david s
Some room air purifiers use only a fan and a filter; they are not very efficient. The best ones use electrostatically-charged plates that attract and hold the tiny dust particles, and lots of other pollutants as well. Some have a UV lamp to help keep the germs down. Avoid the units that generate lots of Ozone. Ozone is a respiratory irritant, regardless of what some folks say. Oreck makes a good unit that is quiet, efficient, and not very costly. I have one; it's great.