I bought 2 pieces at once, because. living area is large. When both were turned on, it showed air pollution 001, that is, everything is clean and the purchase was superfluous. In general, a thing for your reurance and nothing more. They say that with it the air becomes cleaner and there is supposedly less dirt, again, this is just to calm yourself and justify the purchase, in fact nothing has changed. I programmed them to turn on when the robot vacuum cleaner drives, I no longer know why they are needed.