Nitrox

Nitrox refers to any gas mixture composed of nitrogen and oxygen. It is usually used for mixtures that contain less than 78% nitrogen by volume. In the usual application, underwater diving, nitrox is normally distinguished from air and handled differently. 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 body's tissues, thereby extending the practicable underwater dive time by reducing the decompression requirement, or reducing the risk of decompression sickness. The two most common recreational diving nitrox mixes are 32% and 36% oxygen, which have maximum operating depths of about 110 feet and 95 feet respectively.
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. 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.
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

Decompression benefits

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 stops for the same risk. 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 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. The Goldman decompression model predicts a significant risk reduction by using nitrox.

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. 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. 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 HPNS when breathed at high pressures, which does not happen with gases that have greater narcotic potency. However, because of risks associated with 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 or heliox gases are typically used; these gases contain helium to reduce the amount of narcotic gases in the mixture.

Oxygen toxicity

Diving with and handling nitrox raise a number of potentially fatal dangers due to the high partial pressure of oxygen. 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 to ensure a maximum ppO₂ of no more than. 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 breathe 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. Many dive shops, dive operators, and gas blenders require the diver to present a nitrox certification card before selling nitrox to divers.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. This is done by expelling a small amount of air from the diver's tank into an oxygen analyzer. This is to further limit the possibility of oxygen toxicity due to errors in previous testing.
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. The deeper depth, called the "contingency depth", is reached when the partial pressure reaches. Diving at or beyond this level exposes the diver to a greater risk of central nervous system 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.
Divers trained to use nitrox may memorise the acronym VENTID-C or sometimes ConVENTID,, Ears. 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. 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.

Other effects

There is anecdotal evidence that the use of nitrox reduces post-dive fatigue, particularly in older and or obese divers; however a double-blind study to test this found no statistically significant reduction in reported fatigue. There was, however, some suggestion that post-dive fatigue is due to sub-clinical decompression sickness ; 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.
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. 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.

Uses

Underwater diving

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. The main benefit is reduced decompression risk. 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.

Training and certification

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 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.