At about 19 kilometres up the air pressure falls low enough that water boils at human body temperature, so an unprotected person there would feel the saliva on their tongue turn to vapour, which is why aircrew above that altitude wear sealed pressure suits rather than an oxygen mask

At sea level, an oxygen mask and a pressure suit can look like two versions of the same idea. Both seem to give a pilot air.

At about 19 kilometres above Earth, the difference becomes the difference between breathing and remaining physically intact.

This is an account of aerospace physiology and flight protection, not guidance for surviving decompression.

At that altitude, atmospheric pressure falls to roughly 6.3 kilopascals, or 47 millimetres of mercury. That number matters because it is also the vapour pressure of water at 37 degrees Celsius. Exposed water no longer needs to reach 100 degrees to boil. Human body temperature is enough.

The boundary is known as the Armstrong limit, or Armstrong line, after US aviation physician Harry George Armstrong. It does not mark the edge of space. It marks something more intimate: the altitude above which the surrounding atmosphere can no longer keep warm water liquid.

An unprotected person would not explode, instantly freeze or watch all their blood boil away. Skin and blood vessels retain pressure. But moisture exposed to the air, including saliva and tears, can begin turning to vapour. Gas and water vapour can form in tissues, producing swelling known as ebullism. At the same time, the brain is rapidly losing oxygen.

Above this line, oxygen alone is not the complete answer. The body also needs pressure.

The boiling point was never only about temperature

We learn that water boils at 100 degrees Celsius because that is approximately true at sea level. It is not a fixed property of water. Boiling begins when a liquid’s vapour pressure matches the pressure pushing down on it.

Climb a mountain and atmospheric pressure falls, so water boils at a lower temperature. Go high enough and the boiling point reaches the temperature of the human body.

NASA places the Armstrong line at approximately 62,000 feet, or 19 kilometres. Other agency material rounds it to 63,000 feet. The small difference is not a disagreement about a sharp physical wall. Altitude-pressure relationships vary with atmospheric conditions, and 37 degrees is itself an approximation of body temperature.

The governing pressure is about 6.3 kilopascals. Below it, a glass of water warmed to body temperature would boil in the open air. So would a film of saliva on a tongue.

That does not mean the body becomes a kettle. NASA’s history of early pressure suits makes the distinction directly: saliva is exposed enough to boil, while blood remains under sufficient pressure inside the circulatory system to stay liquid. The larger danger is a combination of hypoxia, expanding gas, ebullism, lung injury and loss of consciousness.

In 1966, a suit fitting came loose inside a NASA chamber

The physical limit stopped being an equation for Jim LeBlanc on 14 December 1966.

LeBlanc was a spacesuit technician at NASA’s Manned Spacecraft Center in Houston, now Johnson Space Center. He was testing an Apollo suit inside an eight-foot altitude chamber evacuated to the equivalent of about 150,000 feet. The suit was pressurised. The chamber around it was close to vacuum.

Then an oxygen coupling disconnected.

The contemporary Space News Roundup account published by the centre in January 1967 says the suit pressure fell from its operating level to 0.1 pounds per square inch within ten seconds. LeBlanc saw what looked like steam blowing from his left side as oxygen escaped. He looked at the gauge on his wrist, saw the falling pressure, noticed his vision becoming fuzzy and stumbled backwards.

Later NASA summaries record his last conscious memory as the water on his tongue beginning to boil.

Test conductor Clifford Hess ordered immediate repressurisation. Henry Rotter waited in the adjacent airlock, entered when the chamber had returned to the equivalent of 27,000 feet and went to LeBlanc. The Roundup says repressurisation began ten seconds after the failure. Later NASA accounts estimate that LeBlanc remained conscious for about 14 seconds and recovered as the chamber passed the equivalent of roughly 15,000 feet.

He survived without lasting neurological injury. That outcome did not show that vacuum exposure is safe. It showed how narrow the rescue interval can be when a prepared team begins restoring pressure almost immediately.

What an unprotected person would actually experience

Popular descriptions of vacuum tend to choose between two errors. One imagines a person exploding. The other says exposure is surprisingly harmless for a short time.

Neither is a useful account.

The skin is strong enough to contain the body, so there is no cinematic rupture. Blood does not boil in intact vessels. Heat also does not disappear instantly, because vacuum is poor at carrying it away. But exposed moisture can vaporise, and gas already present in the lungs and digestive system expands as external pressure falls.

If someone holds their breath during rapid decompression, expanding air can damage the lungs. If they exhale, oxygen already carried by the blood may support consciousness for only a few more seconds. The LeBlanc incident suggests roughly 14 seconds, not a guaranteed countdown. Circumstances differ, and useful consciousness can disappear before a person understands what is happening.

Ebullism adds another mechanism. A NASA technical paper on tissue vapour formation describes rapid swelling, cardiovascular obstruction, lung collapse and oxygen deprivation among the possible consequences above 63,000 feet. The water is not being heated. It is changing phase because the pressure holding it in liquid form has vanished.

The first sensation might therefore be wetness becoming vapour, vision narrowing or the sound of gas escaping. The decisive event soon after is loss of consciousness from hypoxia.

An oxygen mask changes the gas, not the world around the body

An ordinary oxygen mask raises the proportion of oxygen in each breath. That works while there is enough total pressure for the lungs to move oxygen into the blood.

Higher up, crews may use pressure breathing, in which oxygen is delivered above the surrounding pressure. But positive pressure inside the lungs creates its own mechanical problem. If pressure is applied only through the airway, the chest must work against it and the rest of the body remains exposed to the low-pressure environment.

By the Armstrong limit, the problem is no longer merely that each breath contains too few oxygen molecules. The ambient pressure is low enough for body-temperature water to vaporise. A mask covering the nose and mouth cannot pressurise the eyes, skin and tissues.

A full-pressure suit can. It encloses the pilot, seals at the helmet and gloves and inflates if cabin pressure is lost. In effect, it moves a small survivable atmosphere with the person.

This is why the suit in our recent account of Alan Shepard’s four-hour wait inside Freedom 7 was more than clothing attached to an oxygen hose. It was a pressure vessel shaped around a body, intended to preserve an atmosphere if the capsule could not.

The U-2 carries a cabin and a suit because one failure cannot be final

Aircraft that operate above the Armstrong limit do not normally leave their crews exposed to the outside pressure. The cockpit is pressurised. The full-pressure suit is another layer, needed if the cabin leaks, the canopy fails or the pilot must eject at altitude.

The U-2 makes the logic unusually clear. The US Air Force says the aircraft is routinely flown above 70,000 feet, more than 21 kilometres up. Its pilot wears a full-pressure suit similar in principle to those worn by astronauts.

The suit does not make the flight comfortable. Historically, the U-2 cabin itself could expose a pilot to a pressure altitude comparable to the summit of Everest. Crews breathe pure oxygen before takeoff to reduce nitrogen in the body and lower the risk of decompression sickness. Long missions still made that risk serious enough for the Air Force to strengthen the cabin and change its pressure regulation.

The Cabin Altitude Reduction Effort increased cockpit pressure, reducing the physiological load on pilots. This is an important correction to the simple image of a yellow suit defeating the stratosphere. Protection is layered: a pressurised cabin for normal operation, oxygen management for decompression sickness, and a sealed suit for the emergency in which the cabin is no longer there.

Not every high-altitude aircrew uses the same equipment, and “above 19 kilometres” does not describe every aircraft’s normal cabin environment. The rule is about exposure. If a person may be exposed to pressure below the Armstrong limit, an oxygen mask by itself is not adequate protection.

A pressure suit is deliberately difficult to bend

Gas pressure saves the body by pushing outward. That same force tries to straighten every joint in the suit.

Early full-pressure garments became stiff when inflated. Bending an elbow meant doing work against the internal gas. Gloves reduced dexterity. Cooling, ventilation, visor seals and emergency connections all had to function while the pilot was strapped into a narrow cockpit.

The alternative, partial-pressure clothing, squeezed the body mechanically rather than surrounding it with a fully pressurised atmosphere. NASA’s history describes the early versions as “get-me-down” suits, emergency equipment designed to keep a pilot responsive long enough to descend rather than an environment for prolonged work.

The engineering problem became even more severe in space. In our account of Alexei Leonov’s first spacewalk, his suit ballooned in vacuum and resisted his attempt to return through the airlock. He had pressure, but pressure also turned the garment into terrain.

Modern suits manage that conflict through restraint layers, joint bearings, carefully chosen operating pressures and life-support systems. They are still compromises. A softer suit would be easier to move in but may provide less stable pressure. A more rigid pressure vessel protects well but makes human work harder.

The Armstrong line is where the atmosphere stops doing invisible work

At sea level, atmospheric pressure is so constant that it feels like nothing. We notice air when it moves, not when it presses on every surface of the body.

The Armstrong line reveals that hidden function. Air pressure is not only part of breathing. It keeps water in its familiar phase. It restrains gases inside tissues. It gives the lungs an environment against which they can work.

Above about 19 kilometres, a crew member needs an artificial replacement for that environment. A sealed aircraft cabin can provide it. A full-pressure suit can preserve it when the cabin fails. An oxygen mask can contribute breathing gas, but it cannot recreate the missing pressure around the rest of the body.

Anyone exposed to rapid high-altitude decompression requires immediate emergency response and medical assessment. The survival of one NASA technician after seconds near vacuum is not a procedure and not a promise.

LeBlanc’s last remembered sensation is what makes the boundary understandable. The water on his tongue was not hot. Nothing had warmed it. The atmosphere around him had simply fallen below the pressure required to keep it liquid.

At that point, the suit was no longer something he wore. It was the only atmosphere he had.