Abstract: A controller is associated with a cabinet which contains apparatus and components for performing operations or conducting processes. Associated with the cabinet is a housing within which is contained an air filtration material. Air is admitted and propelled for flow through that material and delivery into the confined space defined by the cabinet by a blower unit. The blower unit is controlled in response to the rate of flow of air through the housing in order to maintain a selected rate of flow in compensation for changes in impedence to flow of air through the material. Operation also is controlled in response to other parameters, such as pressure differential across the material, pressure differential across a prefilter upstream, a measure of the rate of air flow into the cabinet, ambient temperature within the cabinet, pressure differential between outside atmospheric pressure and each of the housing and the cabinet and relative humidity within the cabinet.