Oxygen Mask Limitation
 

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.

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

pb

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.