Originally Posted by
gazumped
I think you may find crew oxy has 3 selections, Normal, 100%, and Emergency.
If this is standard, I expect this is:
Normal - oxygen/air mix to maintain at equivalent to 10,000ft.
100% - no air mix in the event of loss of cabin pressure - 2mm overpressure to ensure a good supply
Emergency - 4mm pressure in the event of smoke and fumes. It is not true pressure breathing which would be up to 30mm.