[braythwayt]

I believe it's somewhat correct, based on some random diving training.

There is a thing where you deliberately hold your breath, which starts with taking a big lungful of air, which presumably is at normal pressure and contains 21-ish percent oxygen.

Even untrained individuals can hold their breath for a while like this.

There's another thing, which is what happens if the air you're trying to breathe either is at lower pressure, or has lower oxygen proportions. Either way, each breath has fewer oxygen molecules.

If you haven't prepared for this and try to continue breathing, you can quickly become short of breath, and then cognitive abilities become impaired, hampering your attempts to rectify the situation.

I wonder if the problem is that if cabin partially depressurizes by the time the masks drop, you may no longer be able to take a deep breath and hold it while taking your time putting on a mask.

I acknowledge that I do not have training in dealing with cabin depressurization. As luck would have it, my sister has worked for Air Canada for more than 30 years, I will ask her about it.

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I see some other comments about the oxygen actually coming out of your blood into your lungs at low pressure, and the air coming out of your lungs because the cabin is lower pressure than your lungs.

Those make sense, and had I trained for summiting 8,000m peaks instead of tech diving, I would have known that!

[knaq]

It matches diving.

Suppose you're 2000 feet down, breathing hydrox or hydreliox. It's mostly hydrogen, maybe some helium, and less than 1% oxygen. You suddenly ascend to the surface. Instead of changing gas mixture, you try to hold the gas in your lungs and maintain the pressure.

You won't hold it in. As the pressure drops in your lungs, the dissolved gases in your blood will escape into your lungs. That drops the oxygen content in your blood.