Compressed air with higher proportions of oxygen (aka NITROX, EAN) is commonly used in recreational and repetitive diving, due to reduced bodily absorption of nitrogen, which bubbles upon ascent (DCS & DCI risk). N2 is also strongly narcotic at super-atmospheric pressures.
Nitrox use is so common now that we seldom think further.
Questions do arise, such as:
- 1. is it good to breathe a higher proportion of oxygen?
- 2. why is nitrogen bad?
- 3. what else is possible?
Quick answers are easy for an experienced diver. What follows is a bit deeper dive.
Gas molecules
Absorption of pressurized gas molecules into the body has two effects:
- excess absorbed gases need to come back out,
- these gas molecules have effects while inside of us.
All gas molecules have varying properties. These properties affect our bodies in different ways. The ‘air’ compressed into our diving tanks is composed of several different gases:
Q1: Is it good to breathe a higher proportion of oxygen?
The amount of gas molecules that enter and leave our bodies is a function of ambient pressure. Ambient pressure at sea level is ‘one atmosphere,’ (1 atm) containing about ~2.7x1022 total gas molecules per litre of air.
We breathe in 0.21atm worth of oxygen molecules (partial pressure aka ppO2), because the proportion of oxygen molecules in air at sea level is 0.21 (x 1atm). (Using ‘percent’ here is not necessary.) Our lung alveoli do a fantastic job of exposing our blood hemoglobin to these O2 molecules for delivery to cells. Normal ‘blood oxygen saturation’ is already at 95+% with normal air.
Some can survive on as little as 0.07atm of oxygen (Mt. Everest: 0.31atm x 0.21 O2/total) for short periods, after serious acclimatization (though many do not). The typical criterion for a ‘hypoxic gas’ is anything with less than ~0.16atm of O2.
While diving, it is recommended to keep below 1.4 to 1.6atm of O2. This is a 32% nitrox mix at ~33 metres depth [(1.4atm/0.32atm-1atm)x10m/atm]. So there is the maximum operating depth (MOD).
Breathing pure oxygen ‘on land’ is totally fine: 1.0atm of oxygen. That is a ~5 times higher density of O2 molecules than normal air. Our bodies don’t seem to mind. This is helpful for anyone experiencing difficulty breathing.
Most can breathe up to ~2atm of oxygen without ill effects, provided that the exposure is limited to an hour or so. This is standard protocol for a clinical re/de-compression chamber.
Furthermore, oxygen seems to play nicely under pressure with respect to bubble formation–it doesn’t seem to do this much, and that which does might still be metabolized over time. So very little DCS/DCI risk is attributed to oxygen absorption.
A1: No, it is not necessarily ‘bad’ to breathe a higher proportion of oxygen. (Simple answer.)
[Yes, there is a possibility of fatal underwater convulsions if you exceed the recommended limit of 1.4 to 1.6atm O2. So don’t do that. And yes, ’tec’, commercial, or military divers breathing 1.2+atm of O2 for dozens of hours per week can experience pulmonary toxicity.]
But…
Are we wasting oxygen?
Yes. We are wasting gross amounts of oxygen when diving with nitrox.
Our bodies only metabolize about 1 surface-litres worth of oxygen per minute, regardless of depth or what gas mix we are breathing. That is only ~0.05atm of the oxygen present in normal air (of the 0.21 available). So even diving with normal air at 10 metres depth, with a ppO2 of 0.42, the vast majority of oxygen present in inhaled compressed air is not utilized, and simply exhaled back out.
With a 32% nitrox at 20 metres depth, only a tiny fraction of the ~1 atmosphere (0.32atm x 3atm/1atm) of ppO2 is being utilized. The rest of that expensively concentrated or blended enriched oxygen is really only doing one job: crowding out nitrogen molecules. All that extra oxygen is just standing in for an ‘inert gas:’ molecules that take up space in the tank, entering/leaving the body without doing anything.
Nitrogen is ‘inert’ in that it doesn’t chemically react in our bodies, but it definitely does something: narcosis.
Q2: Why is nitrogen bad?
How ‘inert?’
So we know nitrogen is ‘bad’ because it is readily absorbed into our bodies under pressure, and has to come back out, sometimes causing DCS & DCI (the bends) if not done carefully.
It turns out that nitrogen molecules also readily dissolve in our fat tissues and cell membranes, causing interference with neuronal transmission. Alcohol has a related effect.
Without exception, divers will feel more inebriated with increased nitrogen pressures. It begins as a nearly unnoticeable sensation of calmness (or uneasiness, for some), progressing thru a ‘warm blanket’ phase, into a nearly concussed state from which limited awareness or memory escapes. “Martini’s Law” says that every 10 metres of depth is like the effect of one drink. Cheers 🥂
For these reasons, the maximum recommended partial pressure of nitrogen (ppN2) while diving is ~3.16atm, which equates to compressed air at 30 metres (0.79atm * 4atm/1atm). Some suggest a more conservative 2.37atm, an ’equivalent air depth’ of 20 metres (0.79atm * 3atm/1atm), for dives involving anything challenging, cold water, currents, complex tasks, etc.
A2: nitrogen is kinda bad (unless you love martinis & chambers)
What about ‘oxygen narcosis’?
The notion that oxygen is narcotic at high ppO2’s is met with conflicting evidence. People breathing high purity oxygen (certain medical, aviator, analytical grades) can experience no narcosis, even at high ppO2’s up to 1.6 while diving (such as with high-oxygen decompression gases, or rebreathers). The Subsurface dive planner includes an option to include or ignore the notion that oxygen itself is narcotic.
Unfortunately, several other sources of enriched oxygen contain elevated levels of trace narcotic gases, including carbon dioxide and argon. Argon is more than twice as narcotic as nitrogen, and can be concentrated along with oxygen in several types of membrane and pressure-swing nitrox generators. This might explain why agencies such as PADI consider typical nitrox blends to be ’equally narcotic’ to their compressed air counterparts.
As someone who has spent hundreds of hours on rebreathers down to 100+ metres numerous times at a ppO2 up to 1.6atm using high-quality pure oxygen sources, I can say with confidence that the narcosis effect of oxygen itself is not a concern. Granted, in most of these cases the gas mix included plenty of helium as well, whose narcotic factor is essentially zero. This segues nicely into…
Q3: What else is possible?
So oxygen-enriched air nitrox is just using excess oxygen as an ‘inert gas,’ whose not-so-inert properties are nevertheless more desirable than nitrogen’s not-so-inert ones. What else could we use?
Helium, of course! The narcotic effects of nitrogen can be completely avoided by replacing all of the nitrogen with helium, rather than excess oxygen. This also removes the risk of convulsions due to oxygen CNS toxicity at elevated ppO2’s. Helium is almost a fully inert gas, except for high pressure nervous syndrome (HPNS) that can occur at extremely high ppHe’s (>10atm?).
There are three main caveats to using helium in breathing gases:
- Helium is expensive, scarce, and non-renewable
- Helium’s thermal properties make you colder
- The diffusion rate of helium is faster
Helium is a non-renewable gas, largely extracted during oil & gas mining for fuels. Its price and availability in forms pure enough for breathing applications fluctuates.
| Gas | tank fill | contains(approx L) | price/L | vs. ‘air’ |
|---|---|---|---|---|
| air | $10 | 2500 (shop fill 80-100cf) | 0.004 | - |
| EAN32 | $20 | 2500 (shop fill 80-100cf) | 0.008 | 2x |
| O2 | $50 | 3000 (a ‘125’ O2 bank) | 0.016 | 4x |
| 21/35 | $100 | 2500 | 0.04 | 10x |
| 21/79 | $200 | 2500 | 0.08 | 20x |
| 10/70 | $175 | 2500 | 0.07 | 17x |
Even worse than this expense is the wastage of precious, non-renewable gas: with every breath, the overwhelming majority of gas is simply expelled out into the atmosphere, at greater and greater rates as you dive deeper. Two or three full tanks of trimix or heliox can be expended in a single deep dive. Only a tiny minute fraction of these gases ever enter the body.
If you plan to dive with gases containing helium, it becomes quickly clear that using a rebreather is the way to do it.
The faster diffusion rates and coefficients of helium have led some to believe that this is a more dangerous ‘inert’ gas than either nitrogen or oxygen. This probably means that ascents should be done more carefully, since the faster-diffusing gas might more easily form dangerous bubbles if the body is decompressed too quickly. It also might mean that helium might saturate less-perfused parts of the body more quickly. However, one should expect a symmetry to this process: a gas which enters the body quickly should also leave quickly.
A ‘helium penalty’ occurs in current decompression algorithms due to how we consider the potentially additive effects of having both nitrogen and helium dissolved in our bodies at once, as well as a generally more conservative approach to decompression when helium is involved. This typically associates with much deeper diving however, where risks and complications of decompression are more likely.
The benefits and implications of using trimix and heliox mixes for shallower recreational and ’tec-rec’ diving, especially with rebreathers, are encouraging, praised and recommended by most modern ’tec-rec’ diving agencies. TDI for example now recommends trimix for rebreather dives below 40 metres, and I heard that GUE recommends it deeper than 30 metres, under certain conditions.
Conclusion
Oxygen seems to serve just fine as an ‘inert gas’ substitute for recreational diving down to ~30 metres. The benefit of reduced nitrogen absorption really pays off in reducing the risks of repetitive diving in the 20 to 30 metre range, and can ameliorate the ’nitrogen hangover’ effect once can otherwise experience.
These benefits are lost however if you are using nitrox to simply extend your dive times out to longer NDL limits, as most people in fact do on multi-day live-a-shore and live-a-board trips. Be aware that you are still on- and off-gassing significant amounts of nitrogen.
Give trimix and rebreathers a look if you plan to spend a lot of time below ~25 metres!