When your skin is directly exposed to sunlight, a chemical reaction inside your body leads to the formation of vitamin D, an important nutrient that helps you absorb calcium and form strong bones and teeth. However, sunlight that passes through window glass loses its ability to start this reaction, and you'll receive no vitamin D-related benefit from this type of sun exposure.
Sunlight-related vitamin D production is triggered by a component of sunlight called ultraviolet B, or UVB. When UVB wavelengths penetrate your bare skin, they interact with a substance called 7-dehydrocholesterol, which transforms into another substance, called previtamin D3. In turn, other processes in your body turn previtamin D3 into a usable form of vitamin D. The exact amount of vitamin D you get through UVB exposure varies according to factors that include the season of the year, time of day, geographical location of your home, local cloud cover levels and the amount of melanin, or pigment, in your skin.
No Vitamin D
While sunlight can pass through window glass, the UVB wavelengths contained in sunlight cannot, according to the National Institutes of Health's Office of Dietary Supplements. Since you need UVB exposure to jump-start the vitamin D production process, sunlight that passes through a window cannot increase your vitamin D levels, even if it strikes bare skin. Therefore, you'll gain no vitamin D-related benefits from absorbing sunlight through glass.
UVB and Cancer
In addition to triggering the production of vitamin D, exposure to UVB can increase your risks for the development of skin cancer, the Centers for Disease Control and Prevention explain. Since window glass blocks the transmission of UVB, exposure to sunlight that passes through glass may not carry as high an overall risk as exposure to direct sunlight. However, sunlight that passes through glass still contains another form of ultraviolet light, called ultraviolet A, which poses a potentially greater cancer risk than UVB.
If you want to get vitamin D from sunlight, you can meet your requirements for the vitamin with roughly 10 to 15 minutes of direct exposure three times a week, MedlinePlus reports. However, at the time of publication scientists don't know if you can meet your vitamin D requirements without increasing your risks for skin cancer. Indeed, you can get the vitamin more safely by consuming foods such as dairy products and fortified cereals. You can reduce the risks of direct sun exposure by applying a sunscreen product that helps block both UVB and UVA radiation. Consult your doctor for more information on safe, effective ways to obtain vitamin D.