Frequently Asked Questions About Nitrox

What is Nitrox?
In diving terminology, any mixture of nitrogen and oxygen where these two gases represent the major constituents of the gas mix, is termed Nitrox. Note that mixes which contain more than trace levels of other gases in addition to nitrogen and oxygen are not Nitrox. Air, which is what we breathe on land, is considered a Nitrox mix. Nitrox mixes which are hyperoxic (contain more than 21% oxygen) are variously known as Enriched Air and Enriched Air Nitrox (EAN or EANx). For the most part, sport divers will only be interested in hyperoxic Nitrox mixes.

Why would you want to use Nitrox?
The biggest reason you would want to use Nitrox is longer no decompression limits, or NDLs. With this, we get a reduced decompression penalty due to the lower level of nitrogen absorbed during the dive. This may be realized by surfacing according to the Nitrox tables or, as an additional safety factor, by following the standard air tables.

Another reason is shorter surface intervals and longer subsequent dives due to the lower residual nitrogen level following a dive. The surface interval is followed for the equivalent air depth, not the actual depth. Again, it may be used as safety padding to follow the standard air surface interval. Nitrox provides faster off-gassing than air during decompression. It may be used to reduce the length of the decompression penalty by following a Nitrox decompression schedule, or it may be used as padding to increase the safety factor by following an air decompression schedule. This is similar to the use of pure oxygen during decompression. On the surface, it also provides an alternative to pure O2 in situations where it is desirable to breathe a mix with a higher O2 percentage than air.

The reduced level of nitrogen in your system has also been claimed to reduce the feeling of lethargy or tiredness following a dive. This is thought to be due to a reduction in sub clinical DCS, i.e., a reduction in the number of asymptomatic bubbles in our system after a dive.

How do you name a Nitrox mix?
Originally, all Nitrox mixes were named using the nitrogen percentage to the left of the oxygen percentage, e.g., NOAA Nitrox I contained 32% oxygen and 68% nitrogen and was referred to as: Nitrox 68/32 or Nitrox68 for short.

Today, most people quote the oxygen percentage using a term EANx (where x refers to the percentage of oxygen) or ONM (oxygen-nitrogen mixture). For example:

EAN32 = ONM32 = Nitrox68 = NOAA Nitrox I

EAN36 = ONM36 = Nitrox64 = NOAA Nitrox II

Is Air considered Nitrox?
Yes. Air is, roughly, a mixture containing: 78.05% nitrogen + 20.95% oxygen + 1% trace gases including carbon dioxide, carbon monoxide, and various inert gases – mainly argon.

Is Air considered Enriched Air?
No. Since air only contains a normal amount of oxygen, it is not enriched. Only blends with greater than 21% oxygen are considered enriched air.

What are Nitrox I and Nitrox II?
Nitrox I and Nitrox II are standard Nitrox mixes defined by the National Oceanic and Atmospheric Administration (NOAA) in the US. NOAA has been using Nitrox since the 1970s. Nitrox I is defined to be a mix containing 32% oxygen and 68% nitrogen. Nitrox II is defined to be a mix containing 36% oxygen and 64% nitrogen. The tolerance in the oxygen percentage is +/- 1%.

When the Nitrox is made by enriching air with oxygen, the trace gases are included in the percentage nitrogen figure.

If I am not Nitrox Certified can I still use Nitrox Gas?
No. Only properly trained Nitrox divers can use enriched air.

What is Equivalent Air Depth?
The ability to equate the actual depth to an equivalent air depth is one of the fundamental principles underlying Nitrox diving. One of the limitations in scuba diving is the inert gas we absorb while underwater. It governs our decompression obligation. By reducing the fraction of inert gas in our breathing mix we reduce the partial pressure we experience of that gas at any depth when compared to air at that same depth. Since the absorption of the inert gas is controlled by the difference between the partial pressure in our tissues and the ambient partial pressure, it follows that we will absorb less inert gas than we would on air at the same depth over the same period of time. Thus, the equivalent air depth is the depth on air at which we would experience the same nitrogen partial pressure, absorb the same amount of nitrogen, and incur the same decompression penalty for our actual depth on Nitrox.