An apparatus is provided for hypoxic training or therapy. The apparatus provides a user air having a lower oxygen content than the ambient atmosphere. The apparatus reduces the oxygen content of the air by passing the ambient air through a membrane separator which extracts the oxygen therefrom by a vacuum pump attached to the permeate outlet of the separator. Alternatively, the oxygen content can be reduced by adsorption of oxygen from the ambient air or by adding a low oxygen gas to air taken from the environment. This low oxygen gas may be stored nitrogen or stored gas which is low in oxygen, or it may be derived from the exhaled air from the user. If the exhaled air from the user used, carbon dioxide and water are extracted by a membrane separator or by physical or chemical process which adsorbs carbon dioxide from the gas.