The rebreathing device for controlling a gas mixture inhaled by a user formed of carbon dioxide filtered expired gas and ambient air. The device has a conduit 3, a carbon dioxide sequestration chamber 6, and a gas reservoir 8. The conduit 3 provides fluid communication between the user and the sequestration chamber 6, which is in turn in fluid communication with the reservoir 8. An aperture in the reservoir 8 provides fluid communication with ambient air. There may be no open-cell foam in the reservoir. Deflecting surfaces may be in the reservoir 8, which may form a labyrinth. The volume of the reservoir 8 may be adjustable or may be telescopic. The size or number of apertures may be adjustable. The sequestration chamber 6 may be a replaceable cartridge that changes colour when approaching saturation. The device may allow for simulating altitude training, i.e. intermittent hypoxic training, IHT, by providing inhalation gas mix of reduced oxygen and reduced carbon dioxide when compared to ambient air.
