In medicine<unk>  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).<unk>  generates less airway resistance than air and thereby requires less mechanical energy to ventilate the lungs. "Work of Breathing" (WOB) is reduced. It does this by two mechanisms: # increased tendency to laminar flow; # reduced resistance in turbulent flow.<unk>  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.<unk> '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<unk>  has little effect. The Hagen–Poiseuille equation describes laminar resistance. In the large airways where flow is turbulent, resistance is proportional to density, so<unk>  has a significant effect.<unk>  has been used medically since the early 1930s. It was the mainstay of treatment in acute asthma before the advent of bronchodilators. Currently,<unk>  is mainly used in conditions of large airway narrowing (upper airway obstruction from tumors or foreign bodies and vocal cord dysfunction). There is also some use of<unk>  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 might also have the same beneficial effect on upper airway obstruction. Patients with these conditions may suffer a range of symptoms including dyspnea (breathlessness), hypoxemia (below-normal oxygen content in the arterial blood) and eventually a weakening of the respiratory muscles due to exhaustion, which can lead to respiratory failure and require intubation and mechanical ventilation.<unk>  may reduce all these effects, making it easier for the patient to breathe.<unk>  has also found utility in the weaning of patients off mechanical ventilation, and in the nebulization of inhalable drugs, particularly for the elderly. Research has also indicated advantages in using helium–oxygen mixtures in delivery of anaesthesia.
Heliox
Khat
Tabun 
Riken
Phencyclidine
Medicen
Heroin
Keldur