Guy Alcala wrote:

The difference being the rate at which you ascend from lower altitudes,
although the military's 10,000 foot rule seems very conservativ. IIRR,
even the FAA allows pilots to fly between 12,500 and 14,000 ft. for 1/2 an
hour without O2, and pax can do it indefinitely. Pity the poor fighter
pilots in WW1, who used to climb up to 17-20,000 feet without O2 and
patrol there for an hour or so in open, unheated cockpits. And suffered
massive headaches as a result.


Ah, memories come back ... in my old freight dog days I used to fly with
an older guy who hated to waste fuel on pressurization while crossing
the Rockies in Montana. His only complaint was that he had a hard time
keeping his cigarettes lit when the MEA's were in the flight levels.

Rick