Oxygen Mask Limitation

[Turbinator_PT6A]

Hello Folks! I had a query regarding the altitude limitation on sustained use of Oxygen masks by crew/pax:

A/C service ceiling : 45000
Crew Mask Limitation for sustained Use : 40000
Pax Mask limitation for sustained use: 25000

Looking for an answer as why is there an altitude limiting factor, so long as there is enough supply form the oxygen cylinders on board.

Thanks.

[Capt Pit Bull]

Because even with 100% oxygen as altitude increases the pressure will not be sufficient. Above about 40K requires a pressure breathing system.

pb

[Dariuszw]

FL250 and above 100% ox
FL340 and above at possitive pressure
FL400 and above pressure must be higher then standard ox masks used by airlines.