. 7, 327-335.