An air treatment system (1, 1') is described, for a closable room (2) of a vehicle (3), comprising: an air conditioning device (6) provided with a fan (7) and connectable to the room (2); an inlet opening (9) for outside air, connected to the air conditioning device (6); a filtration device (11) for filtering the outside air entering from the inlet opening; a recirculation port (15, 15'); a valve assembly (16) provided with a shutter member (30, 30'), interposed between the inlet opening (9), the recirculation port (15, 15') and the air conditioning device (6) and movable between a plurality of operating positions, in which it varies the shares of air coming from the inlet opening (9) and the recirculation port (15, 15'); and a control unit (32) to adjust the operating position of the shutter member (30, 30') so as to control the shares of air being drawn from the inlet opening (9) and from the recirculation port (15, 15') and to maintain a predetermined value of overpressure in the room (2) with respect to the outside.