An air treatment system includes a recirculation line (24) through which air from the enclosed space can be recirculated, and a contaminant adsorption assembly (36) is provided in an auxiliary supply conduit (31). In the event that contaminants are detected in atmospheric air, air from the enclosed space can be recirculated for conditioning and then readmitted to the enclosed space. Oxygen that is breathed by occupants of the enclosed space can be replenished by admitting air from atmosphere, which is treated using the adsorption assembly and then mixed with air that is in the enclosed space or being recirculated for admission to the enclosed space.