Similarly, UVB rays are also blocked by ozone and pollution. 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. 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. 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. Yet UVB is reflected and blocked by the atmosphere. The reason is that UVB rays-and only UVB rays-catalyze the production of Vitamin D. But most significantly, the sun cannot stimulate your production of Vitamin D if the sun isn’t high enough in the sky. There are many reasons why you won’t make Vitamin D in the sun-even during the middle of the day. People commonly think that if the sun is up and they are outside in the sun, they are going to produce Vitamin D. There’s a lot of confusion surrounding sunlight and the production of Vitamin D.

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