A room air sterilizer mounts on the wall of the room and traps airborne particulate in a HEPA filter. An ultraviolet germicidal lamp destroys any biocontamination on the trapped particulates. The sterilizer has an elongated vertical housing with a return air grille near or at its lower end and a HEPA filter assembly disposed at an outflow port at its upper end. The HEPA filter is preferably a quarter cylinder and the sterilizer lamp is situated to expose the inner or intake side of the filter to the sterilizing ultraviolet radiation. A prefilter can be situated ahead of the blowers for the sterilizer, and can have an associated ultraviolet sterilizer lamp.