A regenerative filter system includes a regenerative filter material arranged to filter air recirculated from an aircraft interior and means for passing a regeneration air stream through the regenerative filter material to regenerate the filter material by removing adsorbed contaminant therefrom, whereby the means for passing the regeneration air stream comprises means for expanding air exhausted from the aircraft interior and using the expanded air to drive a compressor for compressing and conditioning ram air to provide the regeneration air stream to the filter material.