How long does it take to die in space?

Faced with a sudden loss of pressure, air leaves the lungs whether one wants it to or not; holding one's breath would likely damage the lungs. In the following seconds, body fluids form vapor bubbles, known as ebullism; skin and connective tissue hold the body together, and there is no explosion. Blood inside the vessels doesn't boil either, because the circulatory system keeps it under pressure. Cold isn't an immediate problem either, because a vacuum doesn't conduct heat.

Oxygen is what matters most. If pressure drops, gas exchange in the lungs reverses, and blood gives up oxygen instead of absorbing it; the brain remains operative for only a few seconds, after which loss of consciousness follows. If pressure is restored within that brief window, full recovery is possible: a test subject at NASA's Manned Spacecraft Center suffered a space suit leak in 1965 inside a vacuum chamber, becoming exposed to near-total vacuum; he remained conscious for about 14 seconds, and the last thing he recalled was the water on his tongue starting to boil, after which he regained consciousness. If pressure is not restored, death comes within a few minutes.

That's why all space operations are designed around pressure. Space suits maintain an atmosphere and run on pure oxygen at lower pressure, which requires purging nitrogen beforehand; airlocks depressurize slowly and in a controlled manner, and modules carry leak sensors and hatches that can be closed. A pressure loss of this kind ended up being fatal during the Soyuz 11 return: an equalization valve opened, and the cabin's air escaped in under a minute; since then, all cosmonauts wear a space suit during Soyuz launch and landing.