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As far as certain death in a science fiction plot line goes, being ejected into the vacuum of space is more than a pretty sure thing. A shove out of the air lock by a mutinous lieutenant or a vicious rip in a space suit, and your average movie victim is guaranteed to die quickly and quietly, though with fewer exploding body parts than screenwriters might have you believe.
In reality, however, animal experiments and human accidents have shown that people can likely survive exposure to vacuum conditions for at least a couple of minutes. Not that you would remain conscious long enough to rescue yourself, but if your predicament was accidental, there could be time for fellow crew members to rescue and repressurize you with few ill effects.
"In any system, there is always the possibility of equipment failure leading to injury or death. That's just the risk you run when you are in a hostile environment and you depend upon the equipment around you," says Dartmouth Medical School professor and former NASA astronaut Jay Buckey, author of the 2006 book Space Physiology. "But if you can get to someone quickly, that is good. Often spacewalks are done with two spacewalkers and there is continuous communication. So if someone is having a problem, hopefully the other can go get them and bring them in."
Vacuums are indeed lethal: Under extremely low pressure air trapped in the lungs expands, tearing the tender gas-exchange tissues. This is especially grave if you are holding your breath or inhaling deeply when the pressure drops. Water in the soft tissues of your body vaporizes, causing gross swelling, though the tight seal of your skin would prevent you from actually bursting apart. Your eyes, likewise, would refrain from exploding, but continued escape of gas and water vapor leads to rapid cooling of the mouth and airways.
Water and dissolved gas in the blood forms bubbles in the major veins, which travel throughout the circulatory system and block blood flow. After about one minute circulation effectively stops. The lack of oxygen to the brain renders you unconscious in less than 15 seconds, eventually killing you. "When the pressure gets very low there is just not enough oxygen. That is really the first and most important concern," Buckey says.
But death is not instantaneous. For example, one 1965 study by researchers at the Brooks Air Force Base in Texas showed that dogs exposed to near vacuum—one three-hundred-eightieth of atmospheric pressure at sea level—for up to 90 seconds always survived. During their exposure, they were unconscious and paralyzed. Gas expelled from their bowels and stomachs caused simultaneous defecation, projectile vomiting and urination. They suffered massive seizures. Their tongues were often coated in ice and the dogs swelled to resemble "an inflated goatskin bag," the authors wrote. But after slight repressurization the dogs shrank back down, began to breathe, and after 10 to 15 minutes at sea level pressure, they managed to walk, though it took a few more minutes for their apparent blindness to wear off.
However, dogs held at near vacuum for just a little bit longer—two full minutes or more—died frequently. If the heart was not still beating upon recompression, they could not be revived and the more rapid the decompression was, the graver the injuries no matter how much time had elapsed in the vacuum.
Chimpanzees can withstand even longer exposures. In a pair of papers from NASA in 1965 and 1967, researchers found that chimpanzees could survive up to 3.5 minutes in near-vacuum conditions with no apparent cognitive defects, as measured by complex tasks months later. One chimp that was exposed for three minutes, however, showed lasting behavioral changes. Another died shortly after exposure, likely due to cardiac arrest.
Although the majority of knowledge on the effects of vacuum exposure comes from animal studies, there have also been several informative—and scary—depressurization accidents involving people. For example, in 1965 a technician inside a vacuum chamber at Johnson Space Center in Houston accidentally depressurized his space suit by disrupting a hose. After 12 to 15 seconds he lost consciousness. He regained it at 27 seconds, after his suit was repressurized to about half that of sea level. The man reported that his last memory before blacking out was of the moisture on his tongue beginning to boil as well as a loss of taste sensation that lingered for four days following the accident, but he was otherwise unharmed.
When it comes to exposure to the interstellar medium, you might survive it with timely help but it probably won't be to your taste.