How to Know if You Are Getting Vitamin D from Sun Exposure ?

https://www.youtube.com/watch?v=vS9acVgQjZY&list=UUxscQY7X0mtnaCbkGTjZ17g&index=1&feature=plcp what do you guys think?...is Mercola right about the whole 50 degrees angle thing?

Tudor, that's good, practical advice in the vid; something Mercola's good at.

Can't answer your question, but have been reading lately on the same topic. You might find this 2010 paper interesting. Although it primarily discusses skin pigmentation of evolving populations in correspondence with the UV exposure in geographical regions, it has some good observations about vitamin D absorption and UV.

http://www.pnas.org/content/107/suppl.2/8962.full

Some quotes-

"Vitamin D3 is made in the skin when UVR penetrates the skin and is absorbed by 7-dehydrocholesterol (7-DHC) in the epidermis and dermis to form previtamin D3. This reaction only occurs in the presence of wavelengths of 290–310 nm in the UVB range, with peak conversion occurring at 295–297 nm. Photosynthesis of vitamin D3 in the skin depends upon the solar zenith angle, which changes with season, latitude, and time of day, and is further controlled by the amount of pigment and thickness of the skin "

"At the equator and within the tropics, average UVB is high, with two seasonal peaks at the equinoxes (49). Average UVA at the equator and within the tropics is also extremely high... Outside of the tropics, average UVB levels are much lower and exhibit a single peak at the summer solstice (49). The extremely low average UVB in northern Eurasia and North America is matched with a high CoV... Average UVA outside of the tropics is lower, but exhibits much less variation in its pattern."

Among other factors, Ozone thickness in a particular region and season also dictates time required for sun exposure for Vitamin D. The thinner the ozone layer, other environmental factors not varying, the less time required for the daily sunlight dose. This might be because a thinner ozone cover would possibly allow more UVB to enter the lower atmosphere.

A handy map to check your ozone thickness-
http://exp-studies.tor.ec.gc.ca/e/ozone/Curr_allmap_g.htm