Vitamin D isn't used directly in tanning beds, but the beds emit ultraviolet radiation that helps induce the production of vitamin D. UVB rays, as opposed to UVA rays, are the ones responsible for triggering the synthesis of the vitamin D precursor once the skin is exposed to it.
UVB rays feature shorter wavelengths, which causes them to penetrate only the surface layers of the skin. This also causes them to burn the skin quickly, which is why tanning beds utilize only 3 to 6 percent UVB rays. Both types of rays carry the risk of developing skin cancer, although UVA rays are more dangerous. Therefore, it is recommended to use a low-pressure tanning bed that utilizes a higher percentage of UVB rays.