Mountain sickness, also called altitude sickness, has been in the written record since at least the 16th century, when Spanish missionary José de Acosta described it. Traveling from the east coast of South America to the west-coast city of Lima, Peru, in the 1500s, Acosta traversed mountain passes as high as 14,000 feet. He and his traveling companions — and their pack animals — were unprepared for what hit them at such heights: extreme shortness of breath, nausea, vomiting, confusion and debilitating fatigue.
After watching the pattern repeat at each high pass, Acosta concluded that mountain air was too "delicate" and "not proportionate to the human respiratory system."
Though not a scientist, Acosta was nearly right. But his findings were followed by centuries of misunderstanding.
His fellow missionaries concluded that mountain air diminished the mental capacity of the Andean natives. They also believed that fatigue occurred at high altitude because the decreased pressure meant muscles had to work harder to keep skeletons from collapsing. (This was based on the belief that a certain amount of air pressure was necessary to keep bones in place.)
It was 300 years before scientists offered a more sophisticated explanation for altitude-induced symptoms. Working at the Sorbonne in Paris, French physician Paul Bert placed animals in chambers and manipulated the air inside. Low pressure, he concluded, wasn't the direct cause of altitude sickness. The real culprit was oxygen deprivation. Mountain air has the same fraction of oxygen as sea-level air, but the air pressure of that oxygen is lower, which means fewer molecules make it into the lungs on each breath.
In the decades that followed, researchers in Europe and the Americas began to more carefully document the human body's response to altitude, many of them accompanying trekkers on mountain hikes.
Mexican physiologist Daniel Vergara Lope accompanied more than 100 of his compatriots into the mountains, taking detailed measurements on the size of their thoraxes, the lengths of their sternums, the levels of their red and white blood cells and their breathing rates. He found that the climbers' breathing rates increased over time, as did their red blood cell counts. The body, he concluded, acclimatizes to high altitudes; it automatically adapts to compensate for too little oxygen.
Acclimatization, researchers now know, doesn't stop with increased respiration and red blood cells. Pressure in the blood vessels in the lungs also increases, which helps pump blood to parts of the lungs not normally used during sea-level breathing. And the body boosts production of an enzyme that helps oxygen move from hemoglobin in the blood to the tissues that need it.
Such changes are temporary, reversing as lowlanders descend from high altitude back to sea level. Among high-altitude natives, however, more permanent adaptations are common.
Some Andean natives in Peru have high levels of oxygen-carrying hemoglobin in their blood. Some Tibetans, by contrast, have lower levels of hemoglobin, but their hemoglobin binds more efficiently to oxygen
Worldwide, about 140 million people live at altitudes of 8,200 feet or higher. Most live in Asia and in the Andean mountain range.
After the ban on high-altitude soccer matches was announced in May, tempers flared in some parts of South America. The ban was intended to protect the health of players residing in lower altitudes and eliminate the advantage of home-team players accustomed to thin air. There's good reason for concern: Many endurance athletes suffered at the 1968 Olympic Games in Mexico City. At just over 7,000 feet, the city sits right where altitude-induced symptoms usually begin setting in.
But the soccer restriction was ultimately loosened. As of late last month, international matches can be played in stadiums up to 9,800 feet. A special exception allows matches in soccer-crazed La Paz, Bolivia, which at 12,000 feet will likely leave many top-notch athletes gasping for breath.