Breathing oxygen from an oxygen generator if CO concentration is high

O2 concentrator doesn’t have any kind of scrubber, so it doesn’t remove carbon from either CO or CO2 which means these gases stay in circulation until vented.

One of the reasons why emergency O2 for flight crews comes under additional pressure (if selected) is to prevent and reduce effects of toxic gases inhalation.

The bypass % with a cannula is huge – probably 90% plus.

The masks in jets are pressure fed because – as posted above – the ambient O2 partial pressure at say FL300 is too low to sustain life even if you are breathing 100% O2. At MSL the O2 partial pressure is about 3psi (20% of 15 psi i.e. 20% of 1 bar) and at FL300 it is only about 0.5psi. So there the mask is fed entirely out of a cylinder, with 100% O2, and probably at about 2psi. In military jets they sometimes use O2 generators.

GA masks, like the well known Mountain High ~€500 one with a mike, are not 100% O2 either; there is still a big bypass.

Peter wrote:

the ambient O2 partial pressure at say FL300 is too low to sustain life even if you are breathing 100%.

This is not correct. Ambient ppO2 is low at FL300 (0.21 × 0.3 = 0,063 bar which is really low and deathly hypoxic) but breathing 100% O2 at FL300 is perfectly ok. It’s 0.3 bar (total pressure at FL300) times 1 (O2 fraction) which gives ppO2 0.3 bar which is more than enough since we’re OK with anything above 0.18 ppO2. Even at FL360 ppO2 is still 0.227 bar which is above 0.21 (ppO2 in air at sea level). FL380 is where the problems start and where pressure is needed because sealing starts to be critical. However, at any of these heights fast donning is critical.

20%

It’s actually 21%

Depends on how you deliver it. With a cannula, I think Peter is right (massive bypass). With a non-specialty mask Emir is right (else no one would make it to the top of Mt Everest at 29K ft without long adaptation) but you need a lot of flow (which is why no one hangs around for long at that top unless you are a sherpa and think O2 is for wimps).

FL360 with pure O2 is scratching it even with a mask and you are already below normal sea level conditions I believe. Air pressure at FL360 is 170 mmHg (0,227 bar) and you’re supposed to subtract water vapour partial pressure (47 mmHg at body temp) because the air gets humidified, so that’s 123 mmHg ppO2 inhaled – compare with 760-47=713*0.21=150 mmHg normal air inhaled at sea level. Probably you’re supposed to adjust the water vapour pressure for altitude as well but I’m not clever enough for that. And the partial pressures in the lungs are lower than the partial pressures in the atmosphere due to mixing. It all gets messed up fast :-)

Perhaps it’s worth considering firefighter equipment or even scuba gear for this specific emergency scenario (CO intox) if those exist in small form factor – those can obviously provide sealing and you’d even be fine with normal air if you’re not too high. But I don’t know whether either works as intended at altitude.

With a cannula…

With cannula it’s not breathing the oxygen – it’s breathing the mixture of ambient air and oxygen. Breathing O2 assumes sealed mask.

…and you’re supposed to subtract water vapour partial pressure…

I don’t know how you came to this conclusion but I think it’s wrong. Ambient pressure at FL360 is 0.227 bar. What’s the ppO2 of 100% O2 inhaled at FL360? Well it’s ambient pressure times fraction of O2 in breathing mixture. Since fraction is 1 (100%) and ambient pressure is 0.227, actual ppO2 is exactly 0.227 bar (assuming no leaks and good venting of exhaled gas).

With cannula it’s not breathing the oxygen – it’s breathing the mixture of ambient air and oxygen. Breathing O2 assumes sealed mask.

No disagreement but as we are discussing oxygen concentrators I thought it was relevant since those are only good for cannulas – none of them provide enough flow for anything better, not even close (you can hook one up to a regular mask of course but it’s not going to be of any added value, and I suppose you can technically hook one up to a sealed mask as well but depending on the quality of the seals I think you might suffocate ).

All of this is theoretical if you and your plane can do FL360 of course, but a cannula will be problematic much below that which is probably why the FAA restricts them to below 18000 ft. I don’t know whether EASA has a similar rule actually.

I don’t know how you came to this conclusion

Because the ppO2 that matters is the one in your alveoli deep in your lungs, not at the level of your mouth or nose (where your numbers are correct). The air you breathe gets saturated with moisture and this is the 47 mmHg you ‘lose’ in terms of gradient available for diffusion. Every text I’ve seen on respiratory physiology shows the numbers like that (see for example here). As I wrote you probably need to adjust the water vapour pressure for altitude as well, and need to account for mixing effects in the alveoli, but all that makes my head hurt Happy to learn though.

a cannula will be problematic much below that which is probably why the FAA restricts them to below 18000 ft.

There is much inconclusive discussion re the 18k figure. It originates from the FAA scene and applies only to installed O2 systems, in the form of an AFMS for the said system. There is no regulation requiring a mask above 18k if using a portable O2 system.

The above is additional to, and separate from, the general requirement for using O2 above (something like) 11k and (in the FAA scene) make it available to passengers above 15k. In Europe it is different. Not sure what the latest is… I have NCO.OP.190 here from 2016.

FWIW I have been to 21k with just a cannula, and the O2D2 regulator. It was probably set to the “mask” setting though, for extra gas

The problem is that above the ~20k figure you have to breathe quite deliberately, and have a good sniff before calling ATC. Whether this drove the 18k AFMS figure I don’t know, but knowing how this aviation scene works (a lot of stuff was worked out on the back of a fag packet in 1903) I doubt it

Every text I’ve seen on respiratory physiology shows the numbers like that (see for example here). As I wrote you probably need to adjust the water vapour pressure for altitude as well, and need to account for mixing effects in the alveoli, but all that makes my head hurt Happy to learn though.

Thanks for the explanation. I’ll check and report if not clear.