Oxygen Masks for Aviators: A T-37 pilot wearing a mask designed for both diluter- and pressure-demand breathing. Inner view of a military aviators mask showing face seal, facepiece and inhalation valves. Three main kinds of oxygen masks are used by pilots and crews who fly at high altitudes: continuous flow, diluter demand, and pressure demand. In a continuous-flow system, oxygen is provided to the user continuously. It does not matter if the user is exhaling or inhaling as oxygen is flowing from the time the system is activated. Below the oxygen mask is a rebreather bag that collects oxygen during exhalation and as a result allows a higher flow rate during the inhalation cycle. Diluter-demand and pressure-demand masks supply oxygen only when the user inhales. They each require a good seal between the mask and the user's face. In a diluter-demand system, as the altitude increases (ambient pressure, and therefore the partial pressure of ambient oxygen, decreases), the oxygen flow increases such that the partial pressure of oxygen is roughly constant. Diluter-demand oxygen systems can be used up to 40,000 feet. In a pressure-demand system, oxygen in the mask is above ambient pressure, permitting breathing above 40,000 feet. Because the pressure inside the mask is greater than the pressure around the user's torso, inhalation is easy, but exhalation requires more effort. Aviators are trained in pressure-demand breathing in altitude chambers. Because they seal tightly, pressure-demand-type oxygen masks are also used in hyperbaric oxygen chambers and for oxygen breathing research projects with standard oxygen regulators