[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.