Helps reduce white dust, which is a deposit of minerals caused by hard water. For best results replace filter after every 30-40 fillings under normal use or 1-2 months or when the humidifier has not been operated for an extended period of time. To install, simply replace the tank cap of the water tank with the filter cartridge. Place full water tank back on the water basin and wait fifteen minutes for the water to pass through the filter before re-starting the humidifier. The green indicator light on the humidifier will show that enough water is in the base and the filter ready to function.