Nitrox

Nitrox refers to any gas mixture composed (excepting trace gases) of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon. 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 (also known as the bends). Nitrox refers to any gas mixture composed (excepting trace gases) of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon. 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 (also known as the bends). 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 risk 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. 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 (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. The Goldman decompression model predicts a significant risk reduction by using nitrox (more so than the PADI tables suggest). 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. Diving with and handling nitrox raise 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 ft) to ensure a maximum ppO2 of no more than 1.4 bar (140 kPa). 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. 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 1.4 bar (140 kPa). The deeper depth, called the 'contingency depth', is reached when the partial pressure reaches 1.6 bar (160 kPa). 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 Vision (blurriness), Ears (ringing sound), Nausea, Twitching, Irritability, Dizziness, and 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. A solution to either is to ascend to a shallower depth.