Nitrox Diving Safety

July 09, 2018 11:03 PM | Anonymous
Divers training had a good article a few years ago on Nitrox issues and since most of us dive Nitrox, it is important to know some risks. 

The article is quite long, so I cut to the key sections. Here is the link for the full version.

To Breathe or Not to Breathe: Exploring the nitrox controversy

By Alex Brylske

The issue of recreational nitrox diving has been at the forefront of the diving community for the past several months. There are no shortages of opinions about the topic but there seems to be very little objective information about the subject. This article is a milestone in that it addresses both the pros and cons of the activity in an objective and straightforward manner.

When we originally planned a nitrox article for Dive Training, we envisioned it as a two-part series, much like the trilogy published about the dive tables. However, we felt a two-part series increased the chances of confusion, and opted for a single, comprehensive report. We hope you will enjoy the results.


Recently an increasing number of recreational divers have begun purposely altering the air they breathe. Instead of using special gas mixtures to attain certain advantages while diving. These exotic mixtures go by such names as heliox, trimix, and nitrox. By far, the most common alternate breathing mixture is nitrox.

Some of you maybe already know about nitrox diving. Others perhaps have heard about it, but know very little. Even if you’ve never heard of nitrox diving, you certainly soon will. Industry estimates are that from 1985 to 1991, recreational divers engaged in 30,000-50,000 nitrox dives. And the numbers are growing. As altering the diver’s breathing mixture involves serious practical and legal questions, nitrox diving is becoming a hotly debated topic. In this article we’ll examine the issues involved I this new and controversial form of recreational diving. Hopefully, we can put the subject into perspective.


Why, you might ask, would divers want to alter the air they breathe? After all, humans have been breathing what Mother Nature has seen fit to provide us with for millions of years. Theoretically, the answer is simple. For land-based animals who breathe at normal atmospheric pressure, good ol’ regular air does the job of sustaining life quite well. But, when we venture either to altitude or underwater, there are certain disadvantages to regular air. At altitude we’re all aware that the reduced atmospheric pressure robs us of precious oxygen. This is why pilots must breathe oxygen when flying at high altitudes and why the cabins of jet aircraft are pressurized.

When we venture underwater, air continues to have certain limitations. But these limitations have less to do with the oxygen component of air than the nitrogen. It all centers around a topic we have explored extensively in past issues of Dive Training –decompression. The length of time a diver may remain at depth, or the amount of decompression he must undergo if exceeding the no-decompression limits, depends upon the amount of nitrogen absorbed. If a diver breathes air, he breathes a gas mixture containing 79 percent nitrogen. In an EAN mixture, oxygen is used to replace some of the nitrogen. So, instead of breathing a mixture containing 79 percent nitrogen, an enriched air mixture might contain only 68 percent to 64 percent nitrogen. Therefore, as the diver is breathing a gas containing less nitrogen, he absorbs less nitrogen in his body. This means both an extension of the decompression limits and –if required—reduced decompression time. Reduced decompression time is the primary—though not the only—benefit of using EAN.


As in most human endeavors, there is an up side and a down side to the EAN issue. Let’s examine the advantages before looking at the problems. As stated previously, the greatest advantage of using EAN is the extension of no-decompression limits for three of the most popular air dive tables, along with the limits for NOAA Nitrox I and II. As you’ll see, EAN can often more than double the no-decompression limits of the air tables.

Additionally, using EAN can shorten the required surface interval between repetitive dives. Or the diver can make a longer repetitive dive with the same surface interval as a comparable air dive. Either option is possible, again because the diver absorbs less nitrogen than on a comparable air dive.

While the extension of no-decompression time can be a real benefit, it is not—in the opinion of this writer—the primary advantage of using EAN. The real advantage of enriched air is that it can provide recreational divers with an additional safety margin when used with regular air dive tables or computers. Air tables and computers assume the diver will breath air containing 79 percent or 64 percent nitrogen. This means the diver actually absorbs far less nitrogen than the air tables or computer calculates. Thus, the diver’s actual nitrogen absorption will be far below what the tables or computer shows.

Using EAN in this way can be an ideal method of automatically building In the conservatism so many authorities advise when using tables and computers. It might also be a way of overcoming the risks from such unquantifiable factors such as age, obesity, cold, fatigue, and dehydration. In addition, susing EAN with air tables could help decrease the decompression sickness (DCS) rish among dive professionals—instructors and divemasters—who dive continually as part of their duties. Some diver resorts, who maintain EAN filling stations, have already implemented the policy of having their dive guides use EAN for an added measure of safety.

While using EAN with air decompression schedules offers great promise, it’s not a panacea for DCS. We still know far too little about the disorder to make any solid claims about the certainty of avoiding the bends.

Other advantages of EAN have been reported, but haven’t yet been fully scientifically documented. The first involves nitrogen narcosis. The theory is this: The breathing mixture contains less nitrogen than normoxic air, and nitrogen is responsible for narcosis. Thus, as the diver breathes less nitrogen, he is less susceptible to nitrogen narcosis than when breathing air at the same depth. Many EAN divers have confirmed this hypothesis, while others have seen no noticeable difference between air and EAN.

Another benefit experienced by many EAN divers is what can be called the “feel goods.” Quite often divers who use EAN report noticeable lack of post dive fatigue. In many cases, excessive post dive fatigue is attributable to what is termed “sub-clinical” DCS. The theory is that the higher percentage of oxygen in EAN reduces or eliminates these symptoms. The “feel goods” might also result from better oxygenation of the tissues by the enriched air-breathing environment.

A final benefit of EAN, oddly enough, is assumed to occur if a diver is stricken by decompression injury. Because the breathing gas contains a higher-than-normal level of oxygen, it’s theorized that tissues affected by these disorders will survive longer than if the diver was breathing air. Evidence also suggests that breathing EAN can help significantly reduce asymptomatic or silent bubbles after a dive.


EAN is not a magical answer to the physiological problems facing divers. For instance, EAN divers are still subject to the same effects of Boyle’s law – squeezes and lung overexpansion. And while the reduced percentage of nitrogen will increase no-decompression time, EAN divers are not immune to DCS. In addition, EAN creates a few unique problems of its own.

We have known since the late 18th century that humans cannot tolerate breathing pure oxygen at high pressure with eventually falling victim to a disorder called central nervous system (CNS) oxygen poisoning. The symptoms of this disorder include: tunnel vision, ringing in the ears, nausea, facial twitching, irritability, and dizziness. But the most serious effect of CNS oxygen poisoning is the onset of epileptic-type convulsions. Normally, convulsions are not a life-threatening event—if they occur on land. A diver who convulses underwater, however, could drown. Thus, divers must avoid any circumstance where convulsions might arise. (This is why people with seizure disorder are usually disqualified as candidates for diving.)

A complicating factor is that individuals vary greatly in their susceptibility to CNS oxygen poisoning. Even the same individual can vary in his own susceptibility from day to day. Based on years of experience and tens of thousands of dives, both the U.S. Navy and NOAA long ago determined specific oxygen tolerance limits. The modern EAN diving community has followed suit and also adopted these limits. Recently, though, some authorities have recommended a reduction in these limits for recreational divers.

The current recommended oxygen tolerance limits for diving are determined by calculating the partial pressures of oxygen breathed under pressure. These tolerance limits are established in order to prevent divers from encountering CNS oxygen poisoning at depth. You’ll remember that, according to Dalton’s law, each gas within a gas mixture exerts a pressure proportionate to the surrounding pressure. This explains why as a diver descends, the partial pressure of oxygen he is breathing increases. If the diver continues his descent, oxygen toxicity, due to increasing partial pressures, will occur. The depth at which CNS oxygen poisoning occurs is directly related to the amount of oxygen in the diver’s breathing gas. The more oxygen in the mix, the shallower the depth for the oxygen tolerance limit.

When using normal air, the oxygen toxicity limit has no impact on divers who restrict their diving to 130 feet or less. This is because the partial pressure of oxygen in normal air does not reach toxic partial pressures until depths of more than 210 feet. However, EAN mixtures, because of their increased oxygen content, reach this limit at much shallower depths. The chance of CNS oxygen poisoning, therefore, becomes a very real concern even at recreational diving depths. For example, the Maximum Operating Depths (MODS) for NOAA Nitrox I and NOAA Nitrox II are 130 and 110 feet respectively. Exceeding these depths exposes divers to the same risk of oxygen poisoning as breathing air beyond 210 feet!

Oxygen tolerance is the reason for one of the most important rules when using enriched air: “The diver must closely adhere to depth limits.” As you’re aware, the maximum depth for recreational diving is 130 feet. This is because of the possibility of severe nitrogen narcosis beyond that depth.

However, it’s the onset of CNS oxygen poisoning that determines the depth limit for EAN, not the effect f nitrogen. Unlike nitrogen narcosis, CNS oxygen poisoning does not always come about gradually. While the diver might experience minor symptoms before convulsions occur, convulsions often begin with no prior symptoms!

Dr. Lee Sommers, diving safety coordinator at the University of Michigan, sums up the matter quite well. He states in an article in the recent issue of NAUI (National Association of Underwater Instructors) journal Sources, “I suggest that, unlike nitrogen narcosis, which appears to manifest itself progressively from mild to severe impairment, oxygen toxicity can be a much greater threat to the diver. The simple fact that the onset of oxygen-induced convulsions with no preceding symptoms is possible adds another unpredictable dimension to (enriched air) diving. Oxygen may prove to be far less forging than nitrogen!”

The conclusion is both harsh and simple: While a diver breathing air might get away with exceeding the recreational diving limit f 130 feet, it’s unlikely he’ll live to tell about it if he exceeds the depth limits using EAN.


Some in the recreational diving industry vehemently opposed the proliferation of EAN diving. The reality is, however, that this is like trying to push water uphill. The question is no longer should recreational divers use nitrox; the fact is that they are using it. ANDI and IAND have, to date certified almost 300 instructors and more than 4,000 divers. Furthermore, authorities expect this number to double within the next year! These figures, of course, don’t account for those using EAN who have not been formally trained to do so. How many divers this involves is difficult to determine.

Rather than trying to resist the inevitable, it’s more useful to ask the question: Are the advantages of EAN worthwhile given the problems it presents? Frankly, this depends on the type of diving one does and how EAN is used.

For divers of less than 60 feet, there really is no particular advantage to using EAN over air—if increasing your no-decompression time is what you’re after. Although EAN will theoretically extend the no-decompression limits greatly, this has little practical effect. Unless a diver wears double tanks, he’d run out of air long before he reached the EAN no-decompression limits.

For dives below 130 feet, EAN provides no advantage to recreational divers. In fact, because of the increased partial pressure of oxygen, NOAA Nitrox I cannot be used safely below 130 feet. And NOAA Nitrox II can’t be used safely below 110 feet.

The advantage of using EAN to extend no-decompression time, as Table 1 shows, occurs on dives in the 60- to 130-foot range. EAN probably gives a significant enough advantage to consider its use in this depth range if you’re properly trained and closely adhere to EAN diving procedures. (See the “EAN Do’s and Don’ts “sidebar.)

Still, the primary benefit of using EAN is that it can probably enhance diving safety considerably when used in conjunction with air tables or computers. For this reason alone, enriched air deserves a close and thoughtful examination by all divers.


Be trained and certified for EAN diving: Never dive with enriched air if you haven’t completed a sanctioned course. The dangers of enriched air are both subtle and insidious. Also, certified EAN divers should never encourage friends who are not EAN-certified to use enriched air without proper training
Secure EAN from a reputable source, and never dive using a “home brew”: All divers must be certain of the quality of their breathing air. For EAN diving this means not only avoiding contaminates, but also verifying the mixture’s oxygen content. Ask whomever is providing your fill to give you a tour of the compressor/storage system, and ask them to explain the operating procedures and safeguards in place. Above all, never try to mix your own EAN.

Always personally analyze your gas before use: Only an analysis can confirm the actual percentage of oxygen in an EAN mixture. Never use a cylinder containing enriched air unless you analyze it first. And be sure to use at least two in-line analyzers. Multiple analyzers validate the results of one another.
Never exceed the Maximum Operating Dept (MOD) for the mixture you are using: The maximum depth for using EAN is not approximate or flexible. Remember, convulsions from CNS oxygen poisoning can come without warning. Know the Maximum Operation Depth for the mixture you’re using and don’t dive beyond that limit.
Use only dedicated oxygen clean and compatible cylinders: Use of non-dedicated cylinders results in a high risk of explosion, and could subject an unknowing diver to oxygen poisoning. Follow the proper labeling procedures.

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