I was watching a documentary about a group of doctors doing high altitude research while climbing Mt Everest. It was claimed in that documentary that breathing

I was watching a documentary about a group of doctors doing high altitude research while climbing Mt Everest. It was claimed in that documentary that breathing pure oxygen only increases actual oxygen intake by 5% at those sort of altitudes. Presumably they were talking about conventional oxygen masks and not the tight fitting, high pressure masks that pilots use. In any case I was fairly shocked that breathing pure oxygen made such a small difference at altitude, but the difference it does make can be critical at the limits of human endurance.