work of breathing to intolerable levels, and can cause carbon dioxide retention at lower densities. helium is used as a component to reduce density as well as to reduce narcosis at depth. like partial pressure, density of a mixture of gases is in proportion to the volumetric fraction of the component gases, and absolute pressure. the ideal gas laws are adequately precise for gases at respirable pressures. the density of a gas mixture at a given temperature and pressure can be calculated as: ρm = (ρ1 v1 + ρ2 v2 + .. + ρn vn) / (v1 + v2 + ... + vn) where ρm = density of the gas mixture ρ1 ... ρn = density of each of the components v1 ... vn = partial volume of each of the component gases since gas fraction fi (volumetric fraction) of each gas can be expressed as vi / (v1 + v2 + ... + vn ) by substitution, ρm = (ρ1 f1 + ρ2 f2 + .. + ρn fn) hypobaric breathing gases astronaut in an orlan space suit, outside the international space station 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. medical breathing gases medical use of breathing gases other than air include oxygen therapy and anesthesia applications. oxygen therapy main article: oxygen therapy a person wearing a simple face mask for oxygen therapy oxygen is required by people for normal cell metabolism. air is typically 21% oxygen by volume. this is normally sufficient, but in some circumstances the oxygen supply to tissues is compromised. oxygen therapy, also known as supplemental oxygen, is the use of oxygen as a medical treatment. this can include for low blood oxygen, carbon monoxide toxicity, cluster headaches, and to maintain enough oxygen while inhaled anesthetics are given. long term oxygen is often useful in people with chronically low oxygen such as from severe copd or cystic fibrosis. oxygen can be given in a number of ways including nasal cannula, face mask, and inside a hyperbaric chamber. high concentrations of oxygen can cause oxygen toxicity such as lung damage or result in respiratory failure in those who are predisposed. it can also dry out the nose and increase the risk of fires in those who smoke. the target oxygen saturation recommended depends on the condition being treated. in most conditions a saturation of 94-98% is recommended, while in those at risk of carbon dioxide retention saturations of 88-92% are preferred, and in those with carbon monoxide toxicity or cardiac arrest the saturation should be as high as possible. the use of oxygen in medicine become common around 1917. it is on the world health organization's list of essential medicines. the cost of home oxygen is about us$150 a month in brazil and us$400 a month in the united states. home oxygen can be provided either by oxygen tanks or an oxygen concentrator. oxygen is believed to be the most common treatment given in hospitals in the developed world. anaesthetic gases a vaporizer holds a liquid anesthetic and converts it to gas for inhalation (in this case sevoflurane) an anaesthetic machine. bottles of sevoflurane, isoflurane, enflurane, and desflurane, the common fluorinated ether anaesthetics used in clinical practice. these agents are colour-coded for safety purposes. note the special fitting for desflurane, which boils at room temperature. the most common approach to general anaesthesia is through the use of inhaled general anesthetics. each has its own potency which is correlated to its solubility in oil. this relationship exists because the drugs bind directly to cavities in proteins of the central nervous system, although several theories of general anaesthetic action have been described. inhalational anesthetics are thought to exact their effects on different parts of the central nervous system. for instance, the immobilizing effect of inhaled anesthetics results from an effect on the spinal cord whereas sedation, hypnosis and amnesia involve sites in the brain.: 515 an inhalational anaesthetic is a chemical compound possessing general anaesthetic properties that can be delivered via inhalation. agents of significant contemporary clinical interest include volatile anaesthetic agents such as isoflurane, sevoflurane and desflurane, and anaesthetic gases such as nitrous oxide and xenon. administration anaesthetic gases are administered by anaesthetists (a term which includes anaesthesiologists, nurse anaesthetists, and anaesthesiologist assistants) through an anaesthesia mask, laryngeal mask airway or tracheal tube connected to an anaesthetic vaporiser and an anaesthetic delivery system. the anaesthetic machine (uk english) or anesthesia machine (us english) or boyle's machine is used to support the administration of anaesthesia. the most common type of anaesthetic machine in use in the developed world is the continuous-flow anaesthetic machine, which is designed to provide an accurate and continuous supply of medical gases (such as oxygen and nitrous oxide), mixed with an accurate concentration of anaesthetic vapour (such as isoflurane), and deliver this to the patient at a safe pressure and flow. modern machines incorporate a ventilator, suction unit, and patient monitoring devices. exhaled gas is passed through a scrubber to remove carbon dioxide, and the anaesthetic vapour and oxygen are replenished as required before the mixture is returned to the patient.