https://employees.faa.gov/org/linebusiness/ato/news/features/story/index.cfm?newsId=59708
Really nasty stuff, it can creep up on you..... The guy who ran the FBO and was my mechanic used to be a crew chief on the KC97 tankers. They would be flying at 18,000 ft and the guys in the back didn't have oxygen. He said they would sit in a circle and you were supposed to keep an eye on the two guys beside you and if one of them started to nod off your job was to wake him up... Jim was sitting there thinking this was stupid, he was wide awake and was shaking his buddies from time to time... Next thing he knew he was the one being shaken....
Flying in the flight levels on oxygen is not a very good idea, in my opinion. It's just too easy for a single point of failure to cause a big problem. I used to fly all the time in the mid teens on oxygen, which isn't so bad, but if you go much above that, it can get bad very quickly.
