Originally Posted by USD

... 290nm was chosen as the distinction between UVB and UVC because UVR in the shorter wavelengths are unlikely to be present on Earth's surface, other than at high altitudes . The division between UVA and UVB does seem to be rather arbitrary however, as I have yet to read a definitive explanation. More recently what does seem to becoming clear now, however, is that electromagnetic radiation occurring at wavelengths shorter than 320nm are thought to be generally more active photobiologically than longer wavelength UVR.