Where you live. The further away from the Equator you live, the less vitamin D–producing UVB light reaches the earth’s surface during the winter. Residents of Boston, for example, make little if any of the vitamin from November through February. Short days and clothing that covers legs and arms also limit UVB exposure.
Air quality. Carbon particles in the air from the burning of fossil fuels, wood, and other materials scatter and absorb UVB rays, diminishing vitamin D production. In contrast, ozone absorbs UVB radiation, so pollution-caused holes in the ozone layer could end up enhancing vitamin D levels.
Use of sunscreen. Sunscreen prevents sunburn by blocking UVB light. Theoretically, that means sunscreen use lowers vitamin D levels. But as a practical matter, very few people put on enough sunscreen to block all UVB light, or they use sunscreen irregularly, so sunscreen’s effects on vitamin D might not be that important. An Australian study that’s often cited showed no difference in vitamin D between adults randomly assigned to use sunscreen one summer and those assigned a placebo cream.