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Studies have been conducted in a sealed environment altitude chamber to study the chronic effect of reduced barometric pressure without reduced oxygen tension on human respiratory function and other physiological processes. Experiments were conducted at ground level, at 18,000 ft equivalent altitude (pO2 150 mm Hg), at 27,000 ft equivalent altitude (pO2 243 mm Hg), and at 33,500 ft equivalent altitude (pO2 174 mm Hg). Results were as follows: 1) Tests of pulmonary functions which are dependent on high volume flow rates are increased at altitude. These tests include maximum breathing capacity (increased 37% at 33,500 ft) and per cent of forced vital capacity during the first second of effort. 2) Expiratory minute volume was unchanged at altitude when oxygen partial pressure was maintained. 3) Forced vital capacity was reduced 3.1% at 18,000 ft, 2.9% at 27,000 ft, and 7.6% at 33,500 ft. This result was not due to hypoxia or abdominal distention and confirms the earlier findings of Rahn and Hammond. The cause of this change remains obscure but may be related to decreased lung volume. 4) The magnitude of changes observed appears to be somewhat proportional to altitude. Evidence is presented to relate increases in the flow-dependent tests to reduced density and reduced turbulent flow in the respiratory passages. respiratory physiology; sealed environment; spacecraft; atmosphere selection; space medicine Submitted on October 22, 1963

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