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Soyez un bon plongeur : prise en compte de l'oxygène

Hovering over a reef, camera in hand - You see that Juvenile Spotted Drumfish that you've been chasing for the last half hour. Finally, he has come out into the open and is parked in front of this spectacular purple sponge and bright orange anemone. Your frame is set; lighting is good. Your underwater model is cooperating. "BEEP BEEP BEEP", your computer is telling you that your entering a danger zone and at risk for oxygen toxicity. It says your "PO2 is 1.4". Not critical, but a good alert nonetheless. Glad it went off. Fortunately, the alert didn't scare away your Drumfish. You posture to take the picture. SNAP! Your strobes light up the reef, and your Drumfish heads back into the protective crevices of the Boulder Coral below. As you peer into the camera's view-screen, you think, this one is going on the wall on the office. A great shot! It's time to head back up to a safer depth and make your computer happy.

The air we breathe, both while walking on land or normally compressed into a Scuba tank, is comprised of two major gases and a small collection of trace gases. The most prevalent gas in our breathable air is Nitrogen which equals 79%. Oxygen, which is the gas that our life depends on, is a mere 20.9% of the gas we breathe. The .1% left over is made up of a bunch of trace gases, many of which, like Nitrogen, are inert and have little effect on us, normally, while walking on land. While this blend is harmonious and life-enabling, it all changes once you add pressure while Scuba diving.

Nitrogen - the evil gas!
From the first day you strap a Scuba tank to your back, you hear about Nitrogen Narcosis and Decompression sickness. In some cases, instructors scare you into memorizing the causes to reinforce the notion of managing your NITROGEN. Simply put, Nitrogen Narcosis is caused by the combination of depth and nitrogen absorption. When you reach a depth, usually below 100 feet, if your physiology isn't cooperating, you may get "Narced". An intoxicating feeling that may impair your judgment. The deeper you go, the more susceptible you become to Narcosis. At some depth, everyone gets Narced. Decompression Sickness is more severe and requires proper planning in order to avoid getting the "Bends". Decompression Sickness is not caused by depth, it is caused by loading Nitrogen into your body. The deeper you go, the faster you will load Nitrogen. At some point, you will load enough Nitrogen, where you will need to control the off-gassing prior to surfacing. These we call Decompression Stops. If you load enough Nitrogen and forego a Decompression Stop, it is highly likely you will get Bent. Even a mild case of the bends is painful and perhaps even life threatening. A severe case can, at best, cause permanent neurological damage and highly possible you could die from it.

Thus, from your first day, you are taught by your instructor to manage your nitrogen levels using a dive computer or a by using dive planning tables for "No Decompression Limits". In either case, you are planning your dive so that you are not needing Decompression Stops. Nitrogen management means you have planned your dive to not max out your Nitrogen loading. Additionally, you are taught the symptoms and cures, for you and your buddy, on how to combat, avoid and relieve Nitrogen Narcosis. By the end of your beginner Scuba Diver class, you are proficient in Nitrogen Management, staying away from getting Narced and planning your dives to stay away from even the mildest case of the Bends.

Oxygen - the good gas?
If Nitrogen is the bad gas and Oxygen is the life giving gas, then Oxygen is the good gas, right? Well, Oxygen is generally the good gas. You definitely need it to live. However, Oxygen epitomizes the saying, "too much of a good thing can be bad for you". In fact, there are times when too much Oxygen can be as deadly and hit your faster than Nitrogen. Oxygen is the good gas with some really bad character traits. However, we as a new Scuba diver, we never hear about these bad traits. In fact, until we start looking into Enriched Air or Technical Diving, we never even hear that Oxygen could be a bad thing. Oxygen has toxic characteristics when it has a high partial pressure. This is called CNS Oxygen Toxicity. Oxygen can also have toxic characteristics is you have prolonged exposure to high partial pressures. This is called Pulmonary Oxygen Toxicity. In either case, the effect is generally the same, getting a "Tox Hit" usually means that you will have a convulsion. In fact, in hospital wards, this happens fairly frequently. Most people have no problem surviving them. However, underwater a convulsion can be deadly. A convulsion generally leads to a diver's regulator falling out of their mouth, passing out and drowning.

Oxygen Toxicity and the New Diver
At this point, you have to be asking, why didn't I hear about the adverse effects of Oxygen in my Open Water or Beginner Scuba Diver certification class? Good question; and some agencies are changing that and making it a part of the beginner lesson plan. However, some agencies are not teaching it because they feel the depth at which Oxygen Toxicity, when diving on air, becomes an issue is way too deep for a new diver. And, if a new diver is that deep, they have more to be concerned about that Oxygen Toxicity. A fair argument since most new divers are using air (21% oxygen / 79% nitrogen) and the toxic depths of air is generally below 200 feet. However, this theory is flawed when dealing with the buddy system or diving off a boat with divers who may be using enriched air or Nitrox blends in excess of 21% oxygen. In this scenario, even with the non-nitrox diving buddy needs to be aware of the risks surrounding him in the water. A diver using a common blend of 32% Nitrox (32% Oxygen / 68% Nitrogen), can have a Tox Hit within the recreational limits of 130 feet. Now Oxygen Management is a concern for everyone diving with that buddy.

Enriched Air Versus Nitrox
We've been throwing a lot of terms at you. Nitrox, Air, Enriched Air, etc. Air is simply what you are breathing while surfing the Web reading this article. It is comprised, simply put, of 21% Oxygen and 79% Nitrogen. Nitrox is a "slang term" for any Nitrogen/Oxygen blend. This means, technically, air is a Nitrox blend. However, Nitrox has become a common replacement for Enriched Air. Enriched Air is any blend of Nitrogen and Oxygen exceeding 21% Oxygen in the mix. That means a Nitrox blend of 23% Oxygen and 77% Nitrogen is Nitrox.

Hypoxia and Hyperoxia and how it relates to Scuba Diving
The human body needs a specific amount of oxygen to survive. It can't be too little and it can't be too much. Hypoxia is a condition in which tissues are deprived of an adequate supply of Oxygen. On land, this usually occurs, in folks with normal physiology, when the Oxygen content in a breathing mix drops below 16%. People are considered Hypoxic when this happens.

Hyperoxia is a condition caused by an excess of oxygen in tissues and organs. On land, this usually occurs, in people with normal physiology when the oxygen content in a breathing mix is above 160% or have been breathing high O2 mixes (like pure O2) for an extended period of time. It is near impossible for someone to become Hyperoxic on land. Hyperoxia requires pressure to get the breathing mix to a comparable 160%. Of course, the Scuba environment is the perfect place to experience pressure.

On your first pool session, you felt the effects of pressure on your body from water. Probably in your ears. This pressure has a physiological effect that you don't necessarily feel. It is compounding the effects of Oxygen. In fact, at 33 feet (salt water), the effects of oxygen is doubled. As mentioned above, air is 21% Oxygen. However, physiologically at 33 feet, it has the same effect on your body as 42% oxygen. This effect is compounded the deep your go. At 99 feet, Air (21%) is four times more effective and comparable to 84% Oxygen (almost pure O2). Now you can see, with depth, how Hyperoxia can occur while Scuba diving. For "air" divers, Hyperoxia is less of a concern compared to Narcosis or Decompression Sickness. However, for "enriched air" divers, their high O2 blends can create high partial pressures which can easily lead to hyperoxia. Imagine someone with an Enriched Air Blend with 40% Oxygen and 60% Nitrogen. At 100 feet (salt water), that O2 content is equivalent to 160%. This diver is well within recreational limits and breathing a recreational "Nitrox" blend. You, as his buddy, may, not understanding the dangers, lead him to depths below 100feet. Now you understand the importance of Oxygen management.

What is all this Partial Pressure Stuff?
Partial Pressure is a term used in gas laws. Without getting into a physics lesson: in a gas mixture, each gas has a partial pressure which is equal to the fraction of gas represented in the mix multiplied by the pressure. Thus, in the examples above, Air has a 21% fraction of Oxygen. At sea level, or one atmosphere (as the air pressure is measured), it has a partial pressure of .21. Sometimes you will see this written as PPO2 = .21. In salt water, every 33 feet is equivalent to weight of an additional 1 atmosphere (34 feet for fresh water and you have to add 1 atmosphere for the air on top of the water). So at 33 feet, you have the equivalent weight of 2 atmospheres of pressure. 66 feet, is the equivalent of 3 atmospheres, 99 feet is the equivalent of 4 atmospheres and 132 feet is the equivalent of 5 atmospheres. Thus, in our example above, using air, at 33 feet, you would multiply the fractional gas of 21% times 2 atmospheres which would equal .42 (or 42%). That means, air is twice as effective at 2 atmospheres or 2 ATA (atmospheres absolute)

Using the 40% mix example above. Taking 40 percent to 3 ATA or 66 feet is equivalent to have 120% Oxygen or a partial pressure of 1.2. Another example is taking the most common blend of Enriched Air Available, 32% Nitrox and taking that to depth. At 33 feet, Enriched Air with 32% (usually written EAN32) has a partial pressure .64. Taking EAN32 to 4 ATA or 99 feet, means that it has a partial pressure of 1.28. Or if you take that same 32% blend to 5 atmospheres or 132 feet, you will have a partial pressure of 1.6 - which is equivalent to 160% Oxygen. As mentioned, previously you risk the chance of CNS Oxygen Toxicity when it reaches that level.

Optimal Partial Pressures While Scuba Diving
When Scuba diving, you always want to keep your Oxygen partial pressures between .16 (16%) and 1.6 (160%). If you fall below or go above this range you risk serious injury and death. Even on air, foolish people can exceed a partial pressure of 1.6. That is why we recommend an additional safety margin and tell our divers not to exceed a partial pressure blend 1.4. If you are planning a dive that will result in a 1.5 or 1.6 partial pressure mix, drop the O2 mix down a percentage point or two. As always, plan your dive (conservatively and within your limits) and dive your plan.

Because we are constantly dealing with pressure underwater, it is very possible that you could exceed a 1.6 partial pressure. However, it is very difficult to go below .16. While not exclusively, this usually occurs only in Trimix Diving. Oxygen management in Trimix Diving is a more complex discussion, although the fundamentals you learn in Nitrox Diving remain the same.

Bottom line, don't exceed a 1.4 partial pressure and stay safe!

Is that a freight train or barking dogs I hear?
As mentioned, the effect of Hyperoxia while diving is generally a convulsion. And while this can come on without warning, it has been reported that several precursors may reveal themselves prior to the convulsion. If you experience one of these precursors, you should end your dive immediately. Unlike Nitrogen Narcosis, going up a few feet will not dispel the oncoming convulsion. You are toxic! These precursor events to a Tox Hit include Visual Disturbances (like tunnel vision), Ear Ringing or strange sounds like a barking dog, Nausea, Muscle Twitching (usually in the face), Irritability, Dizziness. Note: a convulsion does not necessarily have to have a precursor symptom.

If at any time you do feel 100% fine, regardless of your Partial Pressure, you should surface - safely - without hesitation. No dive is worth being your last.

# 1 Rule in Enriched Air Diving and Oxygen Management
Obviously, the key to Oxygen Management is partial pressure awareness. Knowing your depth and knowing your gas blend are the critical components to partial pressure awareness. There is only one way to truly "know" your gas blend and that is to analyze it. If you are a Nitrox Diver, you should have an analyzer, be trained to use it and do so. Don't trust anyone on telling you what is the blend. Analyze it yourself. Thus, the number 1 rule in Enriched Air Diving is to always (and I mean always) analyze your own mix. Once you have that, you can calculate your partial pressures and know exactly what depth equals a 1.4 partial pressure with your blend. And, of course, you know your maximum operating depth at 1.6. Once you have done this, using masking or duct tape, write your name, mix and maximum operating depth on the tape and stick on your tank.

Final and Most Important Thoughts
To truly understand Enriched Air or Nitrox Diving, you need training! This article is only a quick overview on Oxygen Management and a quick introduction to Enriched Air Diving. Do not attempt, in any way, to use this article or any of the construed advice as training. Do not dive Enriched Air unless you are certified and qualified to do so. That means a certified Enriched Air Scuba Instructor has signed off that you are trained to dive Enriched Air. Scuba diving is a great sport, but dangerous if you do not dive within your training limits.