A method and apparatus for removing airborne contaminants in residential or commercial buildings injects filtered air into these enclosures. Prior to be injected in these enclosures, the air is filtered for chemical, biological or radioactive hazards. The filtering of the air ensures that the enclosure is a safe haven. The apparatus includes an enclosure, a filtration assembly for removing airborne biological, chemical or radioactive agents mounted with the enclosure. A series of expansion slots can be mounted within the enclosure to assembly a multi-stage filtering process. A blower is also included in the apparatus for overcoming the resistance of the filtration assembly and increases the air pressure inside the building to achieve protective overpressure. A control system within the apparatus monitors the pressure and adjusts the airflow in the system in order to maintain the protective overpressure.