An air treatment system for at least partially removing at least one gaseous contaminant contained in indoor air of a room structured for human occupants. The system may comprise an air treatment assembly having an indoor air inlet configured to receive indoor airflow directly from a room, a regenerable adsorbent material configured to adsorb at least one gaseous contaminant contained in the indoor airflow, at least one airflow element for directing the indoor airflow to flow through the air treatment assembly, an indoor air outlet for expelling the indoor air, from the air treatment assembly back into the room, a purge air inlet configured to receive and direct purge air over and/or through the adsorbent material for removal of at least a portion of the at least one gaseous contaminant, and a purge air outlet for expelling the purge air out of the air treatment assembly.