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There are important differences between Ultraviolet A (UVA) and Ultraviolet B (UVB).Some of these differ-ences have only recently been recognized. There are 2 definitions of UVB. The physics definition is 290 to 315nanometers, the medical definition is 290 to 320 nanometers. Clinically 320 nm was chosen as the cut off be-cause it is cut off by ordinalv window glass.I will use the medical definition.