Nitrox refers to any gas mixture composed out of nitrogen and oxygen. This includes normal air ("AIR") which is approximately 78% nitrogen and 21% oxygen, with around 1% inert 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).

The Number 1 benefit of using NITROX is to safely extend your bottom time Nitrox 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.

It doesn't matter which gas mix you use, if you dive to the limits, you will most like injure yourself 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 most likely is a placebo effect and 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 utilise NITROX at greater depths where more pronounced narcosis symptoms are more likely to occur.

It is a common myth that NITROX is associated with deep or technical diving. There is anecdotal evidence that the use of NITROX reduces post-dive fatigue, particularly in older and or obese divers.  This is known as an "O2 Buzz." While many divers swear it to be true, there is no evidence to prove the myth.  But, if it makes you feel better, who are we to say it didn't work.

NITROX has many names!

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 Nitrox68, which contains 68% nitrogen and 32% oxygen. When one figure is stated, it refers to the nitrogen percentage, not the oxygen 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).

Common Use

The two most common recreational diving NITROX mixes contain 32% and 36% oxygen, which have maximum operating depths (MODs) of 34 metres (110 ft) and 29 metres (95 ft) respectively when limited to a maximum partial pressure of oxygen of 1.4 bar. 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 - 130 feet. 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 standard NITROX recommendations, the maximum operating depth for EAN45 would be 21 metres (69 ft) and the maximum dive time available at this depth even with EAN36 is nearly 1 hour 15 minutes.
Most divers could not make a standard 80 cu ft tank last that long.

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.  This is covered in depth with the Advanced Nitrox Course.

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 organisations, 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.

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