For sale: Air-O-Swiss humidifier AOS W2255 These humidifiers evaporate the water with revolving disks. Biggest benefit of these humidifiers over the vaporizer type is that only water is dissolved into your living or bedroom air. Vaporizers actually vaporize both water and other chemicals (Chlorine etc) that are in Beijing water. This causes coughing etc. Besides humidifying, it’s also an air cleaner that removes house dust and pet dander from the air by trapping it in the water. One humidifier can cover an area up to 50m2. Does not require filters, just uses water. We normally fill each tank once or twice per day depending on humidity and setting. Tank capacity: 7 liters. Very quiet, both in day and night setting (<25dB) 3 years old, all in great condition; no scratches (inside some scaling due to the hard Beijing water) New price on TMall / Taobao is 1,700-2,000 RMB