If maintenance of normal respiratory function were the only consideration, a cabin atmosphere of about sea-level composition and pressure might be an ideal and straightforward choice for manned spacecraft. In fact, this atmosphere has been used in the manned space flights conducted by the U.S.S.R. No other atmosphere has been shown to be more satisfactory from the physiological point of view, and the tedious respiratory studies which should accompany the use of other atmospheres can be avoided. Nevertheless, the formidable problems of spacecraft design and the necessary precautions for safeguarding the crew from accident require that other atmospheric compositions and pressures be considered. For example, if a cabin at 1-atm pressure were decompressed to space suit pressure (0.3 atm), the occupants would develop decompression sickness; i.e., "bends." Several engineering considerations argue for low cabin pressures and pure oxygen composition. Among these are structural design, weight of atmospheric gas storage and control equipment, and the difficulty of contriving pressure suits which allow operation at pressures near one atmosphere. Such departures from the normal human gaseous environment, however, require the demonstration of an acceptable level of safety and physiological performance. The limits of the composition and pressure of acceptable cabin atmospheres are then set by— (1) A pure oxygen atmosphere at a pressure which will provide an alveolar oxygen partial pressure equal to that provided by air at sea level (2) A mixed gas (oxygen and inert gas) atmosphere having a pressure and composition that will allow decompression to the highest acceptable suit pressure without the risk of bends A numerical value for the lower limit (1) is approximately 0.2 atm of pure oxygen. The upper limit (2) is determined by the operating pressure and composition of the space-suit atmosphere and may be of the order of 0.5 atm for a cabin atmosphere of 50 percent oxygen. It is necessary to determine the astronaut’s ability to survive and perform his duties in any atmosphere selected. Atelectasis and Pulmonary Edema Localized or diffuse collapse of alveoli in the lungs may, if the condition persists, lead to arterial hypoxia which may be extremely undesirable under the stresses of space flight. The alveoli are probably unstable when pure oxygen is breathed; they tend to collapse if there is blockage of the airways, especially at low pressures. This collapse occurs because each of the gases present in the alveoli (oxygen, water vapor, and carbon dioxide) is subject to prompt and complete absorption from the alveoli by the blood. The alveoli are normally stabilized against collapse by the presence of inert and relatively insoluble gas (nitrogen) and an internal coating of lipoprotein substances with low surface tension.