Heliox is a breathing gas mixture of helium (He) and oxygen (O2). It is used as a medical treatment for patients with difficulty breathing because mixture generates less resistance than atmospheric air when passing through the airways of the lungs, and thus requires less effort by a patient to breathe in and out of the lungs. It is also used as a breathing gas diluent for deep ambient pressure diving as it is not narcotic at high pressure, and for its low work of breathing. Heliox has been used medically since the 1930s, and although the medical community adopted it initially to alleviate symptoms of upper airway obstruction, its range of medical uses has since expanded greatly, mostly because of the low density of the gas. Heliox is also used in saturation diving and sometimes during the deep phase of technical dives. Medical uses In medicine heliox may refer to a mixture of 21% O2 (the same as air) and 79% He, although other combinations are available (70/30 and 60/40). Heliox generates less airway resistance than air and thereby requires less mechanical energy to ventilate the lungs. "Work of Breathing" (WOB) is reduced by two mechanisms: increased tendency to laminar flow; reduced resistance in turbulent flow due to lower density. Heliuox 20/80 diffuses 1.8 times faster than oxygen, and the flow of heliox20/80 from an oxygen flowmeter is 1.8 times the normal flow for oxygen. Heliox has a similar viscosity to air but a significantly lower density (0.5 g/L versus 1.25 g/L at STP). Flow of gas through the airway comprises laminar flow, transitional flow and turbulent flow. The tendency for each type of flow is described by the Reynolds number. Heliox's low density produces a lower Reynolds number and hence higher probability of laminar flow for any given airway. Laminar flow tends to generate less resistance than turbulent flow. In the small airways where flow is laminar, resistance is proportional to gas viscosity and is not related to density and so heliox has little effect. The Hagen–Poiseuille equation describes laminar resistance. In the large airways where flow is turbulent, resistance is proportional to density, so heliox has a significant effect. There is also some use of heliox in conditions of the medium airways (croup, asthma and chronic obstructive pulmonary disease). A recent trial has suggested that lower fractions of helium (below 40%) thus allowing a higher fraction of oxygen