The greatest increase in melanomas has occurred in

Dear Dr. Mirkin: You recommend sunlight for vitamin D, but isn't skin cancer a greater concern?

A single sunburn can cause malignant melanoma, but since 1940, the greatest increase in melanomas has occurred in office workers, not in people who work outdoors.
FDA researchers believe that low vitamin D levels may be responsible (Medical Hypothesis, January 2009).

Ultraviolet light is classified by wavelength into UVA and UVB.
UVB rays cause skin to make vitamin D which helps the body to prevent cancers by inhibiting uncontrolled cell growth and restoring programable cells death called apoptosis.

Since window glass block UVB almost completely, indoor office workers get up to nine times less UVB than people who spend more time outside and therefore, have far lower levels of vitamin D.
Since window glass allow UVA to pass through it, indoor workers have exposure to UVA which causes DNA damage and also breaks down what little vitamin D indoor workers get.
The authors found indoor solar UVA irradiation to be 25 percent of what a person gets outdoors.

So being indoors and exposing skin to the sun mostly through window glass reduces exposure to UVB that causes skin to make the vitamin D that prevents cancer, and increases relative exposure to UVA that destroys vitamin D in the skin and therefore increases cancer risk.