As usual - Advanced Aeronautical Sciences at Wikipedia Institute helped.
Time of Useful Consciousness - Wikipedia, the free encyclopedia
What Wilipedia does not say, is that above 62,000 feet, you blood will boil.
I used to scare myself in F-104Gs... 1964-1968.
Some idiots dashed above 55/60,000' - without pressure suits...
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Opinion of passenger oxygen masks above 25,000 feet = WORTHLESS...
Oxygen masks for pilots (747) - Might save you up to 33,000 ft.
But get lower ASAP...
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Happy contrails