Abstract: Apparatus for producing a hypoxic gas mixture having a higher nitrogen-to-oxygen ratio than atmospheric air has a diaphragm compressor feeding air under pressure to an oxygen-depleting unit having a perforate tube extending axially of a cylindrical chamber having an outlet in one side. The annular space between the perforate tube and the wall of the chamber is filled with axially extending hollow fibers of poly-4-methyl-penthene-1. Ends of the chamber are closed except for the interiors of the hollow fibers which are open to the atmosphere. The walls of the hollow fibers are more permeable to oxygen than to nitrogen so that part of the oxygen escapes through open ends of the hollow fibers, thereby increasing the nitrogen-to-oxygen content of the gas.