Ultraviolet rays react with in the skin to begin the process of forming vitamin d.

Ultraviolet rays react with cholesterol in the skin to begin the process of forming vitamin D, Cholesterol is a sterol, which is found in body tissues and in the blood plasma, it is transformed, by the sun UV rays into ergocalciferol, a vitamin D, necessary for the absortion of other vitamins such as calcium, important for bone health, and to regulate the leves os phosphorus in blood. A deficiency of vitamin D in the body could generate great bone damages such as rickets, osteomalacia or even osteoporosis in a long term.