here’s a lot of confusion surrounding sunlight and the production of Vitamin D. People commonly think that if the sun is up and they are outside in the sun, they are going to produce Vitamin D. This is not the case!There are many reasons why you won’t make Vitamin D in the sun—even during the middle of the day. But most significantly, the sun cannot stimulate your production of Vitamin D if the sun isn’t high enough in the sky.
UVB Rays And Vitamin D
The reason is that UVB rays—and only UVB rays—catalyze the production of Vitamin D. Yet UVB is reflected and blocked by the atmosphere. The more atmosphere UVB rays have to travel through, the more UVB rays are prevented from reaching the surface of the earth; and the lower the sun is in the sky, the more atmosphere these UVB rays travel through. If the sun is low enough, UVB rays are completely blocked by the atmosphere before they even reach you, and Vitamin D production is not possible.In addition to the atmosphere, clouds also block UVB rays, so if it is cloudy or overcast you probably aren’t going to produce any Vitamin D. Similarly, UVB rays are also blocked by ozone and pollution. In some cities excessive pollution can be enough to severely limit the amount of UVB rays reaching the ground and, therefore, the potential to make Vitamin D.
As a side note, UVB rays also don’t penetrate glass (or clothing). If you are in your car or behind a glass window Vitamin D production isn’t possible. UVB rays have to strike your exposed skin to make Vitamin D creation possible. Sunscreen lotion also blocks the production of Vitamin D.