Dangers of a Vacuum

The vacuum chamber that Jim LeBlanc was in
when his spacesuit lost all pressure.

Recently, a reader asked “What happens to the human body in a vacuum? For example, if an astronaut removed his space suit.”
   This reminds me of a scene from the movie 2001: A Space Odyssey. In the movie, HAL has figured out that Dave is planning to disconnect him when he returns to the ship, so he refuses to let Dave back in. Dave is forced to go in through the unpressurized emergency airlock, but there’s a problem: he doesn’t have his space helmet. 

  Terrifying, but Kubrick got the science right. Short-term exposure to the vacuum of space would not make your body explode or freeze solid as some movies have depicted. If you don’t try to hold your breath, exposure to space for about 15 seconds would cause no permanent injury. Holding your breath would be bad, though, because in a vacuum your lungs collect gas from your bloodstream and expands with the drop in pressure. Holding your breath would cause your lungs to overinflate and possibly rupture. This is similar to how scuba divers need to exhale when rising to the surface or risk damaging their lungs. 
   Temperature would not be an immediate problem because although space is very cold, a vacuum is a perfect insulator. You would only gradually radiate away your body heat. Exposure to direct sunlight would give you a sunburn. Your saliva and tears would quickly evaporate and you might have eardrum troubles.
   After about 15 seconds, oxygen-deprived blood from the lungs reaches the brain causing you to lose consciousness. 
   At such low pressures, your body fluids will boil away. Moist surfaces such as the eyes, mouth and airways experience this immediately. Fluids inside your body also start to vaporize. This happens rapidly in the lungs and under the skin. Bubbles of water vapor that form in the bloodstream will interrupt the circulation. This is called ebullism. No one knows how long the human body can withstand the vacuum of space—perhaps a couple of minutes. 
   In 1965, this actually happened to Jim LeBlanc while working at the NASA Manned Spacecraft Center (now called the Johnson Space Center). He was testing a space suit in their vacuum chamber when the tube that was pressurizing his suite came loose and his suit was almost completely depressurized within seconds. He stayed conscious for about 14 seconds and they began repressurizing the chamber right after he passed out. After regaining consciousness, he recalled that he could hear and feel the air leaking out of his suit, and the last thing he remembered was the saliva on his tongue starting to boil.