Abstract: An apparatus for removing gaseous contaminants from an air flow which divides the air flow into a first air flow and at least one other air flow. A cleaning device cleans the first air flow and the cleaned airflow combines with all other air flows to form an outlet air flow. The first air flow is small when compared to the sum of the first air flow and each other air flow. The gas phase cleaning device passes the entire first air flow through a cleaning bed having a thickness of greater than the Critical Thickness for the contaminant of concern.