| Be sure to weigh risks and benefits before diving with the gas in the green-and-yellow tanks. Photograph by Steve Simonsen.|

By John Francis

Nitrox has been promoted as an almost miraculous improvement in recreational diving. Among the claims you'll hear for the gas: safer diving and longer dives, a tremendous margin of safety, reduced gas consumption, and more energy and stamina. In actuality, nitrox (that artificial mix with generally less nitrogen and more oxygen than air) is a special-purpose breathing gas. It was developed by the National Oceanic and Atmospheric Administration (NOAA) to save time and money on scientific diving projects. It's good for some divers and some diving situations, but it's not for everybody all the time. Tech divers carry it specifically for long decompression and switch to it at their stops, because it can substantially reduce the time they have to spend hanging in the water column. But if you believe all the claims, all of us should be using nitrox on all our single-tank, no-stop dives. In fact, for everyday recreational diving, many of the benefits claimed for nitrox are insignificant or misunderstood. Let's get them out of the way now:

What Nitrox Won't Do

Claim #1
Nitrox reduces narcosis. This sounds reasonable. If nitrogen causes narcosis and you're breathing less of it, you should have less narcosis. The trouble is, the depth range where you start worrying about narcosis (100 to 130 feet) is also where you have to stop using nitrox because of the risk of oxygen toxicity. Dr. Peter Bennett, who co-edited The Physiology and Medicine of Diving and wrote the chapter on inert gas narcosis, says the preventive value of nitrox is "very small, so marginal that I think it should be discounted."

Claim #2
Less gas consumption. This sounds reasonable too. If there's more oxygen in each breath, presumably you don't have to take another one so soon and your gas consumption rate is lower. Unfortunately, it doesn't work that way because most of the oxygen you breathe is exhaled unused anyway. What drives the next breath is the need to exhale carbon dioxide, and that's not affected by what's in your tank.

Claim #3
Less fatigue. "Those long and tiring drives returning from a day of diving are over!" enthuses one dive shop's web site on the benefits of diving on nitrox. Is it really the "feel good" gas? The theory here is that the work of offgassing nitrogen is a major cause of diving fatigue, so less of it should leave you less tired. Many nitrox divers swear it's true, but Bennett cites a blinded study that proved otherwise. Using unmarked tanks, one group of divers was given nitrox, another was given air, and both were asked later how they felt. "There was no difference," says Bennett. "It's a placebo effect."

Claim #4
Deeper dives. Some divers think those green-and-yellow cylinders look serious and "techie," and associate that with going deep. In fact, nitrox introduces a new depth floor that's often shallower than the 130 feet we're used to. PADI's recommended depth limits are 110 feet for 32 percent nitrox and 95 feet for 36 percent nitrox. The penalty for going below the floor is serious, too. Many of us would risk chasing that eagle ray down to 150 feet on air, but wouldn't think of it on nitrox.

What Nitrox Will Do

Breathing nitrox can reduce your decompression stress. You absorb less nitrogen than you would breathing air for the same time at the same depth. Less nitrogen absorbed means less you have to offgas, therefore less risk of decompression sickness (DCS).

That's the "tremendous margin of safety" with nitrox that you hear about so often. Trouble is, it's not so tremendous. For one thing, if a diver breathing air follows the rules his risk of DCS is already so low there's not much room for improvement by switching to nitrox. Since Divers Alert Network (DAN) began collecting records, the risk of DCS has averaged two or three cases in 10,000 dives, whether on air or nitrox, and those few are mostly the screwups. DCS and embolism are caused by running out of breathing gas, losing control of your buoyancy, ascending too fast, holding your breath, and blowing off the safety stop-diver error, and the breathing gas is nearly irrelevant to that. If you're one of those who likes a belt with suspenders for a "tremendous margin of safety," you're going to love diving nitrox to air limits.

Many of us, however, will dive nitrox to nitrox limits. And that brings up the other problem with the "tremendous margin of safety" claim. Take two divers, Nitrox Ned and Air Allen, breathing their respective gases. If Ned stays down longer than Allen until he reaches his nitrox no-deco limit, he now has the same nitrogen load as Allen does at his air limit, and the same risk of DCS. The DCS risk is lower with nitrox than with air only if you leave more no-deco time on the table. Nitrox offers more safety from DCS or more no-deco time or some of both. But if you max out one benefit, you get none of the other.

"Safe Air"?

Before leaving the safety question, we have to consider the downside of nitrox-the chance that the higher concentration of oxygen could cause a convulsion and actually kill you. To be fair, there's not a big chance of an oxygen toxicity convulsion. But the chance exists, and if a convulsion happens, you will probably lose consciousness, drop your reg and drown.

Oxygen is a highly reactive chemical. It's eager to bond with a wide range of other chemicals, and the result is all around us, from rust to fire to explosion. Oxygen is highly active in your body too, for both good and ill. At normal atmospheric concentration and pressure, our bodies have evolved complex mechanisms for using oxygen's good effects and protecting from its toxic ones. Increase both concentration and pressure, as we do when we dive with nitrox, and those mechanisms may not work well.

One result of diving with nitrox is that oxygen can eventually attack your lungs. That's called pulmonary oxygen toxicity. Fortunately, it happens only after long exposures, like many hours in a recompression chamber. Recreational divers using single tanks really don't have to worry about it. They do have to worry about central nervous system poisoning, CNS ox-tox. This one is the killer because brain tissue is unusually sensitive to oxygen poisoning and can succumb to it within the dive time a single tank gives you. Besides a grand-mal seizure, symptoms include dizziness, spasms, tingling, twitching, ear ringing, irritability, anxiety, euphoria and unconsciousness-and they can come in any order.

Ox-tox occurs when you go deep enough to increase the partial pressure of oxygen beyond a threshold. Unfortunately, no one is sure exactly where that threshold is. Statistics suggest it's somewhere around 1.3 to 1.6 atmospheres. NOAA and most training agencies suggest a limit of 1.6 atmospheres, which corresponds to maximum depths of 130 and 110 feet for the popular nitrox mixes (32 percent and 36 percent). PADI is more conservative, recommending 1.4 atmospheres and maximum depths for the same mixes of 110 and 95 feet.

What's your safe partial pressure limit? Nobody really knows. People vary considerably in their susceptibility to CNS ox-tox, and the same person's risk can vary from day to day. Excess carbon dioxide, decongestants like Sudafed, aspirin, caffeine and maybe even Viagra can make you more vulnerable.

Nevertheless, the safety record of recreational nitrox is actually good, perhaps because a single tank of it gives you so little bottom time at depths where you approach the ox-tox threshold-whatever it is.

The risk of ox-tox is low in quantity but high in quality-like Russian roulette. We face situations like that all the time, where the consequence would be catastrophic, but the chance of it seems small and the benefit worthwhile. Most of us fly in airplanes without the slightest idea why they stay up in the air, and drive at 70 mph only a few feet away from total strangers. You can't say nitrox is safer than air because it doesn't necessarily reduce your total risk. You do have to say it trades a minuscule and known risk of bends, which is treatable, for a minuscule but unknown risk of oxygen toxicity, convulsion and drowning, which is not.

What Is Nitrox Good For?

The real value of nitrox is that it gives you more minutes before you have to ascend to avoid decompression stops, often a lot more. Compare no-decompression limits: at 100 feet, tables issued by NOAA give 25 minutes on air, 30 minutes on 32 percent nitrox and 40 minutes on 36 percent nitrox. In some cases, nitrox can double your permitted no-deco time, and the difference can be even more dramatic on repetitive dives.

But twice the no-deco time is not the same as twice the dive time. Unless you use very large capacity tanks or doubles, you sometimes won't be able to use all the extra no-deco time that nitrox gives you. Besides, how often do you end your dive when you run out of no-deco time? Do you go directly to the surface even though you have gas left? Or, like most of us, do you ascend until your computer gives you more minutes, use those, then ascend to get some more, riding your no-deco limits upward until you've used your gas supply? Because most recreational dives are multilevel, even those on air can be limited by tank capacity or cold or boredom, not by no-decompression time.

You can trade that extra no-deco time for a shorter surface interval or a deeper second dive. Or you can spend more of your tank in the deeper part of your depth range. That may be important where there's nothing worth seeing between the wreck and the surface, for example. But along a wall or over a sloping reef, the nitrox advantage is not more dive time, it's more flexibility in where you spend it.

Benefit vs. Risk

How do you balance the benefit of nitrox against the risk? Nitrox offers the most benefit if you want to maximize your number of dives within a limited depth range. Say, four or five dives a day in the 50- to 80-foot range. Above that range, no-deco limits on air are already generous. Below it, the benefit decreases and the risk of oxygen toxicity enters the equation.

How the benefit and risk pencil out depends a lot on how you dive. If most of your diving is fairly shallow, if the water is cold and dark, maybe you don't use all the no-deco time air already gives you. If you want the option to go below 100 feet and your gauge discipline is not faultless, maybe you don't want the ox-tox risk. On the other hand, if you're a big customer of live-aboards or a tropical dive guide humping tanks between dives or a research diver-or anyone who pushes the no-deco limits pretty hard-maybe nitrox makes sense. Just make sure you understand the risks as well as the benefits and stick to the depth limits religiously.