Sunshine is the main source for vitamin D. Sun’s rays produces three types of ultraviolet (UV) radiation: UVA, UVB, and UVC. Both UVA and UVB reach the Earth’s surface.
Short exposure to UVB starts the cascade of events that produces vitamin D. However, too much exposure to UVB and UVA may lead to skin cancers and aging of the skin. Protecting the skin with sunscreen also blocks production of vitamin D.
Many other factors influence your ability to obtain adequate vitamin D production from the sun.
Time of year: the angle of the sun varies throughout the year and creates different intensities of radiation. In most parts of the United States, the intensity of the sun necessary to produce vitamin D is only available between the months of May through September.
Time of day: the midday sun is the most intense.
Where you live: the greater the distance from the equator, the lower the sun intensity. Higher altitude with lesser atmosphere increases the sun intensity.
Weather: clouds, fog, or air pollution will decrease UVB radiation.
Don’t count on the sun to give you adequate vitamin D levels year-round. In the summer months, your sunscreen is blocking the production of vitamin D. In the wintertime, there’s insufficient UVB radiation to produce vitamin D.