Electromagnetic radiation is defined by its wavelength and frequency. Based on its biological activity, dermatologists subdivide the UV region into UVB (290-320 nm), UVA2 (320-340 nm), UVA1 (340-400 nm), and UVC (100-290 nm) radiation. UVC radiation is almost completely absorbed by stratospheric ozone, does not reach Earth's surface, and is not used therapeutically