What Is This Nitrox Stuff?


Larry "Harris" Taylor, Ph.D.

This is an electronic reprint of an article that appeared in The Flipper (Sept/Oct. 1993, p. 6-8). This material is copyrighted and all rights retained by the author. This article is made available as a service to the diving community by the author and may be distributed for any non-commercial or Not-For-Profit use.  

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Simply put, Nitrox is any binary mixture of nitrogen and oxygen. The air you are currently breathing is nitrox-21. (The number refers to the percentage of oxygen in the mix.) The Nitrox mixes most often encountered are NOAA Nitrox I (32% O2) and NOAA Nitrox II (36% O2). Collectively, all nitrogen-oxygen binary mixes are known as "enriched air nitrox" (EANx) or oxygen enriched air to emphasize that the O2 concentration is higher than air. One Nitrox training agency (ANDI) uses the marketing trademark of "safe air." Like the emergence of buoyancy compensators and submersible pressure gauges, the entrance of Nitrox into the sport diving community has promulgated much discussion.  As such, there is an abundance of mythology surrounding the topic. Let's examine some of these myths.

Myth: Nitrox is a deep diving gas:  

Absolutely false! Nitrox is used primarily to extend bottom times in the 60-90 fsw range. NOAA I has an operating depth limit of 130 fsw; NOAA II has a limit of 100 fsw.

Myth: Nitrox is a new, untested gas mix. 

No! Nitrox has been successfully used in the scientific, commercial and military diving communities since 1879.

Myth: If you use this gas, you will die.

As with any diving, there are risks. The increased O2 concentration increases risk (relative to air) for oxygen toxicity at depths below 100 fsw. The use of this gas requires that divers to rigorously monitor depth and time. 

Myth: If I use this gas, then I cannot be treated for bends in a chamber. 

There is no medical reason for non-treatment. Oxygen "dose" is measured in OTU's (Oxygen Tolerance Units). A typical sport dive accumulates less than 150 OTU's. It has been medically accepted that humans can tolerate a daily dose of 1440 OTU's. Since a typical chamber run uses about 400 OTU's (with 600 OTU's being about max per treatment), a typical diver, especially in rigidly controlled resort situations, has more than adequate OTU reserve available for treatment. 

Myth: If I use this equipment, I must purchase totally separate scuba gear and everything must be oxygen cleaned. Using Nitrox will destroy my current scuba gear.

It has been established that oxygen mixes less than 40% O2 do not require O2 cleaning. While it might be prudent, as long as O2 concentration is less than 40% there is no need for separate gear. Current scuba gear can be used with EANx. Since some gas mixes are prepared in the scuba cylinder (thus cylinder may be exposed to higher concentrations of oxygen), it is required that Nitrox cylinders be O2 cleaned and used only for nitrox. 

Advantages of Nitrox

The primary component of air is nitrogen. Nitrogen is physiologically inert (not used by the body.) As such it just accumulates in tissues. Decompression schedules (obligations) are based on this accumulation of nitrogen. If we lower the percentage of nitrogen in the breathing gas mix, then we lower our tissue accumulation of nitrogen gas and decrease our risk of decompression problems.

There are two ways to utilize EANx. The first is termed the "physiological" advantage. This means breathing EANx while using whatever sport diver tables/computer (based on air) you currently use. Since you are breathing a mix that employs less nitrogen than air, you will accumulate less nitrogen than you would breathing air on the same dive profile. Since you have accumulated less nitrogen than your air table/computer has calculated, you have a "physiological" safety factor; you are at less risk for a decompression sickness hit than you would be breathing air on the same dive. 

The second way to utilize EANx is termed the "decompression advantage." Since decompression schedules are based on nitrogen accumulation, divers breathing EANx (contains less nitrogen than air) will take longer to absorb a fixed amount of nitrogen. This means that divers can stay at the chosen depth longer on EANx than on air. This difference can be substantial.   

No Decompression Times

The increase in no-D time allowed in the 60-90 fsw range is the primary reason for using the gas as a breathing mix within the scientific, commercial and military diving communities.  


The decrease in nitrogen is accomplished by a corresponding increase in oxygen. This increases the potential for oxygen toxicity "hits." An oxygen toxicity hit may occur with no-warning with the severity of a "grand-mal-type" seizure. Such an event in sport diving gear is not considered to be survivable. It is the potential for oxygen toxicity hits that mandates absolute discipline and adherence to established diving protocols and procedures: the gas must be analyzed and found appropriate for the depth range desired (requires prior planning to establish diving limits), the depth limits of the gas mix MUST BE respected (the consequence of going too deep with EANx is the potential for in-water seizure followed by drowning), and equipment must be properly maintained.


The use of gas mixes other than air is rapidly entering the recreational market. The distinct advantages of EANx for shallow water diving are substantial and will most likely give this gas a permanent place in sport diving. The disadvantages of this gas require that divers understand what they are doing and adhere to a level of discipline that is unfamiliar to many within the sport diving community. The concerns about the use of EANx in the recreational scuba community are real; the penalty of improper use can be severe.  However, the key to successful EANx diving (as with all diving) is knowledge of the risks and adherence to those procedures that have been established to minimize those risks. 

For those (like myself) whose diving is primarily less than 100 fsw, EANx is definitely the current breathing mix of choice. I have said it before and I will continue to preach the "gospel" according to "Harris": In ALL conceivable diving scenarios, the knowledgeable, physically fit diver has more fun! Staying at depth longer, to many, means more enjoyment of Planet Ocean. With proper training and understanding, EANx can be the doorway to more bottom-time and thus more in-water fun! 


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About The Author:

Larry "Harris" Taylor, Ph.D. is a biochemist and Diving Safety Coordinator at the University of Michigan. He has authored more than 100 scuba related articles. His personal dive library (See Alert Diver, Mar/Apr, 1997, p. 54) is considered one of the best recreational sources of information in North America.

  Copyright 2001-2022 by Larry "Harris" Taylor

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