Marchino61.
It's NOT about "holding your breath" or anything. At 30 000+ the oxygen in the blood actually traversers out of the blood and back to the lungs. Pure oxygen has the ability to go into the bloodstream even at those altitudes.
Thus the available contious time at those altitudes are measured in seconds (about 5-10 seconds for most people) before you loose the ability to function rationally. Another 10 seconds and you're unconsious.
THATS why the say that you should put on your own mask first - THEN help others.
Mj A