Editing Nitrox (section)

==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" />