Part of the common marketing practice these days, the listing of the bacteria in the water that are destroyed by the action of the UV water purifiers clearly tells a buyer what to expect when buying such an item. Ultraviolet light is widely considered one of the most powerful disinfectants discovered so far, which would explain the extensive range of applications both in households and industry. What exactly happens in the UV water purifiers that the results are so remarkable? We should begin by saying that the density of micro-organisms in water very much depends on the source, the geographical area as well as the climate.