Why Astronauts Breathe Pure Oxygen Before Every Spacewalk

Every astronaut who has ever stepped outside a spacecraft has spent the hour before doing something that appears almost too mundane to matter: sitting motionless with a mask over their face, breathing pure oxygen, waiting. The routine looks like nothing. That is precisely the point.

The problem it solves is real enough. A spacecraft cabin maintains pressure close to sea level—about 14.7 pounds per square inch. A spacesuit operates at roughly 4.3 psi, low enough that an astronaut can actually bend their arm or flex a glove against the resistance. That gap between cabin and suit is where the danger lives. When a body moves from higher pressure to lower pressure too quickly, the dissolved nitrogen that normally sits harmless in tissue can come out of solution and form bubbles. It is the same physics that makes a shaken soda fizz the moment you pop the cap.

Retired NASA astronaut Mike Mullane described the shuttle-era routine plainly: spacewalkers would breathe pure oxygen to wash out the remaining nitrogen from their tissues, and the cabin pressure itself would be reduced to 10.2 psi about 24 hours before the spacewalk, shrinking the pressure gap the crew would eventually cross. NASA aerospace physiologist Johnny Conkin, whose research underpins the agency's current prebreathe schedules, frames the same goal in technical language: the protocol ensures that partial denitrogenation is achieved before depressurization occurs.

The condition being prevented is decompression sickness—the bends, as most people know it—the same injury that scuba divers guard against by ascending slowly. Nitrogen bubbles in joints and soft tissue cause deep aching pain and fatigue. Under the shuttle protocol, Mullane notes, no astronauts experienced symptoms of the bends, which is exactly what a well-designed prebreathe schedule is supposed to produce: a real danger neutralized before it ever becomes an incident.

It is worth being precise about what this protocol actually prevents, because it is easy to blur two different dangers into one and lose what makes each of them true. Decompression sickness happens across a comparatively modest pressure drop—cabin to suit—and the gas involved is nitrogen, already dissolved in body tissue at ordinary pressure. A far more extreme event, called ebullism, occurs only when pressure falls low enough that water itself starts turning to vapor at ordinary body temperature. That threshold sits around 63,000 feet of equivalent altitude. Different gas, different threshold, different physics. Decompression sickness can take hours to announce itself, joints aching quietly on the ground long after a spacewalk is over. Ebullism announces itself in seconds, if it gets the chance to announce itself at all.

What connects the two is a single rule of human physiology: cross a big enough pressure change too quickly, in either direction, and gas or vapor starts forming somewhere in the body it was never meant to be. Astronauts protect against the mild, well-understood version of that rule every time they suit up. The vacuum-chamber cases—like a 1966 NASA test where an engineer's suit lost nearly all its pressure at once—represent what happens when nobody had the chance to protect against the extreme version.

None of this is dramatic to watch. No moment of crisis, no countdown. Just a person sitting quietly with a mask on, doing something that looks like nothing is happening. That is the whole achievement of good risk management in aerospace medicine. The dangerous version of the story gets prevented so thoroughly that it never has the chance to become a story at all. Astronauts have logged thousands of spacewalk-hours under this exact protocol across the shuttle and station programs, and the routine nature of it is the success itself. Nobody remembers the spacewalks where the prebreathe schedule worked, which is a strange kind of victory—invisible by design.