Abstract: Portable room air purifiers having replaceable and generally cylindrical high efficiency particulate air filters which are mounted to receive incoming air which is drawn therethrough by centrifugal fans positioned adjacent deflectors in such a manner that air is directed radially outwardly through generally annular exhaust openings in a quiet and yet highly efficient manner. The fans, air deflectors and exhaust openings are designed to create outlet air flow patterns which will give a positive physical indication that the air filters should be replaced. In one embodiment of the invention, the filter units are stackable so as to permit increased air flow rates without modifying the centrifugal fan or fan motor.