worth pointing out pax oxy pointless above about 25k.
Must admit I thought (from AvMed lectures at North Luffenham in my fast pointy aircraft days ...) that you can breath pure oxygen oxygen at ambient pressure at up in the high 30,000's of feet....not to be recommended I'm sure but the partial pressure was enough to ensure survival....
The unpressurised 25K limit was, I thought, due to issues with decompression "sickness"...