Aging of the skin is the result of both the intrinsic chronological aging process and extrinsic damage caused by environmental factors. A major role of the skin is that of protection from external environmental factors. Ultraviolet radiation (UVR) is the most significant environmental insult to the skin. UVR comprises the spectrum of electromagnetic radiation between the wavelengths of 200 and 400 nm. UVR is subdivided into three categories, each of which has distinct biological effects: UVA (320-400 nm), UVB (280-320 nm), and UVC (200-280 nm). The stratospheric ozone blocks the radiation whose wavelength is below 290 nm, effectively preventing the entire UVC spectrum and part of the UVB spectrum from reaching human skin. The UVR that does reach the human skin can cause molecular defects including DNA damage, lipid peroxidation, and protein cross-linking, which can lead to premature skin aging or photoaging. Photoaging is a term used to describe the clinical and histological features of chronically UV-exposed skin .
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