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Mehmet Oz, MD, Cardiology, answeredExposure to the sun’s ultraviolet rays has a double effect. On one hand, it activates a chemical in the skin that produces vitamin D, an essential vitamin necessary for many bodily functions. But it also can be damaging: Too much ultraviolet radiation (UVA and UVB rays) can cause melanoma and other types of skin cancer. Not enough sunlight can cause a vitamin D deficiency, a recently proposed suspect in cancer of the colon, rectum, and pancreas.
The sun emits ultraviolet rays along the entire spectrum, from visible light all the way to invisible X-rays. The lowest UV bands, UVA, UVB, and UVC, are considered the most often and are probably the most applicable to daily life. Almost all UVC rays are blocked by the same oxygen gas we survive on. Ozone blocks a significant amount of UVB but only very little of UVA light. All three can damage collagen fibers in the skin and thus cause aging. UVA and UVB can cause other molecules to change into dangerous forms, thus indirectly damaging DNA. UVB and UVC can directly damage DNA by affecting bonds between different molecules in the DNA's structure.