An apparatus for providing controlled flow of inhalation-air having relatively high and low oxygen concentrations from dual chambers 210,215 of an air-reservoir 205 to a mask 130 includes a switch unit 110 and a control unit which includes a housing 115 to receive the inhalation-air from the air-reservoir via a plurality of ducts 120,125 with a valve 135 configured to control the flow of inhalation-air from the plurality of ducts. The switch unit positions the valve to selectively open and close the plurality of ducts for regulating the flow of inhalation-air from the air-reservoir to the housing. The apparatus enables rapid switching between high and low oxygen partial pressures such as for simulating contrasting altitude training.
