page_title: Breathing gas
section_title: Hypobaric breathing gases
hierarchical_section_title: Breathing gas / Hypobaric breathing gases
caption_attribution_description: English: Astronaut Edward M. (Mike) Fincke, Expedition 9 NASA ISS science officer and flight engineer, wearing a Russian Orlan spacesuit, participates in the third of four sessions of extravehicular activities (EVA) performed by the Expedition 9 crew during their six-month mission. Fincke and cosmonaut Gennady I. Padalka (out of frame), commander representing Russia's Federal Space Agency, spent 4 ½ hours outside the Station swapping out experiments and installing hardware associated with Europe’s Automated Transfer Vehicle (ATV), scheduled to launch on its maiden voyage to ISS next year. A cloudy Earth provided the backdrop for the image. According to a label on his chest, he is wearing a Orlan-M suit.
context_page_description: A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common, and only natural, breathing gas. But other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed habitats such as scuba equipment, surface supplied diving equipment, recompression chambers, high-altitude mountaineering, high-flying aircraft, submarines, space suits, spacecraft, medical life support and first aid equipment, and anaesthetic machines.
  Oxygen is the essential component for any breathing gas, at a partial pressure of between roughly 0.16 and 1.60 bar at the ambient pressure. The oxygen is usually the only metabolically active component unless the gas is an anaesthetic mixture. Some of the oxygen in the breathing gas is consumed by the metabolic processes, and the inert components are unchanged, and serve mainly to dilute the oxygen to an appropriate concentration, and are therefore also known as diluent gases. Most breathing gases therefore are a mixture of oxygen and one or more inert gases.
context_section_description: Breathing gases for use at reduced ambient pressure are used for high altitude flight in unpressurised aircraft, in space flight, particularly in space suits, and for high altitude mountaineering. In all these cases, the primary consideration is providing an adequate partial pressure of oxygen. In some cases the breathing gas has oxygen added to make up a sufficient concentration, and in other cases the breathing gas may be pure or nearly pure oxygen. Closed circuit systems may be used to conserve the breathing gas, which may be in limited supply - in the case of mountaineering the user must carry the supplemental oxygen, and in space flight the cost of lifting mass into orbit is very high.
caption: Astronaut in an Orlan space suit, outside the International Space Station
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