Flight Alloy: It is true that limited pressure breathing is possible, but this is limited to just the slight muscular 'overcapacity' the diaphragm muscles are capable of, in the range of fractions of an atmosphere.
Capn Bloggs: Flight Alloy, what are you on about. You have obviously never worn a flight crew mask with Emergency set on the dial.
Trying to get a ball-park figure I found a 2003 Lancet article on the "Physiological hazards of flight at high altitude"
https://www.thelancet.com/journals/l...059-3/fulltext
"Which mentioned "positive pressure breathing oxygen masks that deliver up to 70 mm Hg", that's just under 10% of an atmosphere (760mm Hg).
Any idea what pressure modern flight crew masks actually deliver?