An air filter system and an air filter therefor are provided for use in a building with first and second zones separated by a wall-door system having a wall with a door therein. Weatherstripping is mounted between side and upper edges of the door and a door frame in the wall and between the lower edge of the door and the threshold region of the floor to substantially seal against air flow between the door edges and the door frame and floor when the door is closed. An air flow permitting opening is provided in the wall-door system, sized to permit a controlled amount of airflow between the first and second zones when there is a pre-determined pressure differential between said zones. An air filter is mounted in such opening.