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{{Short description|Breathing gas, mixture of nitrogen and oxygen}}
{{About|the nitrogen–oxygen gas mixture|the method of case hardening metals|wikt:nitrox|and|Ferritic nitrocarburizing#Post-oxidation black oxide}}
'''Nitrox''' refers to any [[breathing gas|gas mixture]] composed (excepting trace gases) of [[nitrogen]] and [[oxygen]].<ref name="dan" /> It is usually used for mixtures that contain less than 78% nitrogen by volume.<ref name="GoodDive" /><ref name="yourdictionary" /> In the usual application, [[underwater diving]], nitrox is normally distinguished from air and handled differently.<ref name=dan/> The most common use of nitrox mixtures containing oxygen in higher proportions than atmospheric air is in [[scuba diving]], where the reduced [[partial pressure]] of nitrogen is advantageous in reducing nitrogen uptake in the [[tissue (biology)|body's tissues]], thereby extending the practicable underwater dive time by reducing the [[decompression (diving)|decompression]] requirement, or reducing the risk of [[decompression sickness]] (also known as ''the bends''). The two most common recreational diving nitrox mixes are 32% and 36% oxygen, which have maximum operating depths of about 110 feet (34 meters) and 95 feet (29 meters) respectively.<ref name="divingfrontiers" />

Nitrox is used to a lesser extent in [[surface-supplied diving]], as these advantages are reduced by the more complex logistical requirements for nitrox compared to the use of simple low-pressure compressors for breathing gas supply. Nitrox can also be used in hyperbaric treatment of [[decompression illness]], usually at pressures where pure oxygen would be hazardous.<ref name="Treatment tables" /> Nitrox is not a safer gas than compressed air in all respects; although its use can reduce the risk of decompression sickness, it increases the risks of [[oxygen toxicity]] and fire.<ref name="Lang 2006" />

Though not generally referred to as nitrox, an oxygen-enriched air mixture is routinely provided at normal surface ambient pressure as [[oxygen therapy]] to patients with compromised respiration and circulation.

==Physiological effects under pressure==
{{See also|Human physiology of underwater diving}}

===Decompression benefits===
{{See also|Decompression (diving)|Decompression theory|Decompression practice}}
Reducing the proportion of nitrogen by increasing the proportion of oxygen reduces the risk of decompression sickness for the same dive profile, or allows extended dive times without increasing the need for [[decompression stop]]s for the same risk.<ref name="Lang 2006" /> The significant aspect of extended no-stop time when using nitrox mixtures is reduced risk in a situation where breathing gas supply is compromised, as the diver can make a direct ascent to the surface with an acceptably low risk of decompression sickness. The exact values of the extended no-stop times vary depending on the decompression model used to derive the tables, but as an approximation, it is based on the partial pressure of nitrogen at the dive depth. This principle can be used to calculate an equivalent air depth (EAD) with the same partial pressure of nitrogen as the mix to be used, and this depth is less than the actual dive depth for oxygen enriched mixtures. The equivalent air depth is used with air decompression tables to calculate decompression obligation and no-stop times.<ref name="Lang 2006" /> The [[Decompression theory#Goldman Interconnected Compartment Model|Goldman decompression model]] predicts a significant risk reduction by using nitrox (more so than the PADI tables suggest).<ref name="Goldman 2013" />

===Nitrogen narcosis===
{{Main|Nitrogen narcosis}}
Controlled tests have not shown breathing nitrox to reduce the effects of nitrogen narcosis, as oxygen seems to have similarly narcotic properties under pressure to nitrogen; thus one should not expect a reduction in narcotic effects due only to the use of nitrox.<ref name="Hesser1978" /><ref name=Brubakk304 /><ref group=note>Oxygen has the potential to be 1.7 times more narcotic than nitrogen – see [[:File:Relative narcotic potency.gif|relevant narcotic potency of gases]].</ref> Nonetheless, there are people in the diving community who insist 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.<ref name="Hamilton1995" /> Although oxygen appears chemically more narcotic at the surface, relative narcotic effects at depth have never been studied in detail, but it is known that different gases produce different narcotic effects as depth increases. Helium has no narcotic effect, but results in [[high-pressure nervous syndrome|HPNS]] when breathed at high pressures, which does not happen with gases that have greater narcotic potency. However, because of risks associated with [[Nitrox#Oxygen toxicity|oxygen toxicity]], divers do not usually use nitrox at greater depths where more pronounced narcosis symptoms are more likely to occur. For deep diving, [[trimix (breathing gas)|trimix]] or [[heliox]] gases are typically used; these gases contain [[helium]] to reduce the amount of narcotic gases in the mixture.

===Oxygen toxicity===
{{Main|Oxygen toxicity}}
Diving with and handling nitrox raise a number of potentially fatal dangers due to the high [[partial pressure]] of oxygen (ppO<sub>2</sub>).<ref name=noaa/><ref name=dan/> Nitrox is not a deep-diving gas mixture owing to the increased proportion of oxygen, which becomes [[oxygen toxicity|toxic]] when breathed at high pressure. For example, the maximum operating depth of nitrox with 36% oxygen, a popular [[recreational diving]] mix, is {{convert|29|m|ft}} to ensure a maximum ppO<sub>2</sub> of no more than {{convert|1.4|bar|abbr=on}}. The exact value of the maximum allowed ppO<sub>2</sub> 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 diving|professional divers]] sometimes being allowed to breathe higher ppO<sub>2</sub> than those recommended to [[recreational diving|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 ppO<sub>2</sub> is known, and the maximum operating depth is not exceeded. Many dive shops, dive operators, and gas blenders (individuals trained to [[Gas blending for scuba diving|blend gases]]) require the diver to present a nitrox certification card before selling nitrox to divers.<ref name="scubadiving.com" />Additionally, it is strongly encouraged for divers to confirm the percentage of oxygen in their tank before every dive, regardless of the specified amount on their tank.<ref name="Scuba doctor" /> This is done by expelling a small amount of air from the diver's tank into an [[oxygen sensor#Diving applications|oxygen analyzer]]. This is to further limit the possibility of oxygen toxicity due to errors in previous testing.<ref name="Lippman and Mitchell 2005" />

Some training agencies, such as PADI and [[Technical Diving International]], 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 {{convert|1.4|bar|abbr=on}}.  The deeper depth, called the "contingency depth", is reached when the partial pressure reaches {{convert|1.6|bar|abbr=on}}.<ref name="scubadiving.com"/> Diving at or beyond this level exposes the diver to a greater 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 may memorise the [[acronym]] VENTID-C or sometimes ConVENTID, (which stands for '''V'''ision (blurriness), '''E'''ars (ringing sound), '''N'''ausea, '''T'''witching, '''I'''rritability, '''D'''izziness, and '''C'''onvulsions). However, evidence from non-fatal oxygen convulsions indicates that most convulsions are not preceded by any warning symptoms at all.<ref name="bennett2003-375" /> Further, many of the suggested warning signs are also symptoms of nitrogen narcosis, and so may lead to misdiagnosis by a diver. A solution to either is to ascend to a shallower depth.

===Carbon dioxide retention===
Use of nitrox may cause a reduced ventilatory response, and when breathing dense gas at the deeper limits of the usable range, this may result in carbon dioxide retention when exercise levels are high, with an increased risk of loss of consciousness.<ref name="Lang 2006" />

===Other effects===
There is anecdotal evidence that the use of nitrox reduces post-dive fatigue,<ref name="Scubaboard Poll" /> particularly in older and or obese divers; however a double-blind study to test this found no statistically significant reduction in reported fatigue.<ref name=Brubakk/><ref name="Harris et al 2003" /> There was, however, 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. In 2008, a study was published using wet divers at the same depth no statistically significant reduction in reported fatigue was seen.<ref name=AAUS2008 />

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 increases exercise tolerance, during aerobic exertion.<ref name="Anderson 2015" /> Though even moderate exertion while breathing from the regulator is a relatively uncommon occurrence in recreational scuba, as divers usually try to minimize it in order to conserve gas, episodes of exertion while regulator-breathing do occasionally occur in recreational 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, and unplanned contingencies due to currents or buoyancy problems. It is possible that these so-far un-studied situations have contributed to some of the positive reputation of nitrox.

A 2010 study using [[critical flicker fusion frequency]] and perceived fatigue criteria found that diver alertness after a dive on nitrox was significantly better than after an air dive.<ref name="Lafere et al 2010" />

==Uses==
[[File:Nitrox tables.JPG|right|thumb|300px|Enriched Air Nitrox diving tables, showing adjusted no-decompression times.]]

===Underwater diving===
{{Main|Underwater diving}}
Enriched Air Nitrox{{r|EANx}}, nitrox with an oxygen content above 21%, is mainly used in [[scuba diving]] to reduce the proportion of nitrogen in the breathing gas mixture. The main benefit is reduced decompression risk.<ref name="Lang 2006" /> To a considerably lesser extent it is also used in surface supplied diving, where the logistics are relatively complex, similar to the use of other diving gas mixtures like [[heliox]] and [[Trimix (breathing gas)|trimix]].

====Training and certification====
<!-- target for redirect [[Nitrox diver]], [[Advanced nitrox diver]], [[Recreational nitrox]] -->
{{visible anchor|Recreational nitrox certification}} (Nitrox diver) allows the diver to use a single nitrox gas mixture with 40% or less oxygen by volume on a dive without obligatory decompression. The reason for using nitrox on this type of dive profile can be to extend the [[no-decompression limit]], and for shorter dives, to reduce the [[decompression stress]]. The course is short, with a theory module on the risks of oxygen toxicity and the calculation of maximum operating depth, and a practical module of generally two dives using nitrox. It is one of the most popular further training programmes for entry level divers as it makes longer dives possible at a large number of popular sites. Gases suitable for this application may be referred to as recreational nitrox.

Advanced nitrox certification ({{visible anchor|Advanced nitrox diver}}) requires competence to carry two nitrox mixtures in separate scuba sets, and to use the richer mix for accelerated decompression at the end of the dive, switching gases underwater at the correct planned depth and selecting the new gas on the dive computer if one is carried. For the purposes of the certification any mixture from air to nominally 100% oxygen may be used, though at least one agency prefers to limit oxygen fraction to 80% as they consider this has a lower risk for acute oxygen toxicity.

===Therapeutic recompression===
{{Further|Hyperbaric treatment schedules|Therapeutic recompression}}
Nitrox50 is used as one of the options in the first stages of therapeutic recompression using the [[Comex therapeutic table CX 30]] for treatment of vestibular or general decompression sickness. Nitrox is breathed at 30&nbsp;[[msw]] and 24&nbsp;msw and the ascents from these depths to the next stop. At 18 m the gas is switched to oxygen for the rest of the treatment.<ref name="Treatment tables"/>

===Medicine, mountaineering and unpressurised aircraft===
The use of oxygen at high altitudes or as [[oxygen therapy]] may be as supplementary oxygen, added to the inspired air, which would technically be a use of nitrox, blended on site, but this is not normally referred to as such, as the gas provided for the purpose is oxygen.

==Terminology==
Nitrox is known by many names: Enriched&nbsp;Air&nbsp;Nitrox, Oxygen&nbsp;Enriched&nbsp;Air, Nitrox, EANx or Safe&nbsp;Air.<ref name=dan/><ref name="Elliott 1996" /> Since the word is a compound contraction or coined word and not an acronym, it should not be written in all upper case characters as "NITROX",<ref name="dan"/> but may be initially 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 first figure is redundant.{{Citation needed|date=January 2009}}

The term "nitrox" was originally used to refer to the breathing gas in a seafloor habitat where the oxygen has to be kept to a lower fraction than in air to avoid long term [[oxygen toxicity]] problems. It was later used by Dr Morgan Wells of NOAA for mixtures with an oxygen fraction higher than air, and has become a generic term for binary mixtures of nitrogen and oxygen with any oxygen fraction,<ref name="dan"/> and in the context of recreational and technical diving, now usually refers to a mixture of nitrogen and oxygen with more than 21% oxygen.<ref name=dan/> "Enriched Air Nitrox" or "EAN", and "Oxygen Enriched Air" are used to emphasize richer than air mixtures.<ref name=dan/> In "EANx", the "x" was originally the x of nitrox,<ref name=noaa/><!--alternative interpretations have been suggested--> but has come to indicate the percentage of oxygen in the mix and is replaced by a number when the percentage is known; for example, a 40% oxygen mix is called EAN40. The two most popular blends are EAN32 and EAN36, developed by NOAA for scientific diving, and also named Nitrox I and Nitrox II, respectively, or Nitrox68/32 and Nitrox64/36.<ref name=noaa/><ref name=dan/> These two mixtures were first utilized to the depth and oxygen limits for scientific diving designated by NOAA at the time.<ref name=Mastro1989 />

The term Oxygen Enriched Air (OEN) was accepted by the (American) scientific diving community, but although it is probably the most unambiguous and simply descriptive term yet proposed, it was resisted by the recreational diving community, sometimes in favour of less appropriate terminology.<ref name="dan"/>

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).<ref name=oxyhackers />

[[American Nitrox Divers International]] (ANDI) uses the term "SafeAir", which they define as any oxygen-enriched air mixture with O<sub>2</sub> concentrations between 22% and 50% that meet their gas quality and handling specifications, and specifically claim that these mixtures are safer than normally produced breathing air for the end user not envolved to the mix production which.{{clarify|typo or grammar error garbage|date=January 2022}}<ref name="SafeAir" /> Considering the complexities and hazards of mixing, handling, analyzing, and using oxygen-enriched air, this name is considered inappropriate by those who consider that it is not inherently "safe", but merely has decompression advantages.<ref name="dan"/>

===MOD===
{{Main|Maximum operating depth}}
Maximum Operating Depth (MOD) is the maximum safe depth at which a given nitrox mixture can be used. MOD depends on the allowed partial pressure of oxygen, which is related to exposure time and the acceptable risk assumed for central nervous system oxygen toxicity. Acceptable maximum ppO<sub>2</sub> varies depending on the application:<ref name="dan" />
* 1.2 is often used in closed circuit rebreathers.
* 1.4 is recommended by several recreational training agencies for ordinary scuba diving.
* 1.5 is allowed for commercial diving in some jurisdictions.
* 1.6 is allowed for technical diving decompression stops, and is the recommended maximum according to NOAA<ref name="noaa" />
Higher values are used by commercial and military divers in special circumstances, often when the diver uses surface supplied breathing apparatus, or for treatment in a chamber, where the airway is relatively secure.

==Equipment==
{{See also|Oxygen cleaning|Oxygen compatibility}}

==Choice of mixture==
{{See also|Scuba gas planning}}
[[File:Decompression Dive-Preparation.JPG|300px|thumb|left|[[Technical diving|Technical divers]] preparing for a mixed-gas [[Decompression schedule|decompression dive]] in [[Bohol]], [[Philippines]]. Note the [[backplate and wing]] setup with [[Side mount diving|side mounted]] [[Diving cylinder#Purposes of diving cylinders|stage tanks]]{{Broken anchor|date=2025-08-02|bot=User:Cewbot/log/20201008/configuration|target_link=Diving cylinder#Purposes of diving cylinders|reason= The anchor (Purposes of diving cylinders) [[Special:Diff/460309596|has been deleted]].|diff_id=460309596}} containing EAN50 (left side) and pure [[oxygen]] (right side).]]
The two most common [[recreational diving]] nitrox mixes contain 32% and 36% oxygen, which have [[maximum operating depth]]s (MODs) of {{convert|34|m|ft}} and {{convert|29|m|ft}} respectively when limited to a maximum partial pressure of oxygen of {{convert|1.4|bar|abbr=on|lk=in}}. Divers may calculate an [[equivalent air depth]] to determine their decompression requirements or may use nitrox [[Dive tables|tables]] or a nitrox-capable [[dive computer]].<ref name=noaa/><ref name=dan/><ref name="Logan1961" /><ref name="Berghage and McCraken 1979" />

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]].<ref name=noaa/><ref name=dan/> The second reason is that richer mixes extend the time the diver can stay underwater without needing [[decompression stop]]s far further than the duration permitted by the capacity of typical [[diving cylinder]]s. For example, based on the PADI nitrox recommendations, the maximum operating depth for EAN45 would be {{convert|21|m|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.

Use of nitrox mixtures containing 50% to 80% oxygen is common in [[technical diving]] as decompression gas, which by virtue of its lower partial pressure of inert gases such as nitrogen and helium, allows for more efficient (faster) elimination of these gases from the tissues than leaner oxygen mixtures.

In deep [[Scuba set#Types|open circuit]] technical diving, where [[Breathing gas#Oxygen|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 (medical)|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.

===Best mix===
The composition of a nitrox mix can be optimized for a given planned dive profile. This is termed "Best mix", for the dive, and provides the maximum no-decompression time compatible with acceptable oxygen exposure. An acceptable maximum partial pressure of oxygen is selected based on depth and planned bottom time, and this value is used to calculate the oxygen content of the best mix for the dive:<ref name="Becker" />

:<math>f_{\text{O}_2,\text{max}} = \frac{p_{\text{O}_2,\text{max}}}{p} = \frac{\text{Maximum acceptable partial pressure of oxygen}}{\text{Maximum ambient pressure of the dive}}</math>

==Production==
{{See also|Gas blending}}
<!--Target for redirect [[Nitrox production]] -->
There are several methods of production:<ref name=dan/><ref name=oxyhackers /><ref name=evil />
* Mixing by partial pressure: a measured pressure of [[oxygen]] is decanted into the cylinder and cylinder is "topped up" with air from the [[diving air compressor]]. This method is very versatile and requires relatively little additional equipment if a suitable compressor is available, but it is labour-intensive, and high partial pressures of oxygen are relatively hazardous.
* Pre-mix decanting: the gas supplier provides large cylinders with popular mixes such as 32% and 36%. These may be further diluted with air to provide a larger range of mixtures.
* Mixing by continuous blending: measured quantities of oxygen are introduced to air and mixed with it before it reaches the compressor inlet. Concentration of oxygen is commonly monitored as partial pressure using an [[oxygen cell]]. The compressor and particularly the compressor oil, must be suitable for this service. If the resulting oxygen fraction is less than 40%, the cylinder and valve may not be required to be cleaned for oxygen service. Relatively efficient and quick compared to partial pressure blending, but requires a suitable compressor, and the range of mixes may be limited by the compressor specification.
* Mixing by mass fraction: oxygen and air or nitrogen are added to a cylinder that is accurately weighed until the required mix is achieved. This method requires fairly large and highly accurate scales, otherwise it is similar to partial pressure blending, but insensitive to temperature variations.
* [[Membrane gas separation|Mixing by gas separation]]: a [[nitrogen]] permeable membrane is used to remove some of the nitrogen molecules from air until the required mix is achieved. The resulting low pressure nitrox is then pumped into cylinders by a compressor. <br />A limited range of mixes is possible, but the equipment is quick and easy to operate and relatively safe, as there is never high partial pressure oxygen involved. A supply of clean low-pressure air at a constant temperature is required for consistent results. This may be supplied from a low pressure compressor or a regulated supply from high-pressure storage or compressor. The air must be free of contaminants that could clog the membrane, and at a constant inlet temperature and pressure to produce a consistent delivered partial pressure of oxygen. The air must be of breathing quality, other contaminants must be filtered out independently. The input air pressure is regulated and pressure over the membrane controlled to adjust the product oxygen fraction. CGA Grade D or E air quality is suitable for supply gas, and is commonly heated to a constant inlet temperature. Heating also reduces the chance of high humidity causing wetting of the membrane. In a typical system supply air enters the thousands of hollow fibres of the membrane at one end, and oxygen preferentially permeates the fibre walls, leaving mostly nitrogen at the discharge end, which is vented from the system as waste.<ref name="Nuvair" />
* [[Pressure swing adsorption]] requires relatively complex equipment, otherwise the advantages are similar to membrane separation. PSA is a technology used to separate gases from a mixture under pressure according to the molecular characteristics and affinity for an [[adsorbent]] material of the gases at near-ambient temperatures. Specific adsorbent materials are used as a trap, preferentially adsorbing the target gases at high pressure. The process then swings to low pressure to desorb the adsorbed material and flush the adsorbent container so that it can be reused.

==Cylinder markings to identify contents==
[[File:EANxDecal.png|right|thumb|One type of nitrox cylinder identification label]]Any [[diving cylinder]] containing a blend of gasses other than standard air is required by most diver training organizations, and some national governments,<ref name="SANS 10019" /> to be clearly marked to indicate the current gas mixture. In practice it is common to use a printed adhesive label to indicate the type of gas (in this case nitrox), and to add a temporary label to specify the analysis of the current mix.

==Regional standards and conventions==

===European Union===
Within the EU, valves with M26x2 outlet thread are recommended for cylinders with increased oxygen content.<ref name="EN144" /> Regulators for use with these cylinders require compatible connectors, and are not directly connectable with cylinders for compressed air.

===Germany===
A nitrox cylinder is specially cleaned and identified.<ref name="Becker" /> According to [[European Standard|EN]] 144-3 the cylinder colour is overall white with the letter '''N''' on opposite sides of the cylinder.<ref name="Muller" /> The fraction of oxygen in the bottle is checked after filling and marked on the cylinder.

===South Africa===
South African National Standard 10019:2008 specifies the colour of all scuba cylinders as Golden yellow with French gray shoulder. This applies to all underwater breathing gases except medical oxygen, which must be carried in cylinders that are Black with a White shoulder. Nitrox cylinders must be identified by a transparent, self-adhesive label with green lettering, fitted below the shoulder.<ref name="SANS 10019" /> In effect this is green lettering on a yellow cylinder, with a gray shoulder. The composition of the gas must also be specified on the label. In practice this is done by a small additional self-adhesive label marked with the measured oxygen fraction, which is changed when a new mix is filled.

The 2021 revision of SANS 10019 changed the colour specification to Light navy grey for the shoulder, and a different label specification which includes [[hazard symbol]]s for high-pressure and oxidising materials.<ref name="SANS 10019:2021" />

===United States===
[[File:Cylinder mod.jpg|thumb|right|Cylinder showing Nitrox band and sticker marked with maximum operating depth (MOD) and oxygen fraction (%O<sub>2</sub>)]]
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 that 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.<ref name="Butler 1992" /> Cylinders marked as 'not oxygen clean' may only be filled with oxygen-enriched air mixtures from membrane or stick blending systems where the gas is mixed before being added to the cylinder, and to an oxygen fraction not exceeding 40% by volume.

==Hazards==
Nitrox can be a hazard to the blender and to the user, for different reasons.

===Fire and toxic cylinder contamination from oxygen reactions===
Partial pressure blending using pure oxygen decanted into the cylinder before topping up with air may involve very high oxygen fractions and oxygen partial pressures during the decanting process, which constitute a relatively high fire hazard. This procedure requires care and precautions by the operator, and decanting equipment and cylinders which are clean for oxygen service, but the equipment is relatively simple and inexpensive.<ref name=oxyhackers /> Partial pressure blending using pure oxygen is often used to provide nitrox on live-aboard dive boats, but it is also used in some dive shops and clubs.

Any gas which contains a significantly larger percentage of oxygen than air is a fire hazard, and such gases can react with hydrocarbons or lubricants and sealing materials inside the filling system to produce toxic gases, even if a fire is not apparent. Some organisations exempt equipment from oxygen-clean standards if the oxygen fraction is limited to 40% or less.<ref name="Rosales et al 2007" />

Among recreational training agencies, only ANDI subscribes to the guideline of requiring oxygen cleaning for equipment used with more than 23% oxygen fraction. The USCG, NOAA, U.S. Navy, OSHA, and the other recreational training agencies accept the limit as 40% as no accident or incident has been known to occur when this guideline has been properly applied. Tens of thousands of recreational divers are trained each year and the overwhelming majority of these divers are taught the "over 40% rule".<ref name=noaa/><ref name=dan/><ref name=padi/> Most nitrox fill stations which supply pre-mixed nitrox will fill cylinders with mixtures below 40% without certification of cleanliness for oxygen service.<ref name=dan/> Luxfer cylinders specify oxygen cleaning for all mixtures exceeding 23.5% oxygen.<ref name="Luxfer 23.5%" />

The following references for oxygen cleaning specifically cite the "over 40%" guideline that has been in widespread use since the 1960s, and consensus at the 1992 Enriched Air Workshop was to accept that guideline and continue the status quo.<ref name="dan"/>
* Code of Federal Regulations, Part 1910.430 (i) – Commercial Diving Operations
* OSHA Oxygen Specifications 1910.420 (1)
* NOAA Oxygen Specifications (appendix D)
* U.S. Navy Oxygen Specifications U.S. MIL-STD-777E (SH) Note K-6-4, Cat. K.6
* U.S. Coast Guard Oxygen Specifications Title 46: Shipping, revisions through 10-1-92. 197.452 Oxygen Cleaning 46 CFR 197.451

Much of the confusion appears to be a result of misapplying PVHO (pressure vessel for human occupancy) guidelines which prescribe a maximum ambient oxygen content of 25% when a human is sealed into a pressure vessel (chamber). The concern here is for a fire hazard to a living person who could be trapped in an oxygen-rich burning environment.<ref name="dan"/>

Of the three commonly applied methods of producing enriched air mixes – continuous blending, partial pressure blending, and membrane separation systems – only partial pressure blending would require the valve and cylinder components to be oxygen cleaned for mixtures with less than 40% oxygen. The other two methods ensure that the equipment is never subjected to greater than 40% oxygen content.

In a fire, the pressure in a gas cylinder [[Amontons' Law|rises in direct proportion to its absolute temperature]]. If the internal pressure exceeds the mechanical limitations of the cylinder and there are no means to safely vent the pressurized gas to the atmosphere, the vessel will fail mechanically.  If the vessel contents are ignitable or a contaminant is present this event may result in a "fireball".<ref name="DAN" />

===Incorrect gas mix===
Use of a gas mix that differs from the planned mix introduces an increased risk of decompression sickness or an increased risk of oxygen toxicity, depending on the error. It may be possible to simply recalculate the dive plan or set the dive computer accordingly, but in some cases the planned dive may not be practicable.

Many training agencies such as [[Professional Association of Diving Instructors|PADI]],<ref name=padi /> [[Confédération Mondiale des Activités Subaquatiques|CMAS]], [[Scuba Schools International|SSI]] and [[National Association of Underwater Instructors|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 planned mix, the diver must either recalculate the dive plan with the actual mix, or else abort the dive to avoid increased risk of oxygen toxicity or decompression sickness. Under [[International Association of Nitrox and Technical Divers|IANTD]] and [[American Nitrox Divers International|ANDI]] rules for use of nitrox, which are followed by dive resorts around the world,{{citation needed|date=March 2012}} filled nitrox cylinders are signed out personally in a blended gas records book, which contains, for each cylinder and fill, the cylinder number, the measured oxygen fraction by percentage, the calculated [[maximum operating depth]] for that mix, and the signature of the receiving diver, who should have personally measured the oxygen fraction before taking delivery. All of these steps reduce risk but increase complexity of operations as each diver must use the specific cylinder they have checked out. In South Africa, the national standard for handling and filling portable cylinders with pressurised gases (SANS 10019) requires that the cylinder be labelled with a sticker identifying the contents as nitrox, and specifying the oxygen fraction.<ref name="SANS 10019" /> Similar requirements may apply in other countries.

==History==
In 1874, [[Henry Fleuss]] made what was possibly the first Nitrox dive using a rebreather.<ref name="Lang 2006" />

In 1911, [[Drägerwerk|Draeger]] of Germany tested an injector operated rebreather backpack for a [[standard diving]] suit. This concept was produced and marketed as the DM20 oxygen rebreather system and the DM40 nitrox rebreather system, in which air from one cylinder and oxygen from a second cylinder were mixed during injection through a nozzle which circulated the breathing gas through the scrubber and the rest of the loop. The DM40 was rated for depths up to 40m.<ref name="Draegerwerk 1899" />

[[Christian J. Lambertsen]] proposed calculations for nitrogen addition to prevent oxygen toxicity in divers utilizing nitrogen–oxygen rebreather diving.<ref name=Lambertsen1941 />

In [[World War II]] or soon after, British commando [[frogman|frogmen]] and [[clearance diver]]s started occasionally 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.{{citation needed|date=July 2015}}<!--This is plausible, as it basically (badly) describes a semi-closed constant mass flow rebreather-->

Lambertson published a paper on nitrox in 1947.<ref name="Lang 2006" />

In the 1950s, the [[United States Navy]] (USN) documented enriched oxygen gas procedures for military use of what we today call nitrox, in the US Navy Diving Manual.<ref name=usn />

In 1955, E. Lanphier described the use of nitrogen–oxygen diving mixtures, and the [[equivalent air depth]] method for calculating decompression from air tables.<ref name="Lang 2006" />

In the 1960s, A. Galerne used on-line blending for commercial diving.<ref name="Lang 2006" />

In 1970, [[John Morgan Wells|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 introduced the concept of Equivalent Air Depth (EAD). He also developed a process for mixing oxygen and air which he called a continuous blending system.  For many years 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.<ref name=noaa/><ref name=dan/>

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.<ref name="Lang 2006" />

In 1989, the Harbor Branch Oceanographic institution workshop addressed blending, oxygen limits and decompression issues.<ref name="Lang 2006" />

In 1991, Bove, Bennett and ''Skin Diver'' magazine took a stand against nitrox use for recreational diving. ''Skin Diver'' editor Bill Gleason dubbed nitrox the "Voodoo Gas". The annual DEMA show (held in Houston, Texas that year) banned nitrox training providers from the show.  This caused a backlash, and when DEMA relented, a number of organizations took the opportunity to present nitrox workshops outside the show.<ref name="Lang 2006" />

In 1992, the Scuba Diving Resources Group organised a workshop where some guidelines were established, and some misconceptions addressed.<ref name="Lang 2006" />

In 1992, [[British Sub-Aqua Club|BSAC]] banned its members from using nitrox during BSAC activities.<ref name="BSACOK" /> IAND's 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.{{citation needed|date=July 2015}} In the early 1990s, these agencies were teaching nitrox, but the main scuba agencies were not. Additional new organizations, including the [[American Nitrox Divers International]] (ANDI) – which invented the term "Safe Air" for marketing purposes – and [[Technical Diving International]] (TDI) were begun.{{citation needed|date=July 2015}} [[National Association of Underwater Instructors|NAUI]] became the first existing major recreational diver training agency to sanction nitrox.<ref name=NAUIhx />

In 1993, the [[Sub-Aqua Association]] was the first UK recreational diving training agency to acknowledge and endorse the Nitrox training their members had undertaken with one of the tech agencies. The SAA's first recreational Nitrox qualification was issued in April 1993. The SAA's first Nitrox instructor was Vic Bonfante and he was certified in September 1993.<ref name="xray-mag.com" />

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.{{citation needed|date=July 2015}} With the new dive computers which could be programmed to allow for the longer bottom-times and shorter residual nitrogen times that nitrox gave, the incentive for the sport diver to use the gas increased.

In 1993, ''Skin Diver'' magazine, the leading recreational diving publication at the time,{{citation needed|date=July 2015}} published a three-part series arguing that nitrox was unsafe for sport divers.<ref group=note>A position which it would formally maintain until in 1995 magazine editor Bill Gleason was reported to say that nitrox was "all right".  ''Skin Diver'' would later go into bankruptcy.</ref>{{citation needed|date=July 2015}} [[DiveRite]] manufactured the first nitrox-compatible [[dive computer]], called the Bridge,<ref name="TDI manual" /> the aquaCorps TEK93 conference was held in San Francisco, and a practicable oil limit of 0.1&nbsp;mg/m<sup>3</sup> for oxygen compatible air was set. The Canadian armed forces issued EAD tables with an upper PO<sub>2</sub> of 1.5 ATA.<ref name="Lang 2006" />

In 1994, John Lamb and Vandagraph launched the first oxygen analyzer built specifically for Nitrox and mixed-gas divers, at the Birmingham Dive Show.<ref name="xray-mag.com"/>

In 1994, BSAC reversed its policy on Nitrox and announced BSAC nitrox training to start in 1995.<ref name=BSACOK/>

In 1996, the [[Professional Association of Diving Instructors]] (PADI) announced full educational support for nitrox.<ref name=padi/> While other mainline scuba organizations had announced their support of nitrox earlier,<ref name=BSACOK /> it was PADI's endorsement that established nitrox as a standard recreational diving option.<ref name="adc" /><!--{{Unreliable source?|date=July 2015|reason=American Dive Center has no "reputation for fact-checking and accuracy" as needed for a reliable source; the statement is too strong to be based on the opinion of an unknown website}} Commented out, but left for reference. Lang 2006 comes to a similar conclusion, and has a reputation as an expert in the field--><ref name="Lang 2006" />

In 1997, [[Professional Technical and Recreational Diving|ProTec]] started with Nitrox 1 (recreational) and Nitrox 2 (technical). A German ProTec Nitrox manual (ref to the 6th edition) has been published.<ref name="Hirschmann" />

In 1999, a survey by R.W. Hamilton showed that over hundreds of thousands of nitrox dives, the DCS record is good. Nitrox had become popular with recreational divers, but not used much by commercial divers who tend to use surface supplied breathing apparatus. The OSHA accepted a petition for a variance from the commercial diving regulations for recreational scuba instructors.<ref name="Lang 2006" />

The 2001 edition of the ''NOAA Diving Manual'' included a chapter intended for Nitrox training.<ref name="Lang 2006" />

==In nature==
At times in the geological past, the Earth's atmosphere contained much more than 20% oxygen: e.g. up to 35% in the [[Upper Carboniferous]] period. This let animals absorb oxygen more easily and influenced their evolutionary patterns.<ref name="Berner and Canfield 1989" /><ref name="Dudley" />

==See also==
*Other {{annotated link|Breathing gas|breathing gases}}
**{{annotated link|Argox}}
**{{annotated link|Heliox}}
**{{annotated link|Hydreliox}}
**{{annotated link|Hydrox (breathing gas)|Hydrox}}
**{{annotated link|Trimix (breathing gas)|Trimix}}
*{{annotated link|Breathing gas analysis}}
*{{annotated link|Diving cylinder}}
*Methods of {{annotated link|Gas blending}}
**{{annotated link|Membrane gas separation}}
*{{annotated link|Equivalent air depth}}
*{{annotated link|Lean air}}
*{{annotated link|Maximum operating depth}}
*{{annotated link|Nitrogen narcosis}}
*{{annotated link|Oxygen toxicity}}
*{{annotated link|Partial pressure}}

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<ref name=Brubakk>{{cite book |title=Bennett and Elliott's physiology and medicine of diving |edition=5th Rev |last=Brubakk |first=A. O. |author2=T. S. Neuman |year=2003 |publisher=Saunders Ltd. |location=United States |isbn=0-7020-2571-2 |pages=800}}</ref>

<ref name=Brubakk304>{{cite book |title=Bennett and Elliott's physiology and medicine of diving |edition=5th Rev |last=Brubakk |first=Alf O |author2=Neuman, Tom S |year=2003 |publisher=Saunders Ltd. |location=United States |isbn=0-7020-2571-2 |page=304}}</ref>

<ref name=BSACOK>{{cite journal |author=Allen, C |title=BSAC gives the OK to nitrox |journal=Diver 1995; 40(5) May: 35-36. Reprinted in South Pacific Underwater Medicine Society Journal |volume=26 |issue=3 |year=1996 |issn=0813-1988 |oclc=16986801 }}</ref>

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<ref name=dan>{{cite book |title=DAN Nitrox Workshop Proceedings |author=Lang, M.A. |year=2001 |publisher=Divers Alert Network |location=Durham, NC |pages=197 }}</ref>

<ref name="DAN">{{cite web |title=Incident Insights - Trust But Verify |url=https://www.diversalertnetwork.org/diving-incidents/trust-but-verify |website=Divers Alert Network}}</ref>

<ref name="divingfrontiers" >{{Cite web |title=About Enriched Air Nitrox |publisher=Diving Frontiers |location=Balcatta, Perth, Western Australia, 6021 |url=https://www.divingfrontiers.com.au/about-enriched-air-nitrox.htm#:~:text=The%20two%20most%20common%20recreational,is%20uncommon%20within%20recreational%20diving. |access-date=16 September 2024 |website=www.divingfrontiers.com.au}}</ref>

<ref name="Draegerwerk 1899">{{cite web |url=http://www.divinghelmet.nl/divinghelmet/1899_Draegerwerk.html |title=1889. Draegerwerk Lübeck |last=Dekker |first=David L. |work=Chronology of Diving in Holland |publisher=www.divinghelmet.nl |access-date=14 January 2017}}</ref>

<ref name="Dudley">{{cite journal |last1=Dudley |first1=Robert |title=Atmospheric oxygen, Giant Paleozoic Insects and the Evolution of Aerial Locomotor Performance |url=http://jeb.biologists.org/content/201/8/1043.full.pdf |journal=Journal of Experimental Biology |year=1998 |volume=201 |issue=8 |pages=1043–1050 |doi=10.1242/jeb.201.8.1043 |pmid=9510518|bibcode=1998JExpB.201.1043D }}</ref>

<ref name="EANx">{{cite book |first1=Gerhard|last1=Hirschmann |title=Nitrox Diver Manual |edition=3nd |language=de |publisher=Alpha Verlag |location=München |date=1996 |isbn=3-932470-01-X |url=https://d-nb.info/959297987 |access-date=15 April 2022 }}</ref>

<ref name="Elliott 1996">{{cite journal |last1=Elliott |first1=D |title=Nitrox |journal=South Pacific Underwater Medicine Society Journal |volume=26 |issue=3 |year=1996 |issn=0813-1988 |oclc=16986801  }}</ref>

<ref name="EN144">EN144-3:2003 Respiratory protective devices - Gas cylinder valves - Part 3: Outlet connections for diving gases Nitrox and oxygen</ref>

<ref name=evil>{{cite journal |title=Compressed breathing air – the potential for evil from within |last1=Millar |first1=I.L. |last2=Mouldey |first2=P.G. |year=2008 |volume=38 |pages=145–51 |journal=Diving and Hyperbaric Medicine |issue=2 |publisher=[[South Pacific Underwater Medicine Society]] |pmid=22692708 }}</ref>

<ref name="Goldman 2013">{{cite web |url=http://moderndecompression.com/?p=294 |title=How SAUL relates to the PADI dive tables |last=Goldman |first=Saul |date=23 September 2013 |work=Modern decompression |access-date=10 September 2014}}</ref>

<ref name=GoodDive >{{Cite web |title=Nitrox definition - GoodDive.com |url=https://www.gooddive.com/scuba-diving-glossary/nitrox.htm |access-date=16 September 2024 |website=www.gooddive.com}}</ref>

<ref name="Hamilton1995">{{cite journal |last1=Hamilton |first1=K |last2=Laliberté |first2=MF |last3=Fowler |first3=B |title=Dissociation of the behavioral and subjective components of nitrogen narcosis and diver adaptation |journal=Undersea and Hyperbaric Medicine |volume=22 |issue=1 |pages=41–9 |date=March 1995 |publisher=Undersea and Hyperbaric Medical Society |pmid=7742709 }}</ref>

<ref name="Harris et al 2003">{{cite journal |last1=Harris |first1=RJ |last2=Doolette |first2=DJ |last3=Wilkinson |first3=DC |last4=Williams |first4=DJ |title=Measurement of fatigue following 18 msw dry chamber dives breathing air or enriched air nitrox |journal=Undersea and Hyperbaric Medicine |volume=30 |issue=4 |pages=285–91 |year=2003 |publisher=Undersea and Hyperbaric Medical Society |pmid=14756231 }}</ref>

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<ref name="Lippman and Mitchell 2005">{{cite book |author=Lippmann, John |author2=Mitchell, Simon J |author2-link=Simon Mitchell |title=Deeper into Diving |publisher=J.L. Publications |location=Victoria, Australia |date=October 2005 |edition=2nd |pages=403&ndash;4 |chapter=28 |isbn=0-9752290-1-X |oclc=66524750}}</ref>

<ref name="Logan1961">{{cite journal |last=Logan |first=J.A. |title=An evaluation of the equivalent air depth theory |journal=United States Navy Experimental Diving Unit Technical Report |volume=NEDU-RR-01-61 |date=1961 }}</ref>

<ref name="Luxfer 23.5%" >{{cite web |url=https://www.luxfercylinders.com/support/faq-scuba-aluminum-cylinders-cleaning-concentrations |title=Why does Luxfer require cleaning for oxygen concentrations above 23.5%? |author=Luxfer gas cylinders |publisher=Luxfer |access-date=2 October 2018}}</ref>

<ref name=Mastro1989>{{cite journal |author=Mastro, SJ |title=Use of two primary breathing mixtures for enriched air diving operations |year=1989 |journal=In: Lang, MA; Jaap, WC (Ed). Diving for Science…1989. Proceedings of the American Academy of Underwater Sciences Annual Scientific Diving Symposium 28 September - 1 October 1989 Wood Hole Oceanographic Institution, Woods Hole, Massachusetts, USA }}</ref>

<!--(this citation is not used in content and was thus giving a cite error)
<ref name="Menduno 2023">{{cite web |url=https://gue.com/blog/hydrogen-dreamin/ |title=Hydrogen, At Last? |website=InDepth |publisher=GUE |first=Michael |last=Menduno |date=1 March 2023 |access-date=8 October 2023}}</ref-->

<ref name="Muller" >{{cite web|author=Beat A. Müller |url=http://www.swiss-cave-diving.ch/PDF-dateien/Tauchen-Normung-EN144_V2.6_15042008.pdf |title=Tauchen und Normung mit spez. Berücksichtigung der EN144-3 (Nitrox-Gewinde M26 x 2) |publisher=Swiss Cave Diving |date=15 April 2008 |pages=33–36 |accessdate=20 December 2023 |archiveurl=https://web.archive.org/web/20220206213018/https://www.swiss-cave-diving.ch/PDF-dateien/Tauchen-Normung-EN144_V2.6_15042008.pdf |archivedate=6 February 2022 |language=de}}</ref>

<ref name=NAUIhx>{{cite web |title=NAUI History |publisher=[[National Association of Underwater Instructors]] |url=https://www.naui.org/history.aspx |access-date=31 December 2015 }}</ref>

<ref name=noaa>{{cite book |title=NOAA Diving Manual: Diving for Science and Technology |edition=Fourth |last=Joiner |first=J.T. |year=2001 |publisher=Best Publishing |location=United States |isbn=0-941332-70-5 |pages=[https://archive.org/details/noaadivingmanual00noaa/page/660 660] |url-access=registration |url=https://archive.org/details/noaadivingmanual00noaa/page/660}}</ref>

<ref name="Nuvair">{{cite web |url=https://www.nuvair.com/products/nitrox |title=Nitrox: How the nitrox membrane system works |website=www.nuvair.com |access-date=29 February 2020}}</ref>

<ref name=oxyhackers>{{cite book |author=Harlow, Vance |title=Oxygen Hacker's Companion |edition=fourth |publisher=Airspeed Press |location=Warner, NH |year=2002 |isbn=0-9678873-2-1}}</ref>

<ref name=padi>{{cite journal |author1=Richardson, D  |author2=Shreeves, K  |name-list-style=amp |title=The PADI Enriched Air Diver course and DSAT oxygen exposure limits |journal=South Pacific Underwater Medicine Society Journal |volume=26 |issue=3 |year=1996 |issn=0813-1988 |oclc=16986801 }}</ref>

<ref name="Rosales et al 2007">{{cite journal |title=Guide for Oxygen Compatibility Assessments on Oxygen Components and Systems. |journal=NASA Johnson Space Center Technical Report |volume=NASA/TM-2007-213740 |year=2007 |last1=Rosales |first1=KR |last2=Shoffstall |first2=MS |last3=Stoltzfus |first3=JM }}</ref>

<ref name="SafeAir">{{cite web |url=http://www.andihq.com/index.php/about-andi/what-is-safeair |title=What is SafeAir |last=<!--not specified--> |year=2015 |publisher=ANDI |access-date=28 July 2016}}</ref>

<ref name="SANS 10019">South African National Standard 10019:2008, Transportable containers for compressed, dissolved and liquefied gases - Basic design, manufacture, use and maintenance, Standards South Africa, Pretoria</ref>

<ref name="SANS 10019:2021">South African National Standard 10019:2021, Transportable containers for compressed, dissolved and liquefied gases - Basic design, manufacture, use and maintenance, Standards South Africa, Pretoria</ref>

<ref name="Scubaboard Poll" >{{cite web |title=How does nitrox make you feel? |publisher=ScubaBoard |year=2007 |url=http://www.scubaboard.com/forums/basic-scuba-discussions/poll-1630-a.html |access-date=21 May 2009 }}</ref>

<ref name="scubadiving.com">{{cite web |last1=Duong |first1=T. |date=2021 |title=Guide to nitrox diving, Scuba Diving. |url=https://www.scubadiving.com/nitrox-scuba-diving-guide-certification |access-date=3 December 2022 }}</ref> 

<ref name="Scuba doctor">{{Cite web |last= |first= |title=Scuba Diving Gas Analysis |url=https://www.scubadoctor.com.au/scuba-diving-gas-analysis.htm#:~:text=Calibrate%20%E2%80%94%20This%20step%20is%20important,in%20the%20gas%20being%20analysed. |access-date=25 December 2024 |website=The Scuba Doctor |language=en-au |publication-place=Victoria, Australia}}</ref>

<ref name="TDI manual">[[Technical Diving International|TDI]], ''Nitrox Gas Blending'' Manual, at pages 9-11</ref>

<ref name="Treatment tables">{{cite book |last1=Berghage |first1=T.E. |last2=Vorosmarti |first2=J. |last3=Barnard |first3=E.E.P. |editor=Miner, W.F |title=Recompression treatment tables used throughout the world by government and industry |date=25 July 1978 |publisher=Naval Medical Research Institute |location=Bethesda, Maryland}}</ref>

<ref name=usn>{{cite book |title=US Navy Diving Manual, 6th revision |year=2006 |publisher=US Naval Sea Systems Command |location=United States |url=http://www.supsalv.org/00c3_publications.asp?destPage=00c3&pageID=3.9 |access-date=24 April 2008 }}</ref>

<ref name="yourdictionary">{{Cite web |title=Nitrox Definition & Meaning {{!}} YourDictionary |url=https://www.yourdictionary.com/nitrox |access-date=16 September 2024 |website=www.yourdictionary.com}}</ref>

<ref name="xray-mag.com">{{cite magazine |first=Rosemary E |last=Lunn |url=https://xray-mag.com/content/john-lamb-%E2%80%98mr-oxygen%E2%80%99-dies-78/ |title=John Lamb - 'Mr Oxygen' - dies at 78 |magazine=X-Ray Magazine}}</ref>
</references>

==Footnotes==
<references group=note/>

==External links==
{{Wiktionary}}

*[https://web.archive.org/web/20041104014946/http://www.hq.nasa.gov/office/codeq/doctree/canceled/1740151.pdf NASA – Safety Standards for Oxygen and Oxygen Handling]

{{Underwater diving|divequ}}

[[Category:Breathing gases]]