Most people meet at least some of their vitamin D needs through exposure to sunlight Ultraviolet (UV) B radiation with a wavelength of 290–320 nanometers which penetrates uncovered skin and converts cutaneous 7-dehydrocholesterol to previtamin D, which in turn becomes vitamin D. Season, time of day, length of day, cloud cover, smog, skin melanin content and sunscreen are among the factors that affect UV radiation exposure and vitamin D synthesis. Ample opportunities exist to form vitamin D from exposure to sunlight during the Spring, Summer, and Fall months, even in the far north latitudes.
Complete cloud cover reduces UV energy by 50%; shade (including that produced by severe pollution) reduces it by 60%. UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D; however, you can still get a sunburn through glass (a window.) Sunscreens with a sun protection factor (SPF) of 8 or more appear to block vitamin D-producing UV rays, although in practice, people generally do not apply sufficient amounts, cover all sun-exposed skin, or reapply sunscreen regularly. Therefore, skin likely synthesizes some vitamin D even when it is protected by sunscreen as typically applied.
It has been suggested by some vitamin D researchers that around 30 minutes of sun exposure between 10 AM and 3 PM at least twice a week on the face, arms, legs, or back without sunscreen usually lead to sufficient vitamin D synthesis. Individuals with limited sun exposure need to include good sources of vitamin D in their diet or take a supplement to achieve recommended levels of intake.