Aussie
we fly at cabin altitude of 8000ft.
the partial pressure of O2 is further decreased by recirculating the air through the cabin.
On long high flights it can result in severe headaches amongst other symptoms.
Anything over eight hours I developed bad ones.
Age paid it's price as well.
Many of our guys used our masks for 10 mins before we started descent on long haul night flights as well as when we were tired on short haul or hung over.
I've also used O2 around the ITCZ.
Once erroneously deviated 100+miles off track (with permission) due to miss reading the wx radar. Putting on the mask helped me to literally see the picture.
The air /O2 quality issue has somewhat been addressed by the dreamliner and O2 scrubbers.
But it goes back to cost and recirculation fans burn less kerosene than bleed air consumption.