Ultraviolet light is split into several different ranges: (near the visible light range) UV-A, UV-B, UV-C, Vacuum-UV (near the x-ray range). The UV-B range is what causes sunburn on humans. UV water purifiers use the UV-C range because of its germicidal abilities. In this range the light light will break molecular bonds in the DNA of viruses and bacteria, rendering them unable to reproduce and effectively killing them.