Around 30 BCE a Chinese text warned Silk Road travelers about the Big Headache Mountains of the Karakoram, and in the late 1500s the Spanish Jesuit José de Acosta, crossing the Pariacaca pass in the Andes, blamed his nausea and blinding headache on air too thin for human needs. This episode explains why: the air stays about 21 percent oxygen at any height, but falling barometric pressure spreads the molecules out, and water vapor in the lungs takes a growing share of the space. It covers why an elite marathoner can struggle while a less fit person acclimatizes, the three altitude bands, and why no one lives permanently above 5,100 meters.

The body fights back with a racing heart, hyperventilation, more EPO and red blood cells, and 2,3-BPG that helps hemoglobin release oxygen. Those same reflexes can turn deadly, as dilated vessels leak fluid into the brain in HACE and constricted vessels in the lungs burst under pressure in HAPE. The episode covers climb high, sleep low, the 300 meter daily sleeping limit, acetazolamide’s trick of acidifying the blood, Aymara coca chewing, and the EPAS1 variant Tibetans likely inherited from Denisovans, then turns to Chilean copper miners who commute from sea level to mines such as Collahuasi.

• Acute mountain sickness, which can begin at 2,000 meters within about 10 hours of arrival
• Roughly 20 percent of fast climbers sickened at 2,500 meters and 40 percent at 3,000 meters
• The Gamow bag, a foot pumped chamber that can simulate a descent of 1,500 meters
• How Andean and Tibetan populations evolved opposite solutions to thin air
• Chile’s laws requiring screening and reassignment to lower elevation without a cut in pay