Portable, rapidly deployable filter unit for use in air filtering methods providing high efficiency filtration of airborne toxins. The filter unit includes a fan section and a filter section that are releasably attachable at their respective lateral ends permitting the filter unit to be reconfigurable to properly handle a variety of threat scenarios involving protecting the occupant(s) and equipment in an enclosure against an airborne release of toxic chemical, biological, or radiological agents that threaten to contaminate the air supply, or, alternatively, for decontaminating the air of an isolation room occupied by contaminated or contagiously ill persons or materials and protecting the occupants or equipment located outside the isolation room.