A ventilation system is disclosed for an enclosure in which people live. The ventilation system comprises a first ducting means for ducting fresh air from outside to inside the enclosure and a second ducting means for ducting stale air. The second ducting means comprises a stale air inlet for receiving stale air containing carbon dioxide from inside the enclosure and an exhaust manifold having a first vent to exhaust air outside the enclosure and a second vent in fluid communication with the first ducting means to return clean air into the first ducting means. The ventilation system is further characterised in that it comprises catalysing means for catalysing hydration of carbon dioxide contained in at least a portion of the stale air into a solution of carbonate ions and hydrogen ions, and thereby removing carbon dioxide from said portion of the stale air and obtaining clean air. Evacuating means are also provided for evacuating the solution out of the ventilation system.