Its worth noting that the only reason we nominate an altitude is as a metric for SPO2 levels. A better yardstick would be density altitude, best still would be SPOs2 levels. When these regulations were formed, pulse oximiters were not readily available. Now they are everywhere. I challenge anyone to find a definitive limit for SPO2 readings. I've looked & looked. This is partly because its a personal thing that varies. I have a figure that I usdes as a minimum. I also find that I need higher oxyhen flows to maintain this than the oxygen regulator (maked in feet altitude) suggests. I also use a pulse oximiter that I calibrated against the lab gear when I did hypoxia training. But it would be pretty easy to suggest that (say) the crew SPO2 reading must remain above 90%. Why cant we be having regulations that are keep up with contemporary technology?