When it arrived, I put it on high and left the room for a few hours.  I could tell the difference the second I re-entered the room! This air purifier works amazingly well, and I could not be more happy with this purchase, as I am an allergy sufferer but was skeptical that a room air purifier would be powerful enough to really make a difference.  There was an immediate and sustained improvement in the air quality. I'm thinking of getting more for other rooms in the house, and wishing I had bought this a long time ago!