I have asthma, and I'm allergic to dust. I'm looking into buying an air purifier, but there are so many on the market, I don't know how to choose the best one. I've read that it should have a HEPA filter, but aren't there different kinds of HEPA filters? Have any of you purchased a purifier, and have you noticed that it improved the quality of the air in your home? Do you seem to breathe better?