and distribution for large-scale use, helium is extracted by fractional distillation from natural gas, which can contain as much as 7% helium. since helium has a lower boiling point than any other element, low temperature and high pressure are used to liquefy nearly all the other gases (mostly nitrogen and methane). the resulting crude helium gas is purified by successive exposures to lowering temperatures, in which almost all of the remaining nitrogen and other gases are precipitated out of the gaseous mixture. activated charcoal is used as a final purification step, usually resulting in 99.995% pure grade-a helium. the principal impurity in grade-a helium is neon. in a final production step, most of the helium that is produced is liquefied via a cryogenic process. this is necessary for applications requiring liquid helium and also allows helium suppliers to reduce the cost of long-distance transportation, as the largest liquid helium containers have more than five times the capacity of the largest gaseous helium tube trailers. in 2008, approximately 169 million standard cubic meters (scm) of helium were extracted from natural gas or withdrawn from helium reserves with approximately 78% from the united states, 10% from algeria, and most of the remainder from russia, poland and qatar. by 2013, increases in helium production in qatar (under the company qatargas managed by air liquide) had increased qatar's fraction of world helium production to 25%, and made it the second largest exporter after the united states. an estimated 54 billion cubic feet (1.5×109 m3) deposit of helium was found in tanzania in 2016. a large-scale helium plant was opened in ningxia, china in 2020. in the united states, most helium is extracted from natural gas of the hugoton and nearby gas fields in kansas, oklahoma, and the panhandle field in texas. much of this gas was once sent by pipeline to the national helium reserve, but since 2005 this reserve is being depleted and sold off, and is expected to be largely depleted by 2021, under the october 2013 responsible helium administration and stewardship act (h.r. 527). diffusion of crude natural gas through special semipermeable membranes and other barriers is another method to recover and purify helium. in 1996, the u.s. had proven helium reserves, in such gas well complexes, of about 147 billion standard cubic feet (4.2 billion scm). at rates of use at that time (72 million scm per year in the u.s.; see pie chart below) this would have been enough helium for about 58 years of u.s. use, and less than this (perhaps 80% of the time) at world use rates, although factors in saving and processing impact effective reserve numbers. helium must be extracted from natural gas because it is present in air at only a fraction of that of neon, yet the demand for it is far higher. it is estimated that if all neon production were retooled to save helium, 0.1% of the world's helium demands would be satisfied. similarly, only 1% of the world's helium demands could be satisfied by re-tooling all air distillation plants. helium can be synthesized by bombardment of lithium or boron with high-velocity protons, or by bombardment of lithium with deuterons, but these processes are a completely uneconomical method of production. helium is commercially available in either liquid or gaseous form. as a liquid, it can be supplied in small insulated containers called dewars which hold as much as 1,000 liters of helium, or in large iso containers which have nominal capacities as large as 42 m3 (around 11,000 u.s. gallons). in gaseous form, small quantities of helium are supplied in high-pressure cylinders holding as much as 8 m3 (approx. 282 standard cubic feet), while large quantities of high-pressure gas are supplied in tube trailers which have capacities of as much as 4,860 m3 (approx. 172,000 standard cubic feet). conservation advocates according to helium conservationists like nobel laureate physicist robert coleman richardson, writing in 2010, the free market price of helium has contributed to "wasteful" usage (e.g. for helium balloons). prices in the 2000s had been lowered by the decision of the u.s. congress to sell off the country's large helium stockpile by 2015. according to richardson, the price needed to be multiplied by 20 to eliminate the excessive wasting of helium. in the paper stop squandering helium published in 2012, it was also proposed to create an international helium agency that would build a sustainable market for "this precious commodity". applications the largest single use of liquid helium is to cool the superconducting magnets in modern mri scanners. estimated 2014 u.s. fractional helium use by category. total use is 34 million cubic meters. cryogenics (32%) pressurizing and purging (18%) welding (13%) controlled atmospheres (18%) leak detection (4%) breathing mixtures (2%) other (13%) while balloons are perhaps the best known use of helium, they are a minor part of all helium use. helium is used for many purposes that require some of its unique properties, such as its low boiling point, low density, low solubility, high thermal conductivity, or inertness. of the 2014 world helium total production of about 32 million kg (180 million standard cubic meters) helium per year, the largest use (about 32% of the total in 2014) is in cryogenic applications, most of which involves cooling the superconducting magnets in medical mri scanners and nmr spectrometers. other major uses were pressurizing and purging systems, welding, maintenance of controlled atmospheres, and leak detection. other uses by category were relatively minor fractions. controlled atmospheres helium is used as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, and in gas chromatography, because it is inert. because of its inertness, thermally and calorically perfect nature, high speed of sound, and high value of the heat capacity ratio, it is also useful in supersonic wind tunnels and impulse facilities. gas tungsten arc welding helium is used as a shielding gas in arc welding processes on materials that at welding temperatures are contaminated and weakened by air or nitrogen. a number of inert shielding gases are used in gas tungsten arc welding, but helium is used instead of cheaper argon especially for welding materials that have higher heat conductivity, like aluminium or copper. industrial leak detection a dual chamber helium leak detection machine one industrial application for helium is leak detection. because helium diffuses through solids three times faster than air, it is used as a tracer gas to detect leaks in high-vacuum equipment (such as cryogenic tanks) and high-pressure containers. the tested object is placed in a chamber, which is then evacuated and filled with helium. the helium that escapes through the leaks is detected by a sensitive device (helium mass spectrometer), even at the leak rates as small as 10−9 mbar·l/s (10−10 pa·m3/s). the measurement procedure is normally automatic and is called helium integral test. a simpler procedure is to fill the tested object with helium and to manually search for leaks with a hand-held device. helium leaks through cracks should not be confused with gas permeation through a bulk material. while helium has documented permeation constants (thus a calculable permeation rate) through glasses, ceramics, and synthetic materials, inert gases such as helium will not permeate most bulk metals. flight because of its low density and incombustibility, helium is the gas of choice to fill airships such as the goodyear blimp. because it is lighter than air, airships and balloons are inflated with helium for lift. while hydrogen gas is more buoyant, and escapes permeating through a membrane at a lower rate, helium has the advantage of being non-flammable, and indeed fire-retardant. another minor use is in rocketry, where helium is used as an ullage medium to backfill rocket propellant tanks in flight and to condense hydrogen and oxygen to make rocket fuel. it is also used to purge fuel and oxidizer from ground support equipment prior to launch and to pre-cool liquid hydrogen in space vehicles. for example, the saturn v rocket used in the apollo program needed about 370,000 m3 (13 million cubic feet) of helium to launch. minor commercial and recreational uses helium as a breathing gas has no narcotic properties, so helium mixtures such as trimix, heliox and heliair are used for deep diving to reduce the effects of narcosis, which worsen with increasing depth. as pressure increases with depth, the density of the breathing gas also increases, and the low molecular weight of helium is found to considerably reduce the effort of breathing by lowering the density of the mixture. this reduces the reynolds number of flow, leading to a reduction of turbulent flow and an increase in laminar flow, which requires less work of breathing. at depths below 150 metres (490 ft) divers breathing helium–oxygen mixtures begin to experience tremors and a decrease in psychomotor function, symptoms of high-pressure nervous syndrome. this effect may be countered to some extent by adding an amount of narcotic gas such as hydrogen or nitrogen to a helium–oxygen mixture. helium–neon lasers, a type of low-powered gas laser producing a red beam, had various practical applications which included barcode readers and laser pointers, before they were almost universally replaced by cheaper diode lasers. for its inertness and high thermal conductivity, neutron transparency, and because it does not form radioactive isotopes under reactor conditions, helium is used as a heat-transfer medium in some gas-cooled nuclear reactors. helium, mixed with a heavier gas such as xenon, is useful for thermoacoustic refrigeration due to the resulting high heat capacity ratio and low prandtl number. the inertness of helium has environmental advantages over conventional refrigeration systems which contribute to ozone depletion or global warming. helium is also used in some hard disk drives. scientific uses the use of helium reduces the distorting effects of temperature variations in the space between lenses in some telescopes, due to its extremely low index of refraction. this method is especially used in solar telescopes where a vacuum tight telescope tube would be too heavy. helium is a commonly used carrier gas for gas chromatography. the age of rocks and minerals that contain uranium and thorium can be estimated by measuring the level of helium with a process known as helium dating. helium at low temperatures is used in cryogenics, and in certain cryogenics applications. as examples of applications, liquid helium is used to cool certain metals to the extremely low temperatures required for superconductivity, such as in superconducting magnets for magnetic resonance imaging. the large hadron collider at cern uses 96 metric tons of liquid helium to maintain the temperature at 1.9 k (−271.25 °c; −456.25 °f). medical uses helium was approved for medical use in the united states in april 2020 for humans and animals. as a contaminant while chemically inert, helium contamination impairs the operation of microelectromechanical systems (mems) such that iphones may fail. inhalation and safety effects neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood. the speed of sound in helium is nearly three times the speed of sound in air. because the natural resonance frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled, a corresponding increase occurs in the resonant frequencies of the vocal tract, which is the amplifier of vocal sound. this increase in the resonant frequency of the amplifier (the vocal tract) gives an increased amplification to the high-frequency components of the sound wave produced by the direct vibration of the vocal folds, compared to the case when the voice box is filled with air. when a person speaks after inhaling helium gas, the muscles that control the voice box still move in the same way as when the voice box is filled with air, therefore the fundamental frequency (sometimes called pitch) produced by direct vibration of the vocal folds does not change. however, the high-frequency-preferred amplification causes a change in timbre of the amplified sound, resulting in a reedy, duck-like vocal quality. the opposite effect, lowering resonant frequencies, can be obtained by inhaling a dense gas such as sulfur hexafluoride or xenon. hazards inhaling helium can be dangerous if done to excess, since helium is a simple asphyxiant and so displaces oxygen needed for normal respiration. fatalities have been recorded, including a youth