Abstract: A system (10) for controlling the quality of the cabin air in an aircraft of the type having a cabin air recirculating system (37, 43) which introduces selectable amounts of fresh outside air, an arrangement (25, 27) for venting controllable quantities of cabin air to the atmosphere, and a source of oxygen-rich air (49). The system (10) includes sensors for monitoring the partial pressure of carbon dioxide (15) in the cabin air, the partial pressure of oxygen (17) in the cabin air, the humidity (19) of the cabin air, and the total air pressure (21) of the cabin air, and an air quality controller (23). The air quality controller (23) includes at least four inputs, one input connected to each of the sensors, and at least two outputs, one output connected in controlling relationship to the source (49) of oxygen-rich air and another (47) connected in controlling relationship to the cabin air recirculating system.