Philips AC4025 Air purifier has advanced filtration system which eliminates harmful agents, bacteria and allergens. The filteration is done in 3 stages . This purifier operates very quietly. The purifier witches to lower activity level, the fan speed and noise level are reduced and energy consumption is minimized. Healthy air protect alert gives you timely warning when to replace the filter. Appliance operates for a set of hours and automatically switches off when set time has elapsed.