Go to
www.sharperimage.com and look at them and then go to Ebay and they sell their returns for a cheaper price.
They are expensive, but since I raise parrotlets (world's smallest parrots) I have many in my house. The other air purifiers I had purchased didn't help and I was on allergy meds plus having to go get breathing treatments, plus listen to everyone nag about me getting rid of my birds, but these allow me to keep my birds, no breathing treatments, no daily medications, and I have only had a few asthma attacks (which use to last for several days) and as soon as my husband cleaned the "filter" I was fine within hours.