Abstract: A training mask for replicating the decrease in oxygen density and the increased breathing effort at higher altitudes. The mask has a nasal portion for covering the user's nose and a lower portion for covering the user's mouth. A peripheral edge conformable with the face forms a chamber with the user's mouth and nose contained therein. A valve-controlled air channel presents a first exterior aperture in communication with the outside ambient air and a second interior aperture positioned adjacent the user's mouth. A fibrous filter, or a plurality of such filters, are releasably engageable within the air channel. Upon inhalation the oxygen density of the ambient air, as drawn through the air channel and through the fibrous filter(s), is decreased. This decrease replicates the decrease in oxygen found in the ambient air at higher altitudes which requires the user to increase the breathing effort so as to deliver a sufficient amount of oxygen to the bloodstream.