Originally Posted by USD

Like you have mentioned, the ultraviolet region of the magnetic spectrum is divided into three different regions, UVA, UVB, and UVC. These three sub-divisions are rather arbitrary, however, as it seems that most disciplines originally defined them differently and have their own definition of what wavelengths fit in which division. Environmental photobiologists (who I tend to trust because they know plants) tend to define the wavelength regions as: UVA= 400-320nm, UVB= 320-290nm and UVC= 290-200nm. 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.