Philips AC4072 Air Purifier with professional filtration system , smart sensor gives you feedback on indoor air quality. Fully automatic operation ensures hassle-free functioning. The built-in sensor measures the air quality in the room and automatically selects appropriate speed. The healthy air protect lock assures you of healthy air always. 4-step light indicators clearly show air quality level: Blue - Very Good; Deep Purple - Good; Purple - Fair; Red – Bad. 5-step fan speed allows you to adjust speed level you want. Under the Night mode, the appliance switches to lower activity level, the lights go out (except Power On/Off light) , the fan speed and noise level are reduced and the energy consumption is minimized. If the room becomes bright again for 5 minutes or longer, the appliance resumes normal operation. Healthy air protect lock function ensures you healthy air always. It shuts off the purifier when healthy air can no longer be guaranteed.