I rode a Navy pressure chamber up to 30+K or so. At around 20K several persons were told to remove their masks and do some simple task. It was only seconds before they went goofy, grinned a lot, and had zero capability to understand or perform. Within a minute or two, they would have been unconscious. A loss of oxygen is an immediate and soon fatal issue unless it is very quickly recognized. When I was a controller in the USAF, we were instructed that if a fighter type began to act weird (more so than normal), we were to tell them,“go 100% oxygen, go 100% oxygen”.