I haven't had my Alen air purifier very long, so I'm not real sure yet. I admit that I have it in an area a little larger than it stated it could handle, and I do think I notice the air is better when I'm closer to where the unit stands.
I've also noticed that my Alen kicks into high mode when my furnace comes on which makes me believe I need to have my furnace ducts cleaned?
I'm feeling positive though and may write a better review after I've had it a while longer.