Chamber run
I did the chamber run of 25'000 about 6 months ago. I lasted about 3 min without O2 before I started to lose my ability to think straight. TUC at that altitude is about 3-4 min if I remember correctly. It got to the point where I did not realize that the instructor had put me back on 02.
Very dangerous to anyone who does not use 02 above 10'000 or 12'500 depending on your regs