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Questions and Answers on
Physiology and Medical Aspects of Scuba Diving


Lawrence Martin, M.D. Copyright 1997


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Myths & Misconceptions
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Brief History of Diving
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The Respiratory System
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Unequal Air Pressures
Decompression Sickness
Oxygen Therapy
Gas Pressure at Depth

Dive Tables & Computers
Stress & Diving
Non-air Gas Mixtures
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Medical Fitness for Diving
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The Great Debate

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Diving with non-air gas mixtures: Nitrox, Heliox, Trimix


Nitrox is a gas mixture of oxygen and nitrogen, but with a higher oxygen percentage than found in ordinary air. As a result of its higher oxygen concentration, the percentage of nitrogen in nitrox is always lower than in air. There are two standard mixtures of Nitrox recognized by NOAA for diving: Nitrox I and Nitrox II (see Table 1).

Air 21 79
Nitrox I 32 68
Nitrox II 36 64
* Included are trace inert gases such as argon, krypton, and neon which together make up less than one percent.

Nitrox is usually prepared by mixing pure oxygen from one source (e.g., a tank of 100% oxygen) with air, until the desired oxygen concentration is reached (either 32% or 36%). Adding oxygen to air always lowers the percentage of nitrogen in the final nitrox mixture, because the sum of gas percentages cannot add up to more than 100%. The process requires quality control to assure the desired oxygen concentration is reached, and that the two gases are thoroughly mixed in whatever container holds the nitrox.

Synonyms of nitrox include "enriched air nitrogen" (EAN) and "oxygen-enriched air" (OEA). No matter what it is called, nitrox is not air and should not be called air. Also, when nitrox is discussed in relation to a specific dive profile it must always be qualified with the exact percentage of oxygen used; nitrox I and nitrox II have different risks.


Nitrox provides a lower percentage of nitrogen than ordinary air. In this way less nitrogen will enter the body at a given depth and decrease the risk of two nitrogen-related problems: decompression sickness (DCS) and nitrogen narcosis. Both DCS and nitrogen narcosis result from increased nitrogen pressure, the former from bubble formation on ascent and the latter from nerve inhibition at depth.

It cannot be over emphasized that nitrox does NOT allow one to go deeper than with air. Instead, the decreased nitrogen percentage provides two advantages, which define the two principal reasons nitrox is used:

1) as a time extender for dives to recreational depths, i.e., ability to dive longer at a given depth than allowed for by standard air tables, without increasing the risk of developing DCS;

2) to lessen the risk of developing DCS for dive profiles that adhere to the standard air tables.


Nitrox evolved from military and commercial diving, both considered professional activities. In professional settings the blending process is tightly controlled, divers are highly trained, and a hyperbaric chamber is likely to be available at the diving site. In non-professional use of nitrox, it has become clear that a principal potential hazard is oxygen toxicity.

Going to any depth increases the partial pressure of oxygen that is inhaled. The recreational diver who goes to 130 fsw on compressed air has a blood oxygen (and nitrogen) pressure almost five times that at sea level. While at depth, the principal hazard of excess nitrogen pressure is nitrogen narcosis. But there is also an increased risk of oxygen toxicity. The oxygen pressure at 130 feet when diving with ordinary air would lead to oxygen toxicity if the diver stayed long enough. Because the recreational dive tables allow for only a short time at 130 feet, oxygen toxicity is not a problem.

When the percentage of inhaled oxygen is increased the risk of oxygen toxicity increases, particularly with deeper dives (100 to 130 feet). With nitrox you can stay at a given depth longer because there is less nitrogen in your body (and hence less risk of narcosis and DCS), but stay too long and you risk oxygen toxicity. Since the first manifestation of oxygen toxicity can be seizures, diving with nitrox can lead to drowning.

Simply put, nitrox is a two-edged sword: less nitrogen good, more oxygen bad. Oxygen toxicity is the limiting factor when diving with nitrox. Thus, to the usual major concerns associated with scuba diving (DCS and AGE), nitrox adds the possibility of oxygen toxicity. Anyone diving with nitrox needs to be aware of this potential hazard. Table 2 compares the atmospheres of oxygen (Atm. O2) inhaled with air and with Nitrox I, when diving to 100 fsw and 130 fsw.

depth air (21 % O2) Nitrox I (32 % O2)
100 fsw 0.80 1.29
130 fsw 0.98 1.58


For several reasons the use of nitrox by recreational divers is controversial. Nitrox has to be specially prepared; proper use requires special training; and there are differences in philosophy about what should be included in the purview of recreational diving.

As nitrox entered into non-professional diving a significant problem became apparent: poor quality control. The same quality control achievable in military, scientific and commercial applications was not always found in facilities promoting nitrox to the recreational diver.

Compressed air for scuba diving is universally available because, basically, air is air. The air compressor used to fill a tank to 3000 psi either works or it doesn't. There is little to go wrong. Impurities can enter compressed air but that problem is preventable by good maintenance and proper positioning of the compressor with respect to environmental exhausts.

Nitrox is different. Nitrox has to be specially prepared, and without good quality control the resulting mixture may not contain the specified amount of oxygen. You can be sure that compressed air has 21% oxygen because that's the composition of air, but you can't be sure a tank of Nitrox I has 32% oxygen, and not 30% or 34%. A couple of percentage points one way or the other could make a difference in the safety of the dive. Nitrox experts recommend that the composition of tank gas be measured before each dive. Because of the need for special equipment (in preparing the mixture and then measuring its makeup), most dive shops are not equipped to offer nitrox.

A second reason nitrox is not routinely used is that it requires additional training. Air-trained divers are not trained to dive with nitrox. Nitrox diving requires more education about the risks of oxygen toxicity and the importance of measuring the gas composition before diving. No air-certified diver should use nitrox unless accompanied by a qualified nitrox instructor (or has otherwise completed an accredited training course).

The third reason has to do with differences in philosophy. Some people in the dive industry believe nitrox complicates the recreational experience, and that it should be used only by the "technical" diver, never by the casual diver who puts on scuba gear once or twice a year. People of this philosophy readily concede the potential benefits of nitrox, but believe the potential hazards far outweigh those benefits for recreational divers and the recreational diving industry. They are openly afraid the casual diver will abuse the mixture, that the typical dive shop will not be able to maintain adequate quality control, and that the overall result will be an increase in diving accidents and bad publicity for the industry. These people don't want to see nitrox promoted to the recreational diver.

On the opposite side are many scuba enthusiasts who see this attitude as regressive, as inhibiting growth of the industry and enrichment of the scuba experience. They are passionate about the benefits of nitrox, and feel anyone who is certified for open water can be properly trained in use of nitrox, and that it should be widely available. They point out that exceeding the limits can (and does) happen with compressed air, and feel nitrox is no more dangerous than air if used properly. They also point out how scuba has changed dramatically in the last three decades, and see nitrox as just one more evolutionary advancement whose "time has come."

In truth, both groups have good arguments. Nitrox would complicate the scuba experience, and not just for the occasional diver, but for the dive operator who must make it available. On the other hand, nitrox does offer advantages if properly used. It is probably just a matter of time before nitrox becomes an option to compressed air at many dive resorts. Like other aspects of recreational diving (for example, the dive computer), nitrox can be a great benefit if divers learn to use it wisely and do so.

For all the reasons indicated, nitrox diving comes under the general heading of technical diving. It is a semantic argument whether or not nitrox should instead be part of "recreational diving." Anyone who chooses to dive with nitrox should just be aware of its benefits and potential hazards, and obtain proper training. As long as those goals are accomplished, it is not particularly relevant what label the activity goes by.


Nitrox has actually received "bad press" from much of the recreational dive community, for reasons explained above. The bad press is largely undeserved because nitrox, per se, is not dangerous. In fact, its purpose is to make diving safer than with compressed air, by lowering the risk of decompression sickness. Problems occur when nitrox is abused, such as staying longer on deep dives or going deeper than allowed. The hazards include:

  • using an improperly mixed gas composition
  • going deeper than allowed
  • staying longer than allowed at a given depth

If all nitrox divers limited themselves to 130 feet and used the standard air tables, nitrox would be safer than diving with air, as the risk of decompression sickness and nitrogen narcosis should be lower. This is a big "if." After all, one of the two reasons divers use nitrox is because it allows them to stay down longer than with compressed air (since less nitrogen is taken up for a given time at depth).

1. You are diving with nitrox I to a depth of 60 feet. Your computer, designed for air use only, states you can stay at this depth for 40 minutes. Since you are using nitrox, you can:
a. stay an extra 5 minutes
b. stay an extra 10 minutes
c. stay no longer than your computer indicates
d. do what you want
2. For nitrox I at recreational depths and times, compared to compressed air, all the following are true except one
a. less risk of nitrogen narcosis
b. more risk of oxygen toxicity
c. less risk of decompression sickness.
d. less risk of arterial gas embolism
e. more risk of improper air mix
3. Valid reasons for objecting to use nitrox in recreational diving include all of the following except one:
a. a greater cost than air
b. more difficult to prepare
c. special knowledge and training required
d. greater risk of Type II decompression sickness
e. greater risk of oxygen toxicity



In the past decade many reliable Nitrox training facilities have opened up around the country. Many of these are regular dive shops that have decided to enter the field of nitrox diving; they train divers under the auspices of a national certification agency.

For a while there were only two agencies which sanctioned nitrox diving and arranged for standards of use: ANDI (American Nitrox Divers International) and IANTD (International Association of Nitrox and Technical Divers). (See Appendix B.) Most recently NAUI and PADI have developed training programs for nitrox. No doubt by the end of the decade there will be many nitrox certifying agencies, probably as many as currently certify for basic open water diving.


Heliox is a mixture of helium and oxygen used for very deep diving, usually to greater than 200 feet. Helium's great advantage is that it does not lead to nitrogen narcosis. Helium diving requires as much or more decompression time as nitrogen, so there is no saving there. Beyond 300 feet heliox may cause the 'high pressure nervous syndrome', a shaking sensation that can be incapacitating. Another disadvantage of helium is that it conducts heat about six times faster than nitrogen, so divers get colder than with air diving. A third problem is caused by the fact that helium is much less dense than nitrogen or air; as a result, the vocal cords vibrate much faster and divers sound like Donald Duck. Professional divers can use voice unscramblers to make their speech intelligible.

Overall, helium offers no advantage for recreational divers. Diving with heliox is strictly for technical and professional divers.


Trimix is a mixture of oxygen, helium and nitrogen. Nitrogen, usually in a small percentage (e.g., 15%), is added back to heliox to create trimix, in order to lessen the risk of the high pressure nervous syndrome seen with helium breathing. Nitrogen slows down nerve conduction.

Trimix is used for the deepest scuba dives, usually greater than 400 feet. Like Heliox, Trimix is strictly for non-recreational use: military, scientific, commercial, and advanced technical diving.


Several other gas mixtures have been used, such as hydrogen-oxygen, argon-oxygen, and neon-oxygen. These mixtures are all in the realm of technical and experimental diving. The goal with any non-air mixture, of course, is to dive deeper or longer than can safely be accomplished with compressed air. It is apparent that, for a long time to come, recreational diving as we know it will be done only with mixtures of oxygen and nitrogen.


1. c. Unless your computer is designed to incorporate nitrox diving, you must follow the standards for air diving.

2. d. Less risk of arterial gas embolism is not afforded by nitrox.

3. d. There is no greater risk of Type II DCS with nitrox.


See references for SECTIONS B-E, plus the following.

Mount, T, Gilliam B: Mixed Gas Diving. Watersport Publishing Co., San Diego, 1993.

Betts EA. Introduction to Enriched Air Diving. American Nitrox Divers Association, 74 Woodcleft Avenue, Freeport, N.Y. 11520; 1994.

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Last updated: 28/10/97