Quiz - Physics - Partial Pressure of Carbon Monoxide

[GJC]

Whilst the partial pressure of CO at 40m depth is definitely 5 times larger than the p.p. at surface, I am absolutely not sure that the effect of CO is proportional to its partial pressure.
Most of us have knowledge about the toxicity of Oxygen, and for this is widely accepted that the toxicity is triggered by its partial pressure. So we evaluate the maximum allowed depth of a gas mixture by ensuring that ppO2 stays below a given threshold, say 1.5 bar.
But the mechanism for the toxicity of CO is very different than the toxicity of O2. CO is toxic because it creates a permanent chemical bond with haemoglobin, so it inactivates it for transporting oxygen in the blood stream. This toxicity is actually at least partially counterfeit by increasing the p.p. of Oxygen (in fact the typical treatment for CO poisoning is hyperbaric oxygen).
This means that, as you dive deep, the larger quantity of oxygen available makes CO LESS TOXIC than at the surface. Of course there is also more CO, but as soon as its quantity is enough for entirely inactivating haemoglobin, the more CO present has no effect on the human body, as all the possible damage is already done.
Instead, the larger availability of O2 dissolved in the blood (instead of being bonded with haemoglobin) can provide oxygen to your brains even if all the haemoglobin has been inactivated.
Considering that the starting point was an already terribly high percentage of CO, my supposition is that breathing such an highly toxic mixture it is better to be deep than at the surface.
Staying deep you still get some oxygen through solution in the blood, while at surface you would will die immediately.

Something like 97% (off the top of my head) of the O2 carried in blood is bound to hemoglobin. Less than 3% is dissolved in plasma.The amount of O2 carried by hemoglobin does not change with increased pressure, only the amount dissolved in plasma changes with pressure.

CO binds to hemoglobin with a stronger bond than O2. It doesn't start detaching from hemoglobin until the CO you are breathing is removed. Increasing inhaled O2 doesn't help until CO is removed from the gas being inhaled. Then higher levels of O2 allow the O2 molecules to get attached to hemoglobin as soon as the CO molecules detach.

Even at 5 times atmospheric pressure, there is not enough oxygen dissolved in plasma for a human to live. You will still die from hypoxia even breathing close to 100% O2 at any pressure (below oxtox levels) unless CO is removed from your breathing gas.

 

[TMHeimer]

Whilst the partial pressure of CO at 40m depth is definitely 5 times larger than the p.p. at surface, I am absolutely not sure that the effect of CO is proportional to its partial pressure.
Most of us have knowledge about the toxicity of Oxygen, and for this is widely accepted that the toxicity is triggered by its partial pressure. So we evaluate the maximum allowed depth of a gas mixture by ensuring that ppO2 stays below a given threshold, say 1.5 bar.
But the mechanism for the toxicity of CO is very different than the toxicity of O2. CO is toxic because it creates a permanent chemical bond with haemoglobin, so it inactivates it for transporting oxygen in the blood stream. This toxicity is actually at least partially counterfeit by increasing the p.p. of Oxygen (in fact the typical treatment for CO poisoning is hyperbaric oxygen).
This means that, as you dive deep, the larger quantity of oxygen available makes CO LESS TOXIC than at the surface. Of course there is also more CO, but as soon as its quantity is enough for entirely inactivating haemoglobin, the more CO present has no effect on the human body, as all the possible damage is already done.
Instead, the larger availability of O2 dissolved in the blood (instead of being bonded with haemoglobin) can provide oxygen to your brains even if all the haemoglobin has been inactivated.
Considering that the starting point was an already terribly high percentage of CO, my supposition is that breathing such an highly toxic mixture it is better to be deep than at the surface.
Staying deep you still get some oxygen through solution in the blood, while at surface you would will die immediately.

Yes I do follow that. We learned in DM course that CO may not kill you at depth because of the extra O2 circulating in the blood--thus the "tied up" hemoglobin doesn't matter like it would at the surface or a shallower depth. Correct?
Not sure any of this has directly to do with the question asked.
I guess it depends on what the question means by having the "same effect". Does it mean how much CO you'd breathe in at 40m or how doing that would affect you?

 

[saxman242]

Not sure any of this has directly to do with the question asked.

Has everything to do with the question that was asked. Probably not the question as it was intended though.

 

[Rickk]

1. That level of CO is very quickly fatal at surface pressure let alone depth. A diver would not survive long enough to get to 40m.

2. If it is 1.5% concentration, it will be 1.5% concentration at any pressure. It will be 1.5% in the tank at 200 bar, the various gasses all come out of the tank in the same relative concentrations as they exist in the tank. ( Trust me on this I am an engineer.)

3. What will change is the partial pressure of the gas concerned.

4. At 40 m the pressure is 5 bar 40/10+1=5 That is one bar surface pressure plus one for every 10 m depth. The partial pressure at depth will then be 5 times the partial pressure at the surface. If we have a regular air tank 21% O the partial pressure at the surface of the O is 21%, at depth it is 5x0.21=1.05 bar. If Nitrox at 40% it is 5x0.4=2 which is ijn the O toxicity range.

5. At 40 m the diver would have his lifeless body exposed to 5 times as many CO molecules as at the surface in every dying breath, but since all other gas molecules would still be 5 times as many, the percentage is the same with no regard for depth.

6. That is a really dumb exam question, it is looking for a partial pressure type of answer but expresses it in percentage terms, uses a fatal level of the gas mentioned when the same logic would apply to any gas in the tank. They could have just as easily said some inert gas in the tank. I don't think the writer of the question has any idea what they are actually writing about.

6. The guy who wrote the question deserves to be fired, his supervisor who approved the question deserves to be given a tank of 1.5% CO to demonstrate how dumb the question really was.

 

[TMHeimer]

Has everything to do with the question that was asked. Probably not the question as it was intended though.

Yes, poorly worded question. I just read it as a gas question. Better wording could be "what effect will it have on the diver".
This may be a good question for a PADI test......?

 

[GJC]

Yes I do follow that. We learned in DM course that CO may not kill you at depth because of the extra O2 circulating in the blood--thus the "tied up" hemoglobin doesn't matter like it would at the surface or a shallower depth. Correct?

That is not correct at all. There is very little extra O2 circulating in the blood because very little of what the body needs is dissolved in plasma. Almost all the O2 in blood is attached to hemoglobin and that amount does not increase with increased pressure. Even at increased pressure, there is not enough O2 dissolved in plasma to support life.

 

[Ukmc]

And what is the official answer? I wonder if overheated compressors burning their own lubricating oil is included as I have met too many owners who didn't know.

the answers on the test were:
A. Improperly maintained air compressor
B. Improperly located air intake on air compressor
C. Oil vapors in the air compressor
D. All of the above

 

[TMHeimer]

That is not correct at all. There is very little extra O2 circulating in the blood because very little of what the body needs is dissolved in plasma. Almost all the O2 in blood is attached to hemoglobin and that amount does not increase with increased pressure. Even at increased pressure, there is not enough O2 dissolved in plasma to support life.

Well, what I learned in DM course must be wrong. I learned that due to increased pressure at depth resulting in more O2 in the plasma, life can be supported. With the decreased pressure upon ascent, that is when people with CO contaminated tanks will pass out--usually all pretty close to the same time if they have the same tank contents.

 

[Pedro Burrito]

Yes, poorly worded question. I just read it as a gas question. Better wording could be "what effect will it have on the diver".
This may be a good question for a PADI test......?

It was a PADI test question.

 

[DandyDon]

It was a PADI test question.

And a reflection of how little thought that Padi gives the risk, I think.