How do people survive up Everest without Oxygen?

Some people go up Everest without oxygen, so how is it possible? Jon mentions a couple people who have gone up Everest without it. Hall claims that if you stay up in the air without oxygen too long, your blood will get thick, and you will be delusional. It is most likely that the people who have made it to the top without oxygen have probably gone up the mountain as fast as possible, and didn’t stay on the summit for long. They probably got to the summit and then proceeded down the mountain immediately. They probably lost millions of brain cells (according to the first chapter), but still made it up and down in one piece.