Exposing yourself to sunlight is the most important source of vitamin D because sunlight is far more likely to provide you with your vitamin D requirement than food is. UV rays from the sun trigger vitamin D production in your skin. Lights from your home are not strong enough to produce vitamin D. Season, geographic latitude, time of day, cloud cover, smog, and sunscreen affect UV ray exposure and vitamin D synthesis. For example, sunlight exposure from November through February in Boston is insufficient to produce significant vitamin D synthesis in the skin.
Obviously any point of north of Boston is worse. Complete cloud cover halves the energy of UV rays, and shade reduces it by 60%. Industrial pollution also filters sun exposure and may contribute to the development of rickets if you have insufficient dietary intake of vitamin D.
Sunscreens with a sun protection factor (SPF) of 8 or greater will block UV rays that produce vitamin D. An initial exposure to sunlight of 10 to15 minutes allows you adequate time for Vitamin D synthesis and should be followed by application of a sunscreen with an SPF of at least 15 to protect the skin. Ten to fifteen minutes of sun exposure at least two times per week to the face, arms, hands, or back without sunscreen is usually sufficient to provide adequate vitamin D. If you have limited sun exposure it is very important that you include good sources of vitamin D in your diet or supplement.