An environmental control system that includes a first environmental control system pack, a first recirculation fan assembly, and a cabin air sensor unit. The first environmental control system pack is arranged to provide conditioned air to an aircraft cabin. The first recirculation fan assembly is arranged to recirculate a first cabin air fluid flow back to the aircraft cabin. The cabin air sensor unit is arranged to provide a first air quality signal indicative of an air quality of the first cabin air fluid flow to a controller.