Breathing exhaled air at depth

[Deefstes]

Hi all, something in another thread prompted this question. I'm not proposing breathing exhaled air at depth, I'm just curious as to the physics involved.

Let's assume for the purposes of this question that we're just breathing normal air, no Nitrox, Trimix or whatever fancies. This air contains 21% O₂ and from what I remember (from some first aid course in my distant past) our exhaled air can contain something like 16% O₂. This is still sufficient to sustain life (ppO₂ = 0.16) even if not for prolonged periods of time. Also, for the purposes of this question, let's keep the workload we're under constant as well. In other words, we're at rest, whether it be at the surface or at 30m depth.

Now I'm curious, what happens when we're breathing and exhaling this same air at higher ambient pressure? In other words, what would the composition of our bubbles be? I'm sure this would be a function of the depth among a host of other variables but I'd just like to understand the principles (if not the exact math and details).

Something tells me that our body needs a certain amount of O₂ which it will metabolise. By 'amount' I'm not referring to percentage but by physical amount (call it the number of molecules if you will). So when breathing air at depth, with every breath we inhale more O₂ just as we inhale more N and your body will use whatever number of molecules of O₂ it needs (same as on the surface) and exhale the remainder. So something tells me that our exhaled bubbles will have a higher percentage of O₂ than 16% (but obviously lower than 21%).

Is my thinking flawed? What does this mean for breathing air from an air pocket trapped in a wreck for instance? It almost seems as if that air pocket might be better able to sustain life than an exhaled breath on the surface (as in the case of CPR for instance). Sure, I would think that it is very unwise to be breathing air at depth of which you don't know the composition. One thing is for sure, the air pocket will have a higher ppCO₂ as well which doesn't sound like the air I'd want to breathe by choice but at least it sounds like:
1. The ppO₂ would be high enough to keep me alive.
2. Over time, the hypercapnia from the elevated ppCO₂ would probably give me nasty headaches and lead to whatever other symptoms of hypercapnia.
3. The ppN would, as far as I can reason, remain unchanged.

Your thoughts? Looking forward to hearing from someone smarter than me (of which there are plenty on SB)

 

[DA Aquamaster]

Breathing out of your BC used to be taught as an emergency procedure way back in the day. From what I recall, beeathing 21%, your exhaled gas contains about 16% O2, and 16% is right on the edge of what you need to stay conscious under any degree of excercise at the surface.

But also you produce CO2 at a rate of about 80% of the O2 you metabolize, and anything over 10% CO is probably going to incapacitate you in short order. So as a rebreather, a BC is extremely limited as it is mostly a race to see if you are incapacited by the CO2 before the O2 drops below the 10% you need to stay conscious even with no exertion.

Assuming you were to breath out of the BC on the way up, the PPO2 would probably be high enough to sustain you at depth, but if it drops below something between 10% and 16%, depending on your level of exertion, the odds of a shallow water blackout are good once the PPO2 falls as you near the surface. So in effect, you gain the advantage of using some of the O2 in the bag and would have more time than on a single breath, but at the expense of rapid CO2 buildup.

Breathing out of an air packet in a wreck or cave is not a good idea. Just because it is a gas does not mean it is air, and even if it used to be air, it may not have much O2 left in it idf the free O2 whar been used by other organisms and you have no way of telling what metabolic by products are in it. A lungful of gas high in hydrogen sulfide will make a BC full of CO2 look wonderful.

 

[HowardE]

Approximately 4% of the Oxygen in each breath is metabolized and converted into CO2. At depth, this percentage of oxygen is only a small fraction of the total gas, as the partial pressure is higher. The minimum amount of Oxygen partial pressure needed to stay conscious is approximately .16 PO2 or 16% o2 at the surface.

So - as DAAquamaster states... Breathing an unknown gas is not a good idea all around. Breathing trapped air in a cave or a wreck may not be air. It may just be an unknown gas bubble (what if it's methane?), and without knowing what the bubble is.

2. Over time, the hypercapnia from the elevated ppCO2 would probably give me nasty headaches and lead to whatever other symptoms of hypercapnia.

This would happen rather quickly. Not over time, but in a matter of minutes, most likely less than 5 minutes. People worry about CO2 in rebreathers and having detection devices to detect this "silent killer" - in reality, a hypercapnic headache would set in very quickly, and would be much nastier than the slight headaches that people on open circuit get from the fabled "CO2 retention" - it's most likely that your would feel the headache from CO2 before the detector detected it?

The bottom line is: Breathing exhaled breath isn't a good idea. Rebreathers use a scrubbing chemical to remove the CO2 from EVERY exhaled breath. The CO2 will probably kill you before the lack of O2 will.

 

[TSandM]

Deefstes, it's a good question, and your analysis is correct. So long as you remain at depth, your exhaled air will contain more O2 molecules than you require for survival. The CO2, however, will build up very quickly (minutes, as stated previously) and create anxiety, shortness of breath, and severe narcosis probably before it gives you a headache -- one of the first symptoms of CO2 retention is panic.

Air pockets are dangerous for the stated reasons. They are sometimes not air at all, or contaminated with other gases which are very toxic (methane, H2S, etc.). If I were faced with a choice between certain drowning and breathing the gas in a pocket, I'd breathe it -- but not before.

 

[DocVikingo]

Breathing out of your BC used to be taught as an emergency procedure way back in the day.

Emergency Breathing from Your BC
http://www.undercurrent.org/UCnow/issues/y1999/UC0999/BCbreathe9909.pdf

Regards,

DocVikingo

 

[Deefstes]

Thanks for all the informative responses. The principle seems to make sense but it's a moot point anyways as there are more significant reasons why you should not breath air from a pocket.

Thanks for the link DocVikingo, it was a very interesting article but I can understand why agencies wouldn't teach this skill. It sounds like a very complex skill if you ask me and the recipe for two OOA divers breathing of the two air supplies of a dead diver. I just think that, if two divers need to approach me for air at the same time:
a. there is something seriously wrong with the way we dive and no amount of skills training is going to allay those problems.
b. they can fight it out for my octo but I'll have my primary thank you very much. Surely they will know how to buddy breathe, so let them go for it. I'm certainly not going to practice this skill just so that two idiot divers who can't manage their gas and can't buddy breathe can have both my air supplies leaving me to breathe off my BC.

Interesting article nonetheless.

 

[MauiScubaSteve]

There is an artificial reef / wreck dive here off Maui called the St. Anthony that has an air pocket just large enough for three calm divers to get their mouths into. This air pocket is approximately 66 fsw, so 3 atm. The odds that the air is anything other than exhaled recreational dive gas are very remote. I always purge my alternate for a few seconds before surfacing into the space, and I don't spend very long there, but I seriously doubt anyone will ever have a problem breathing that gas for a quick look see.

 

[Deefstes]

There is an artificial reef / wreck dive here off Maui called the St. Anthony that has an air pocket just large enough for three calm divers to get their mouths into. This air pocket is approximately 66 fsw, so 3 atm. The odds that the air is anything other than exhaled recreational dive gas are very remote. I always purge my alternate for a few seconds before surfacing into the space, and I don't spend very long there, but I seriously doubt anyone will ever have a problem breathing that gas for a quick look see.

That's the type of scenario I had originally thought of when I asked the question. As you say you would probably be safe to breath from that gas but after this discussion and thinking about it I think I'd rather not - even if there's a 99.9% chance that the air is nothing but exhaled rec gas which means there's enough O₂, I'm not sure I like the idea of taking on such elevated levels of CO₂ - even if just a breath or two.

 

[DA Aquamaster]

Emergency Breathing from Your BC
http://www.undercurrent.org/UCnow/issues/y1999/UC0999/BCbreathe9909.pdf

Regards,

DocVikingo

One thing the article alludes to but does not specifically state is that nearly all regulators will continue to provide at least some air flow through the low pressure inflator after the second stage completely stops delivering air.

For example, a high performance reg like a Mk 25 or mK 17 will supply enough flow through the LP inflator to inflate a 40 pound wing in maybe 30-45 seconds after the second stage has completely stopped delivering air. A lower performance unbalanced reg may offer near normal flow through the LP inflator, even after the second stage stops delivering air.