I've been reading about Time of Useful Consciousness (TUC), and it says that at FL 180, the time of useful consciousness is 20 to 30 minutes at rest.
But I've been to a trip to Tibet, and we went on a bus trip around the places, and at one point the bus came to a mountain pass around 18,500 feet, and we all got off the bus, walked around, took pictures, and even ate lunch, so it was definitely not at rest. It was about 40 minutes to 1 hour, and I only felt slightly uncomfortable/out of breath.
According to the chart, I should have passed out, or at least become "useless". Why didn't this happen? Are the charts simply very conservative and apply to, say, unfit, crumbling, old pilots?