I’ve read about hikers heading up to Mount Everest and other high mountains and this prompts the question: What is the greatest elevation that one can go and not need extra breathing equipment?
The onset of problems with increasing elevation is known as acute mountain sickness, or AMS. The percentage of oxygen in air, about 21 percent, remains unchanged up to an elevation of about 7,000 feet and so it is the lack of oxygen that causes the sickness. Ascending slowly and avoiding strenuous activity is the best way to avoid it. AMS affects about 20 percent of people rapidly going to 8,000 feet and 40 percent at 10,000 feet. The percentage increases to practically 100 percent of people at 16,000 feet. Physical fitness does not decrease the risk of AMS.