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Enriched Air Recreational Diving
by R.W. Bill Hamilton, PhD and Joel Silverstein

During the last ten years there has been significant emphasis placed on diving with gas mixtures other than air. As one read articles, papers and books one may begin to ask, "What’s wrong with diving on air?" Simply, air contains two major components, oxygen (21%) and nitrogen (79%). The nitrogen in normal air limits your bottom time, or the number of dives that can be done in a day, or requires a longer surface interval between dives. Nitrogen is also the cause of decompression sickness.

Enter Enriched Air Nitrox, also called Oxygen Enriched Air, or just nitrox. Enriched Air Nitrox is air that has more oxygen in it. Consequently it has less nitrogen. Since nitrogen is the controlling factor for decompression from no-stop diving, with less nitrogen in the breathing mixture the body will absorb less nitrogen and will have less of a decompression obligation.

The two most commonly used nitrox mixtures are 32% and 36% oxygen. In all cases the other gas in the mix, is nitrogen. These mixes were first introduced by the National Oceanographic and Atmospheric Administration (NOAA) for use in their scientific and shallow water research dives.

Today’s diver is seeking more adventure and exploration, which almost always leads to the desire for more bottom time. Many times the only way to get that longer bottom time is to plan dives that require lengthy and sometimes complicated decompression stops. Using enriched air nitrox makes getting that longer bottom time not only easier by eliminating decompression stops required with air, but this also allows the diver to have shorter surface intervals while still maintaining a minimal level of risk.

Table 1 is a comparison of no-stop dive times, which illustrates why enriched air nitrox is in such great demand.

No-Stop Dive Times in minutes

A diver making a 90 fsw dive using one of these mixtures can increase the typical air no-stop dive time from 25 minutes to as much as 50 minutes. This is a significant increase in no-stop bottom time.

But increased bottom time does not come without its price tag. Enjoying the underwater world comes with limits and safety procedures.

fsw  Air  EAN 32%  EAN 36%
50 80 200 200
60 55 100 100
70 45 60 60
80 35 50 60
90 25 40 50
100 22 30 40
110 15 25 30
120 12 25 n/a

Myths of enriched air nitrox

With all technology come facts and perceived facts. There are some myths that we need to dispel.

"Nitrox is for deep diving." Actually enriched air nitrox has very stringent depth limits due to the higher concentration of oxygen in the mixture. The greatest advantages for no-stop diving are in the 50 to 130 fsw diving range. The two standard mixtures of EAN 32 and EAN 36 have maximum operational depths of 130 fsw and 110 fsw respectively. There are other applications for using enriched air nitrox to accelerate decompression from deep dives, but these are outside the scope of the primary enriched air training courses, but are covered in other courses.

"You can't get decompression sickness." No gas nor diving table can absolutely ensure that a diver will not get decompression illness. Using enriched air nitrox provides significant decompression advantages over air, which may help avoid decompression sickness, but with all diving there is a risk of decompression illness.

"Narcosis is eliminated." It might seem logical that with reduced nitrogen in the breathing mix there would be reduced nitrogen narcosis at depth. The fact is, oxygen can also be a narcotic gas when under pressure. The result is that there is no significant change in narcosis when diving enriched air nitrox as compared to air.

Justification for enriched air nitrox

It almost seems obvious that using enriched air nitrox provides significant benefits of extending bottom times, but not obvious is its use as an effective tool to help prevent decompression illness. By using a gas with less nitrogen in it we can effectively lower the risk of decompression illness. This does not mean that a diver will never get decompression illness, just that with proper management the already small risk of DCI is even smaller when diving similar profiles using enriched air nitrox. 

Enriched air nitrox is of special value when making multi-day repetitive dives. Dive incident statistics show that almost 80 percent of all cases of decompression illness result from repetitive dives. If a diver were to dive the same profiles that he had done while using air using an enriched air mixture instead, not only would the decompression be significantly less but there would be an overall reduction of nitrogen in the body. As a result, many divers report that they are not as physically tired after a series of dives using nitrox as they have been in the past using air.

Repetitive Dive Example

Looking at a typical recreational dive profile we compare two dives, one using enriched air nitrox 32% and the other air.

Diver # 1 breathing 32% oxygen does a no-stop first dive to 100 fsw (30 m) for 22 minutes, waits one hour between dives and then does a repetitive no-stop dive to 70 fsw (21 msw) for 34 minutes. This dive plan does not require any decompression stops. The diver does however add a 3-5 minute safety stop for each dive.

Diver # 2 breathing air does a no-stop first dive to 100 fsw (30 m) for 22 minutes, waits one hour between dives, and then does a repetitive dive to 70 fsw (21 msw) for 34 minutes. This dive plan does not require any decompression stops on the first dive (the 3-5 min safety stop is taken) but requires a very long 18 minute mandatory decompression stop for the second dive. If the diver did not want to conduct a decompression dive on the second dive, a no-stop dive would be only 14 minutes. This is 20 minutes less bottom time than the diver who used EAN 32. By using enriched air nitrox the first diver in the above example has an 18 minute decompression advantage over the second diver who was diving a similar air profile.

If you imagine a typical dive vacation where a diver may be diving two to three times a day for five or six days, it is easy to see that the benefits of using enriched air nitrox become quite significant for either extending bottom time or by reducing required decompression. That is the magic of oxygen enriched air diving. ©

Updated 29 August 2003
last updated 01/19/02
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