A system for improving the air quality in a pressure cabin of an aircraft comprises a recirculation air circuit by means of which a portion of the cabin air, referred to as recirculation air, is withdrawn from the pressure cabin, filtered and then recirculated into the pressure cabin. To increase the oxygen partial pressure of the air fed back into the pressure cabin, the quantity of recirculation air is passed wholly or partially through a nitrogen generator (52) which generates nitrogen and an oxygen-containing waste gas from the recirculation air supplied. The oxygen-containing waste gas of the nitrogen generator (52), together with any remaining quantity of recirculation air untreated by the nitrogen generator (52), is recirculated into the pressure cabin. The nitrogen generator (52), to which only recirculation air is supplied, utilises a pressure gradient existing in flight between the pressure cabin and the outside of the aircraft for nitrogen generation.