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A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. The essential component for any breathing gas is a partial pressure of oxygen 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. Air is the most common and only natural breathing gas. Other gases, either pure gases or mixtures of gases, are used in breathing equipment and enclosed habitats such as SCUBA equipment, surface supplied diving equipment, recompression chambers, submarines, space suits, spacecraft, medical life support and first aid equipment, high-altitude mountaineering and anaesthetic machines.〔(【引用サイトリンク】author=Tech Diver )〕 Most breathing gases are a mixture of oxygen and one or more inert gases.〔〔 Other breathing gases have been developed to improve on the performance of air by reducing the risk of decompression sickness, reducing the duration of decompression stops, reducing nitrogen narcosis or allowing safer deep diving.〔〔 A safe breathing gas for hyperbaric use has three essential features: * it must contain sufficient oxygen to support life, consciousness and work rate of the breather.〔〔〔 * it must not contain harmful gases. Carbon monoxide and carbon dioxide are common poisons which may contaminate breathing gases. There are many other possibilities.〔〔〔 * it must not become toxic when being breathed at high pressure such as when underwater. Oxygen and nitrogen are examples of gases that become toxic under pressure.〔〔〔 The techniques used to fill diving cylinders with gases other than air are called gas blending. Breathing gases for use at ambient pressures below normal atmospheric pressure are usually air enriched with oxygen to provide sufficient oxygen to maintain life and consciousness, or to allow higher levels of exertion than would be possible using air. It is common to provide the additional oxygen as a pure gas added to the breathing air at inhalation, or though a life-support system. == Common diving breathing gases == These common diving breathing gases are used: * Air is a mixture of 21% oxygen, 78% nitrogen, and approximately 1% other trace gases, primarily argon; to simplify calculations this last 1% is usually treated as if it were nitrogen. Being cheap and simple to use, it is the most common diving gas.〔〔〔 As its nitrogen component causes nitrogen narcosis, it is considered to have a safe depth limit of about 40 metres (130 feet) for most divers, although the maximum operating depth of air is 66.2 metres (218 feet).〔〔 * Pure oxygen is mainly used to speed the shallow decompression stops at the end of a military, commercial, or technical dive and is only safe down to a depth of 6 meters (maximum operating depth) before oxygen toxicity sets in.〔〔〔〔 It was much used in frogmen's rebreathers.〔〔 * ''Nitrox'' is a mixture of oxygen and air, and generally refers to mixtures which are more than 21% oxygen. It can be used as a tool to accelerate in-water decompression stops or to decrease the risk of decompression sickness and thus prolong a dive (a common misconception is that the diver can go deeper, this is not true owing to a shallower maximum operating depth than on conventional air).〔〔〔 * ''Trimix'' is a mixture of oxygen, nitrogen and helium and is often used at depth in technical diving and commercial diving instead of air to reduce nitrogen narcosis and to avoid the dangers of oxygen toxicity.〔〔〔 * ''Heliox'' is a mixture of oxygen and helium and is often used in the deep phase of a commercial deep dive to eliminate nitrogen narcosis.〔〔〔 * ''Heliair'' is a form of trimix that is easily blended from helium and air without using pure oxygen. It always has a 21:79 ratio of oxygen to nitrogen; the balance of the mix is helium.〔 * ''Hydreliox'' is a mixture of oxygen, helium, and hydrogen and is used for dives below 130 metres in commercial diving.〔〔〔 * ''Hydrox'', a gas mixture of hydrogen and oxygen, is used as a breathing gas in very deep diving.〔〔〔〔 * ''Neox'' (also called neonox) is a mixture of oxygen and neon sometimes employed in deep commercial diving. It is rarely used due to its cost. Also, DCS symptoms produced by neon ("neox bends") have a poor reputation, being widely reported to be more severe than those produced by an exactly equivalent dive-table and mix with helium.〔〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Breathing gas」の詳細全文を読む スポンサード リンク
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