Nitrox
Nitrox refers to any gas mixture composed (excluding trace gases) of nitrogen and oxygen; this includes normal air which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon. However, in SCUBA diving, nitrox is normally differentiated and handled differently from air. The most common use of nitrox mixtures containing higher than normal levels of oxygen is in SCUBA, where the reduced percentage of nitrogen is advantageous in reducing nitrogen take up in the body's tissues and so extending the possible dive time, and/or reducing the risk of decompression sickness (also known as the bends).

# Purpose
Enriched Air Nitrox, Nitrox with an oxygen content above 21%, is mainly used in scuba diving to reduce the proportion of nitrogen in the breathing gas mixture. Reducing the proportion of nitrogen by increasing the proportion of oxygen reduces the risk of decompression sickness, allowing extended dive times without increasing the need for decompression stops. Nitrox is not a safer gas than compressed air in all respects; although its use reduces the risk of decompression sickness, it increases the risk of oxygen toxicity and fire, which are further discussed below.
Breathing nitrox is not thought to reduce the effects of narcosis, as oxygen seems to have equally narcotic properties under pressure as nitrogen; thus one should not expect a reduction in narcotic effects due only to the use of nitrox. Nonetheless, there remains a body of the diving community that insists that they feel reduced narcotic effects at depths breathing nitrox.
This may be due to a dissociation of the subjective and behavioural effects of narcosis.  However, it should be noted that because of risks associated with oxygen toxicity, divers tend not to utilize nitrox at greater depths where more pronounced narcosis symptoms are more likely to occur.  For a reduction in narcotic effects trimix or heliox gases which also contain helium are generally used by divers.
There is anecdotal evidence that the use of nitrox reduces post-dive fatigue, particularly in older and or obese divers; however the only known double-blind study to test this found no statistically significant reduction in reported fatigue. There has, however, been some suggestion that post dive fatigue is due to sub-clinical decompression sickness (DCS) (i.e. micro bubbles in the blood insufficient to cause symptoms of DCS); the fact that the study mentioned was conducted in a dry chamber with an ideal decompression profile may have been sufficient to reduce sub-clinical DCS and prevent fatigue in both nitrox and air divers.
Further studies with a number of different dive profiles, and also different levels of exertion, would be necessary to fully investigate this issue. For example, there is much better scientific evidence that breathing high-oxygen gases increase exercise tolerance, during aerobic exertion. Though even moderate exertion while breathing from the regulator is a relatively uncommon occurrence in scuba, as divers usually try to minimize it in order to conserve gas, episodes of exertion while regulator-breathing do occasionally occur in sport diving. Examples are surface-swimming a distance to a boat or beach after surfacing, where residual "safety" cylinder gas is often used freely, since the remainder will be wasted anyway when the dive is completed. It is possible that these so-far un-studied situations have contributed to some of the positive reputation of nitrox.

# Naming
Nitrox is known by many names: Enhanced Air Nitrox, Oxygen Enriched Air, Nitrox, EANx or Safe Air. The name "nitrox" may be capitalized when referring to specific mixtures such as Nitrox32, which contains 68% nitrogen and 32% oxygen. When one figure is stated, it refers to the oxygen percentage, not the nitrogen percentage. The original convention, Nitrox68/32 became shortened as the second figure is redundant.
Although "nitrox" usually refers to a mixture of nitrogen and oxygen with more than 21% oxygen, it can refer to mixtures that are leaner in oxygen than air. "Enriched Air Nitrox", "Enriched Air" or "EAN" are used to emphasise richer than air mixtures. In "EANx", the "x" indicates the percentage of oxygen in the mix and is replaced by a number when the percentage is known; for example a 40% EANx mix is called EAN40. The two most popular blends are EAN32 and EAN36 (also named Nitrox I and Nitrox II, respectively, or Nitrox68/32 and Nitrox64/36).
In its early days of introduction to non-technical divers, nitrox has occasionally also been known by detractors by less complimentary terms, such as "devil gas" or "voodoo gas" (a term now sometimes used with pride).
These percentages are what the gas blender aims for in partial-pressure blending, but the final actual mix in such cases will be unique, and so a small flow of gas from the cylinder must be measured with a handheld oxygen analyzer, before the diver breathes from the cylinder underwater.

# Richness of mix
The two most common recreational diving nitrox mixes contain 32% and 36% oxygen, which have maximum operating depths (MODs) of 34 metres (111.5485566 ft) and 29 metres (95.1443571 ft) respectively when limited to a maximum partial pressure of oxygen of 1.4  (Expression error: Missing operand for *. ). EAN32 is common because it is the mixture with the maximum concentration of oxygen that allows the diver to go to the full depth of recreational diving's "No Decompression Limit" for air. Divers may calculate an equivalent air depth to determine their decompression requirements or may use nitrox tables or a nitrox-capable dive computer.
Nitrox with more than 40% oxygen is uncommon within recreational diving. There are two main reasons for this: the first is that all pieces of diving equipment that come into contact with mixes containing higher proportions of oxygen, particularly at high pressure, need special cleaning and servicing to reduce the risk of fire. The second reason is that richer mixes extend the time the diver can stay underwater without needing decompression stops far further than the duration of typical diving cylinders. For example, based on the PADI nitrox recommendations, the maximum operating depth for EAN45 would be 21 metres (68.8976379 ft) and the maximum dive time available at this depth even with EAN36 is nearly 1 hour 15 minutes: a diver with a breathing rate of 20 litres per minute using twin 10 litre, 230 bar (about double 85 cu. ft.) cylinders would have completely emptied the cylinders after 1 hour 14 minutes at this depth.
Nitrox containing 50% to 100% oxygen is common in technical diving as a decompression gas, which eliminates inert gases such as nitrogen and helium from the tissues more quickly than leaner oxygen mixtures.
In deep open circuit technical diving, where hypoxic gases are breathed during the bottom portion of the dive, a Nitrox mix with 50% or less oxygen called a "travel mix" is sometimes breathed during the beginning of the descent in order to avoid hypoxia. Normally, however, the most oxygen-lean of the diver's decompression gases would be used for this purpose, since descent time spent reaching a depth where bottom mix is no longer hypoxic is normally small, and the distance between this depth and the MOD of any nitrox decompression gas is likely to be very short, if it occurs at all.

# Cylinder markings
Any cylinder containing any blend of gas other than the standard air content is required by most diving training organizations to be clearly marked. Some organizations, e.g. GUE, argue that it does not make sense to have a permanent marking on a gas tank that can be filled with any gas.
The standard nitrox cylinder is yellow in color and marked with a green band around the shoulder of the tank, with "Nitrox" or "Enriched air" marked in white or yellow letters inside. 
Tanks of any other color are generally marked with six inch band around the shoulder, with a one inch yellow band on the top and bottom, with four inches of green in the middle. This green band will also have the designation of "NITROX" or something similar inside, in yellow or white letters.
Every nitrox cylinder should also have a sticker stating whether or not the cylinder is oxygen clean and suitable for partial pressure blending. Any oxygen clean cylinder may have any mix up to 100% oxygen inside. If by some accident an oxygen clean cylinder is filled at a station which does not supply gas to oxygen-clean standards it is then considered contaminated and must be re-cleaned before a gas containing more than 40% oxygen may again be added. Cylinders marked as not-oxygen clean may only be filled with enriched oxygen mixtures from membrane or stick blending systems where the gas is mixed before being added to the cylinder.
Finally, all nitrox cylinders should have a tag that, at minimum, states the oxygen content of the cylinder, the date it was blended, the gas blender's name, and the maximum operating depth along with the partial pressure this depth was calculated with. Other requirements may be made as to what is marked on the cylinder, but these markings are considered standard and safe by the diving community, and any cylinders lacking these markings should be considered possibly unsafe.  Training for nitrox certification suggests this tag be verified by the diver himself by using an oxygen analyzer.

# Dangers

## Oxygen toxicity
Diving and handling nitrox raises a number of potentially fatal dangers due to the high partial pressure of oxygen (ppO2). Nitrox is not a deep-diving gas mixture owing to the increased proportion of oxygen, which becomes toxic when breathed at high pressure. For example, the maximum operating depth of nitrox with 36% oxygen, a popular recreational diving mix, is 29 metres (95.1443571 ft) to ensure a maximum ppO2 of no more than 1.4  (Expression error: Missing operand for *. ). The exact value of the maximum allowed ppO2 and maximum operating depth varies depending on factors such as the training agency, the type of dive, the breathing equipment and the level of surface support, with professional divers sometimes being allowed to breath higher ppO2 than those recommended to recreational divers.
To dive safely with nitrox, the diver must learn good buoyancy control, a vital part of scuba diving in its own right, and a disciplined approach to preparing, planning and executing a dive to ensure that the ppO2 is known, and the maximum operating depth is not exceeded. Reputable dive operators and gas blenders insist on the diver having recognised nitrox training (which appears as an extra notation on a certification card) before selling nitrox to divers.
Some training agencies teach the use of two depth limits to protect against oxygen toxicity. The shallower depth is called the "maximum operating depth" and is reached when the partial pressure of oxygen in the breathing gas reaches 1.4  (Expression error: Missing operand for *. ).  The second deeper depth, called the "contingency depth", is reached when the partial pressure reaches 1.6  (Expression error: Missing operand for *. ). Diving at or beyond this level exposes the diver to the risk of central nervous system (CNS) oxygen toxicity.  This can be extremely dangerous since its onset is often without warning and can lead to drowning, as the regulator may be spat out during convulsions which occur in conjunction with sudden unconsciousness (general seizure induced by oxygen toxicity).
Divers trained to use nitrox memorise the acronym VENTID (which stands for Vision (blurriness), Ears (ringing sound), Nausea, Twitching, Irritability and Dizziness), which indicate some of the possible warning signs of onsetting convulsions.  However, evidence from non-fatal oxygen convulsions indicates that most convulsions are not preceded by any warning symptoms at all.  Further, many of the suggested warning signs are also symptoms of nitrogen narcosis, and so may lead to misdiagnosis by a diver.

# Precautionary procedures at the fill station
Many training agencies such as PADI, CMAS, SSI and NAUI train their divers to personally check the oxygen percentage content of each nitrox cylinder before every dive. If the oxygen percentage deviates by more than 1% from the value written on the cylinder by the gas blender, the scuba diver must either recalculate his or her bottom times with the new mix, or else abort the dive to remain safe and avoid oxygen toxicity or decompression sickness. Under IANTD and ANDI rules for use of nitrox, which are followed by most dive resorts around the world, filled nitrox cylinders are signed out personally in a gas blender log book, which contains, for each cylinder and fill, the cylinder number, the measured oxygen percent composition, the signature of the receiving diver (who should have personally measured the oxygen percent with an instrument at the fill-shop), and finally a calculation of the maximum operating depth for that fill/cylinder. All of these steps minimize danger but increase complexity of operations (for example, personalized cylinders for each diver must generally be kept track of on dive boats with nitrox, which is not the case with generic compressed air cylinders).

## Fire and toxic cylinder contamination from oxygen reactions
Diving cylinders are usually filled with nitrox by a gas blending technique such as partial pressure blending or premix decanting (in which a nitrox mix is supplied to the filler in pressurized larger cylinders). A few facilities have begun to fill cylinders with air which has been enriched with oxygen by a pre-mixing process, so that it is pressurized as nitrox for the first time in the diving cylinder. The pre-mixing is accomplished either by a membrane system which removes nitrogen from the air during compression or by a 'stick' blending technique where pure oxygen is mixed with air in a baffled chamber attached to the compressor intake.
With the use of pure oxygen during "partial pressure blending" (where pure oxygen is added from a large oxygen cylinder to the nearly empty dive cylinder until it reaches 300  (Expression error: Unexpected round operator. ) before air is added by compressor) there is an especially increased risk of fire. Partial blending using pure oxygen is often used to provide nitrox for multiple dives on live-aboard dive boats, but it is also used in some smaller diver shops.
However, any gas which contains a significantly larger percentage of oxygen than air is a fire hazard. Furthermore, such gases can also react with hydrocarbons or incorrect lubricants inside a dive cylinder to produce carbon monoxide, even if a recognized fire does not happen. At present, there is some discussion over whether or not mixtures of gas which contain less than 40% oxygen may sometimes be exempt from oxygen clean standards. Some of the controversy comes from a single U.S. regulation intended for commercial divers (not recreational divers) years ago. However, the U.S. Compressed Gas Association (CGA) and two international nitrox teaching agencies (IANTD and ANDI) now support the standard that any gas containing more than 23.5% oxygen should be treated as nitrox (which is to say, no differently from pure oxygen) for purposes of oxygen cleanliness and oxygen compatibility (i.e., oxygen "servicability"). However, the largest training agency - PADI - is still teaching that pre-mixed nitrox (i.e. nitrox which is mixed before being put into the cylinder) below 40% oxygen does not require a specially cleaned cylinder or other equipment. Most nitrox fill stations which supply pre-mixed nitrox will fill non-oxygen clean cylinders with mixtures below 40%. For a history of this controversy see Luxfer cylinders.

# History
In the 1920s or 1930's Draeger of Germany made a nitrox backpack independent air supply for a standard diving suit.
In World War II or soon after, British commando frogmen and work divers started sometimes diving with oxygen rebreathers adapted for semi-closed-circuit nitrox (which they called "mixture") diving by fitting larger cylinders and carefully setting the gas flow rate using a flow meter. These developments were kept secret until independently duplicated by civilians in the 1960s.
In the 1950s the United States Navy (USN) documented enriched oxygen gas procedures for military use of what we today call nitrox, in the USN Diving Manual.
In 1970, Dr. Morgan Wells, who was the first director of the National Oceanographic and Atmospheric Administration (NOAA) Diving Center, began instituting diving procedures for oxygen-enriched air. He also developed a process for mixing oxygen and air which he called a continuous blending system.  For many years Dr. Wells' invention was the only practical alternative to partial pressure blending. In 1979 NOAA published Wells' procedures for the scientific use of nitrox in the NOAA Diving Manual.
In 1985 Dick Rutkowski, a former NOAA diving safety officer, formed IAND (International Association of Nitrox Divers) and began teaching nitrox use for recreational diving. This was considered dangerous by some, and met with heavy skepticism by the diving community. In 1992 the name was changed to the International Association of Nitrox and Technical Divers (IANTD), the T being added when the European Association of Technical Divers (EATD) merged with IAND. In the early 1990s, the agencies teaching nitrox were not the main scuba agencies. New organizations, including Ed Betts' American Nitrox Divers International (ANDI) - which invented the term "Safe Air" for marketing purposes - and Bret Gilliam's Technical Diving International (TDI) gave scientific credence to nitrox.
Meanwhile, diving stores were finding a purely economic reason to offer nitrox: not only was an entire new course and certification needed to use it, but instead of cheap or free tank fills with compressed air, dive shops found they could charge premium amounts of money for custom-gas blending of nitrox to their ordinary moderately experienced divers. With the new dive computers which could be programmed to allow for the longer bottom-times and shorter residual nitrogen times which nitrox gave, the incentive for the sport diver to use the gas increased. An intersection of economics and scientific validity had occurred.
In 1996, the Professional Association of Diving Instructors (PADI) announced full educational support for nitrox. While other main line scuba organizations had announced their support of nitrox earlier, it was PADI's endorsement that put nitrox over the top as a standard sport diving "option."

# Nitrox in nature
Sometimes in the geologic past the Earth's atmosphere contained much more than 20% oxygen: e.g. up to 35% in the Upper Carboniferous. This let animals absorb oxygen more easily and influenced evolution.