Open systems such as bottled water dispensers have a human factor – someone has to replace the bottles. They may introduce dirt and germs into the system, by touching the top of the bottle or the connection point, sneezing on it, or allowing dust and germs in the air to come into contact with the water supply. Dirt and other impurities and germs, bacteria, fungi, and biofilm may end up in contact with the water. Compare closed system water coolers, such as bottleless water, which have internal water supplies and always give clean and healthy water.