Airfree is the only air purifier manufacturer that is not based around a HEPA filter. In fact, their models have no filters at all, relying instead on a unique patented technology for the sterilization and incineration of allergens. Thus there is no maintenance costs associated with any Airfree model, and we have had excellent experience with their reliability. Since Airfree purifiers rely on natural air flow in a room because they do not have a fan, we see a single unit as better for a small room. They are also a manufacturer that backs up their claims with serious verifiable research. We recommend the AirFree Sterilizer primarily for mold and dust mite control, viruses and bacteria.