I keep the air400 running on Auto 24/7. I purchased it in fear that my air quality was very poor, I have 3 dogs and 1 cat in a 3000square foot house. How could it not be bad, right? Well, I plugged it in and let it run for hours... we stayed at a 3. I was very impressed that I just let it run. It spikes when we make ethnic food for dinner but within a couple of hours, back to a 2 or 3. I would recommend everyone having at least one in their house.