Abstract: A filtering unit for a bus, the unit comprising one outer housing of a size to fit between the row of seats in the bus and a forwardly facing wall of a bus lavatory behind the seats, the housing having air inlet and air outlet openings for communication with ducts opening in upper and lower portions of the lavatory. The housing comprises a main housing body with an interior horizontal passageway of consistent cross sectional shape extending parallel to the rear of the outer housing, the main housing body having a horizontally facing opening at the outer end thereof which is covered by a panel which is movable to expose said opening, and a drawer unit slidable into or removable from said passageway through said opening. The drawer contains at least an air filter for removing at least lavatory odor-causing gaseous and particle materials from the air stream passing therethrough and an air circulating fan and drive motor therefor.