"The Air Force recently grounded some of its newest aircraft, F-35A strike fighters, due to incidents in which pilots became physiologically impaired with symptoms of oxygen deficiency while flying. Although the root cause of the F-35 incidents has not yet been established, the grounding has renewed attention on hypoxia, a physical condition caused by oxygen deficiency that may result in temporary cognitive and physiological impairment and possible loss of consciousness. Hypoxia has affected pilots of F-22, F/A-18, and T-45 aircraft in recent years. Military aircraft (and jet aircraft generally) operate at altitudes where there is not enough oxygen for humans to retain consciousness without supplemental oxygen supplies. For several decades, combat aircraft relied on canisters of compressed liquid or gaseous oxygen to provide pilots sufficient oxygen. However, the finite oxygen supply limited mission duration, and its handling complicated aircraft maintenance. This led to the development of On-Board Oxygen Generation Systems, or OBOGS. Developed in the early 1980s, OBOGS was conceived as a source of limitless oxygen."
CRS Insight, IN10723
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html