UV is usually subdivided into near UV (200-380 nm wavelength) and extreme or vacuum UV (10-200 nm).
Ultraviolet light is the cause of [skin cancer]?s such as melanoma. The radiation ionises DNA molecules in skin cells, causing mutations which can result in cancerous growths. As a defence to this the body tan?s when exposed to moderate (depending on [skin type]?) levels of radiation by releasing the brown pigment melanin?. This helps to block UV and prevent damage to the vulnerable skin tissues deeper down.