Volunteers walked into a steel decompression chamber in Natick, Massachusetts, and the door was sealed behind them.
Every day, a pump drew out a little more air.
After 40 days of descent, the pressure inside matched what a climber feels standing on the summit of Everest.
Nobody had climbed anything. Yet the gas in their blood had reached numbers that would drop an unprepared person within minutes.
What the blood does when the air thins out slowly
The secret of the whole experiment is the word slowly. Rapid ascent to high altitude causes serious problems for climbers, skiers and aviators, while gradual ascent lets humans function where the unacclimatized cannot. The body does not notice the change hour by hour. It adjusts one system at a time, like a ship trimming its sails to a wind that shifts a degree per hour rather than hitting all at once.
The first thing to move is breathing rate. The lungs push faster and deeper, driving carbon dioxide out of the blood and making room for what little oxygen the thin air carries. That drop in carbon dioxide shifts the blood slightly alkaline, which nudges the hemoglobin molecule into a shape that grips incoming oxygen more tightly.
Within days, the kidneys read the low oxygen signal and prompt bone marrow to produce more red blood cells, so each unit of blood carries more cargo. Alveolar ventilation keeps climbing until it is several times the sea level rate. That sustained overbreathing is not a side effect of the stress; it is the main thing keeping a man upright at an altitude that would floor him if he arrived by helicopter.
The steel room at the Army research chambers in Natick
The chamber at the Army’s environmental medicine institute in Natick was not a closet. It was a large decompression chamber with several connected rooms, laboratory space alongside it, and instruments threaded in on cables and through pressure locks. From the outside it looked like an industrial boiler. From the inside, it was an apartment that was slowly losing its air.
The pressure was walked down on a schedule modeled on the pace of successful Everest summit parties, ending at a barometric pressure of 240 Torr, a simulated altitude of 29,028 feet, with an inspired oxygen pressure of 43 Torr. The question was where the body’s capacity to adapt runs out, and whether the summit sits just inside or just outside that limit.
The answer was barely inside, and only if the climb is slow enough.
The numbers the instruments recorded
In October 1985, eight subjects and 27 investigators gathered at the Natick altitude chambers for the 40 day simulated ascent known as Operation Everest II. At the top, maximal oxygen uptake fell to 1.2 liters per minute, with arterial oxygen pressure at 30 Torr, carbon dioxide at 11 Torr and arterial pH at 7.56. The men’s own sea level maximum had been about 4.0 liters per minute, so they were working on less than a third of their usual aerobic capacity.
That is roughly the aerobic ceiling of a brisk walk, applied to a body already fighting to keep its core chemistry stable. Arterial oxygen saturation at maximum effort fell to around 46 percent by ear oximetry.
The pH reading tells the rest of the story. Normal blood runs near 7.4, and a shift of that size changes the working conditions of nearly every enzyme in the body. The alkaline swing also helps the lungs load oxygen onto hemoglobin, buying the tissue a few extra molecules with every heartbeat.
Eight men began the ascent and six reached the summit equivalent, conscious and able to pedal a cycle ergometer at pressures where an unacclimatized person has only a couple of minutes of useful consciousness. The slow ramp is the treatment, and the chamber made it possible to study that ramp apart from cold, wind and exhaustion.
What an earlier experiment had already found, and what this one added
The project built on a smaller wartime era forerunner. In 1946 the same lead researcher ran a Navy decompression chamber study in which four volunteers were decompressed slowly over weeks to a summit equivalent altitude, and the result was that men could still work there after a gradual ascent.
The Natick version four decades later added Swan-Ganz catheterization and inert gas studies at several altitudes. Those showed that cardiac function held up and that pulmonary vascular resistance rose and was not relieved by breathing oxygen. A ventilation perfusion mismatch in the lung increased throughout. The lung, not the heart, was the weak link.
The summit pressure measured on the mountain itself is somewhat higher than the standard atmosphere model predicts, which is part of why the real summit stays inside the survivable range for a very fit, slowly acclimatized climber.
What the chamber proved about the body’s limits and what it still could not answer
The experiment had a ceiling of its own. Investigators could control the pressure and the tests, but not which men would cope best. Individual differences persisted all the way up: the spread of maximal oxygen uptake narrowed as pressure fell, yet the rank order among the subjects stayed the same.
Sustained hyperventilation was what separated performance from collapse. The broader question the chamber studies left open is whether the ceiling of human acclimatization is mostly fixed or partly trainable, and no single measurement has settled it. For a look at how a different kind of isolation probed the opposite end of adaptation, the piece on 24 volunteers in a tilted head down ward shows how bed rest strips the body of gains that altitude slowly builds.
What the Natick project established without ambiguity is simpler and stranger: given enough time and a gentle enough ramp, the human body can keep working where the air holds barely a third of the oxygen it does at sea level, inside a sealed steel room, without ever looking at a mountain.
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