An aircraft environmental control system comprising a bleed air or compressed ambient air input and a RAM air input, heat exchanger means for receiving bleed air from the bleed air input and RAM air from the RAM air input and using RAM air to cool the bleed air, and means for providing the cooled bleed air to an interior of the aircraft; the system further comprising means for feeding back exhaust air emitted from the interior of the aircraft to combine with the RAM air to further cool the bleed air or compressed ambient air.