Abstract: There is provided an apparatus for preparing a ventilation gas mixture that comprises a gas mixing device, a first gas feed arranged to supply a first gas to the gas mixing device, an air inlet configured to receive exhaled air from a person, a gas reservoir arranged to store carbon dioxide from the air received in the air inlet and further arranged to supply the stored carbon dioxide to the gas mixing device via a second gas feed, wherein the gas mixing device is arranged to combine the first gas with the carbon dioxide in order to prepare the ventilation gas mixture.