An aircraft environmental control system includes means for mixing and conditioning bleed air from a bleed air input and recirculation air from an aircraft interior to provide mixed, conditioned air to the aircraft interior. The system also includes a first contaminant removal device and a second contaminant removal device arranged in a path of at least part of the recirculation air, prior to the means for mixing and conditioning, and a valve (SV1) arranged to alternate flow of recirculation air through the first and second contaminant removal devices.