One hypoxia event, I remember approaching the IAF on a high penetration and feeling a bit warm and tingly. Since it was a nice sunny day in the T-37, I figured I was just getting warm. Which was dumb, because I was at 19,000 ft and normally you have to have the heater on at that alt. In any case, I started the approach and the whole way down on the outbound leg, my focus and cross check kept returning to the compass because I couldn't remember if I had turned inbound or not. That's when I finally figured it out, through the haze of the hypoxia, that something was wrong. We descended and the fog lifted after about 15 minutes below 10,000 ft. Turns out the oxygen regulator had failed.
Ever since then, my first hypoxia symptoms have remained constant, a warm skin feeling sort of like sitting just inside a window in direct sunlight, with slight tingling especially in the hands.
Another time, I had an air conditioner turbine in the T-38 come apart just before takeoff, filling the cockpit with toxic smoke. We considered going to a spare aircraft but even 30 minutes after the event I still felt my hypoxia symptoms so we went to the hospital instead. My blood O2 was just a bit below normal but they doc said that I probably absorbed a bunch of carbon monoxide and other chemicals like cyanide that interfere with blood oxygen transfer and transport, so I got an overnight stay in the hospital. If I hadn't recognized the symptoms, I might have re-attempted the flight the same day and who knows how bad my G tolerance or air pressure tolerance would have been. It was planned as a very strenuous air to air formation training sortie, so flying it while degraded in that way could have been a disastrous choice.
