At the summit of Mount Everest, 29,032 feet up, the air holds the same 20.9 percent oxygen as at sea level, yet breathing it can kill. This episode explains the death zone, which begins at 8,000 meters, about 26,246 feet, and covers only 14 peaks in the Himalaya and Karakoram. Swiss doctor Edouard Wyss-Dunant first called it the lethal zone in 1953. The problem is pressure: it falls from about 1,013 millibars at sea level to under 356, too little to push oxygen across the lungs into hemoglobin. The body fights back with EPO and extra red blood cells, thickening the blood and shutting down digestion.

Above 8,000 meters acclimatization stops working and the body burns oxygen faster than it can be replaced. Fluid leaks into the lungs and brain in HAPE and HACE, and failing judgment shaped disasters like Everest in 1996 and K2 in 2008. Researchers in Bolivia point out that a fetus develops at oxygen pressure effectively identical to the Everest summit, thanks to sticky fetal hemoglobin. The episode then traces the oxygen debates, Griffith Pugh’s 1953 protocol, and the climb that physiologists said was impossible.

• The highest permanent human habitation cited is 5,950 meters, where someone lived for two years
• Pugh’s four week acclimatization plan, timed to red blood cell production
• Tom Bourdillon and Charles Evans turned back about 300 feet short when moisture froze their closed circuit oxygen valves
• Edmund Hillary grew clumsy and slow within 10 minutes of removing his mask on the summit
• Reinhold Messner and Peter Habeler reached the top without supplemental oxygen in 1978