What really happens if you're thrown out of an airlock?

The full story

The episode's narration, word for word. Numbers link to the sources below; timestamps play that moment.

The airlock 0:00

In the films, an open airlock is the end of the line. The hatch slides back, someone tumbles out, and you brace for the worst: will they swell, burst, freeze solid in an instant? But what would really happen to a human body in a vacuum? And how do we know, when nobody runs that experiment on purpose? The answer is stranger than the films, and much less messy ¹.

Eyes popping on Mars 0:27

Here's the famous one. In the 1990 film Total Recall, characters end up outside on the surface of Mars ^2,3. As the critic Roger Ebert described it, they immediately begin to expand, their eyes popping and their faces swelling ². Ebert pointed out that, as Arthur C. Clarke had written, a man would not explode even in the total vacuum of deep space ². Then he waved it away: quibbles like that, he said, were largely irrelevant to the film ². So who's right, and why?

Mostly water 1:04

Start with what space takes away, which is pressure ⁴. At sea level, air presses on every part of you ⁵. Take that away, and you might expect the body to balloon like a party balloon. But you are mostly water, and water hardly changes size when the squeeze comes off ^4,6. Losing one whole atmosphere lets water grow by less than five thousandths of one per cent ⁶. If your whole body were seventy litres of water, it would gain about three millilitres ⁶. Your skin and connective tissue push back as well, so nothing bursts ^4,1.

The parts that do care 1:42

The parts of you that do care are the pockets of gas: your lungs, your gut and your middle ears ^4,1. Gas, unlike water, expands freely as the pressure drops, so air trapped at sea-level pressure would try to grow about a hundred and forty-seven times larger at a near-vacuum like the one coming up in our story ⁷. That's why a held breath is the dangerous case ¹. Air sealed in the lungs can damage them, the same risk scuba divers face as they rise ¹. With the airway open, the air simply rushes out ⁴.

Boiling without heat 2:18

Now the strange part: boiling ⁴. Water boils when its own vapour pushes back as hard as the air around it ^4,8. On a mountain five kilometres up, by our calculation, water boils at about eighty-three degrees ^8,5. Drop the pressure far enough and water boils at thirty-seven degrees, which is body temperature ^4,9. That happens at about a sixteenth of sea-level pressure ⁸. On Earth, the air only gets that thin about nineteen kilometres up ⁵.

Ebullism 2:49

Well below that line, the water in your soft tissues starts turning to vapour ⁴. Doctors call it ebullism ⁹. The vapour makes the body swell, perhaps to as much as twice its normal volume, unless a suit holds it in ⁴. Your arteries hold out at first, because your heart keeps the blood above the outside pressure ¹⁰. But vapour soon forms in the veins too, and within a minute the blood stops circulating usefully ^7,4.

Houston, December 1966 3:19

So how do we know any of this for people? Partly from an accident ⁷. On the fourteenth of December 1966, at NASA's Manned Spacecraft Center in Houston, a test subject in a pressure suit was inside a vacuum chamber ⁷. A connection on the hose feeding his suit came apart ⁷. According to a report written for NASA two years later, the pressure in his suit fell to almost nothing in about half a second ⁷.

The water on his tongue 3:49

He later said he could feel and hear the air leaking out ¹. His last conscious memory was of the water on his tongue beginning to boil ¹. He lost consciousness after twelve to fifteen seconds ^7,1. Within about twenty seconds his suit and the chamber were being refilled, and he came round as the pressure rose ^7,1. Afterwards, his eardrums were reddened, but his heart, lungs and brain tests came back normal ⁷. For a few days his sense of taste was dulled, then it came back ⁷.

The fifteen-second clock 4:21

Why did he have about fifteen seconds, and not one? With no air in your lungs, the blood flowing through them picks up no oxygen ¹. Worse, oxygen leaks out of the blood into the empty lungs ¹. But your brain is still running on blood that left your lungs a few seconds earlier ¹. NASA's account says fourteen seconds is roughly how long that oxygen-poor blood takes to reach the brain ¹. NASA's Bioastronautics Data Book expects some consciousness for nine to eleven seconds, and says it's very unlikely anyone would have more than five to ten seconds to help themselves ⁴.

Space is a thermos 5:01

And freezing solid? Space has almost no matter to touch you, so it can't pull heat away the way cold water does ^1,10. That's the same trick a thermos uses, though a real thermos also silvers its walls to stop heat glowing across the gap ^10,11. Your skin can only lose heat by glowing in the infrared and by water evaporating ^4,12. Our calculation, for bare skin in shadow, comes to about a thousand watts ¹². That cools a seventy-kilogram body by only about a fifth of a degree per minute ^12,13. The exception is wet surfaces, where evaporation can chill the mouth and nose towards freezing ⁴.

Mars is thin enough 5:42

Back to Total Recall, which isn't even in a vacuum ¹⁴. Mars has air, but it's very thin: a few hundred pascals near the surface, about seven hundred by one NASA model ¹⁴. The boiling line for body water is about nine times higher than that ^8,14. So for this question, Mars's air might as well be a vacuum ^4,14. The film gets one thing right: a body out there really would start to swell ⁴. Popping eyes and stretching, distorting faces are where it leaves the physics behind: tissue swells, but it doesn't burst ^4,1.

Motors, rubber and balloons 6:18

And they did it on purpose. The Los Angeles Times reported in 1990 that the effects artist Rob Bottin and director Paul Verhoeven planned exactly how the decompression should look ¹⁵. Stung by criticism of the violence in RoboCop, they chose against gory bursts of blood ¹⁵. Instead they went for flesh stretching and malforming, built from tiny motors, rubber muscles, cables and little inflatable balloons ¹⁵. It's a horror effect, built on a real one: swelling ^4,15.

How long, really? 6:53

So what's the real timeline? Nobody has measured it in people, so the Data Book reasons from animal studies of the 1960s ⁴. In one series, none of the dogs held at near-vacuum for up to ninety seconds died, but beyond two minutes, deaths were frequent ⁷. Some chimpanzees recovered after as long as three and a half minutes ⁷. For a person, the Data Book judged that, with help at hand, getting back to a tolerable pressure within sixty to ninety seconds could mean survival, though, in its words, 'there is, of course, no guarantee' ⁴.

Survivable, sometimes 7:30

A more recent review points the same way ⁹. A 2013 review in the journal Aviation, Space, and Environmental Medicine found the research on ebullism, in people and animals, extremely limited ⁹. But it concluded that the injuries are predictable, often treatable, and that many exposures may be survivable ⁹. The man in Houston was in near-vacuum for around twenty seconds, and he recovered ⁷. That's the gap between the films and the reality: seconds matter, but they are seconds, not an instant ^4,1.

Squeeze, not air 8:07

Here's a neat clue about what really protects you. The Data Book reports that a properly fitted elastic garment can entirely prevent ebullism, at pressures as low as about a third of the boiling line ⁴. The researcher Paul Webb called his version an elastic leotard for spacewalks ¹⁶. It works because the problem is the missing squeeze, not the missing air ⁴. You still need oxygen to breathe, but your skin just needs something pushing back ⁴.

The film that got it right 8:38

Which brings us to the film that got it right. In 2001: A Space Odyssey, from 1968, the astronaut Dave Bowman is outside the ship without his helmet ¹⁷. To get back aboard, he jettisons the door of his small pod and is propelled by the escaping air across the vacuum into the airlock ¹⁷. In 1997, someone asked NASA whether he should have blown up ¹. NASA's answer: you do not explode ¹. Geoffrey Landis, a scientist at NASA's Glenn Research Center, writing on his own website, put it simply: Clarke got it about right ¹⁰.

Why it works 9:17

What the scene gets right is the idea ¹⁰. A few seconds in vacuum is a crisis you can act through, not an instant death ⁴. You have a handful of seconds of useful consciousness, and you don't explode or freeze ^4,1. Arthur C. Clarke, who wrote the film with Stanley Kubrick, made the same point in an essay: a man would not explode ^2,17. The physics doesn't spoil the scene; it makes it tenser.

Why films choose otherwise 9:46

So why do most films go bigger? A person quietly swelling and fainting doesn't read on screen as danger in the way an exploding body does. Total Recall's makers, after criticism of RoboCop's violence, chose a stretching, rubbery transformation over blood ¹⁵. And as Ebert said, the physics hardly mattered to that film ². Films bend the rules for a reason; our job is just to know where they bent.

What really happens 10:17

So, thrown out of an airlock? The air rushes out of your lungs ⁴. The water on your tongue begins to boil, and your body starts to swell ^4,1. You stay conscious for something like ten to fifteen seconds, and nothing explodes or freezes ^4,1. After that, it's a race against the clock that only a quick rescue can win ⁴. Next time, from tiny people to giant ones: could a giant monster even stand up?

Sources

Every factual claim in the episode is tied to one of these. Spotted an error? Tell us.

1. NASA Goddard, Imagine the Universe!, Ask an Astrophysicist, "Human Body in a Vacuum"… — web.archive.org
2. Roger Ebert, "Total Recall" (review), RogerEbert.com, 1 June 1990, archived copy — web.archive.org
3. Wikipedia, "Total Recall (1990 film)" (plot and production context; orientation) — en.wikipedia.org
4. C. E. Billings, "Barometric Pressure", chapter 1 of Bioastronautics Data Book, 2nd… — ntrs.nasa.gov
5. NASA Glenn Research Center, "Earth Atmosphere Model - Metric Units" — grc.nasa.gov
6. R. Nave, HyperPhysics, "Bulk Elastic Properties" (Georgia State University) — hyperphysics.phy-astr.gsu.edu
7. Emanuel M. Roth, "Rapid (Explosive) Decompression Emergencies in Pressure-Suited… — ntrs.nasa.gov
8. NIST Chemistry WebBook, SRD 69, "Water: Phase change data, Antoine Equation… — webbook.nist.gov
9. D. H. Murray, A. A. Pilmanis, R. S. Blue et al., "Pathophysiology, prevention, and… — pubmed.ncbi.nlm.nih.gov
10. Geoffrey A. Landis, "Human Exposure to Vacuum", personal web page (revised 7 August… — geoffreylandis.com
11. Wikipedia, "Vacuum flask" ("Heat transfer by thermal radiation may be minimized by… — en.wikipedia.org
12. R. Nave, HyperPhysics, "Radiation: Cooling Mechanisms for Human Body" — hyperphysics.phy-astr.gsu.edu
13. R. Nave, HyperPhysics, "Table of Specific Heats" — hyperphysics.phy-astr.gsu.edu
14. NASA Glenn Research Center, "Mars Atmosphere Model - Metric Units" — grc.nasa.gov
15. Torene Svitil, "Rob Bottin: A Wizard in the World of Special Effects", Los Angeles… — latimes.com
16. Paul Webb, "The space activity suit: an elastic leotard for extravehicular activity",… — pubmed.ncbi.nlm.nih.gov
17. Wikipedia, "2001: A Space Odyssey" (plot section, for the airlock scene; cross-checked… — en.wikipedia.org

Researched and scripted with AI assistance, fact-checked claim by claim, with synthetic narration and diagrams drawn in code. How we make episodes.