Astronauts need space suits to stay alive. You could only last 15 seconds without a space suit, you would die of suffocation or you would freeze to death. If there is any air left in the lungs, they will rupture. A human has fifteen seconds of oxygen in their body.
This means that once your body loses oxygen, you're likely to lose consciousness. That said, the WORST thing a human can do in space without a space suit would be to hold their breath. Due to the lack of pressure in space, the oxygen located inside the lungs would expand and cause the lungs to rupture, inherently causing death. If your body were locked in a space suit, it would decay, but only as long as the oxygen lasted.
Without it, a spacewalker would suffocate due to the lack of breathable air and would suffer from boiling, in which a reduction in pressure causes the boiling point of body fluids to drop below normal body temperature. For example, a study conducted in 1965 by researchers at Brooks Air Force Base in Texas showed that dogs exposed almost to vacuum at a pressure of up to three hundred and eighty parts of the atmosphere at sea level for up to 90 seconds always survived. When it comes to certain death in a sci-fi plot, being expelled into the vacuum of space is more than just a safe thing to do. In the new sci-fi movie Sunshine, an astronaut named Mace must leave his spaceship without a protective suit.
This could provide some protection against temperatures in space that can range from minus 200 to 200 degrees Fahrenheit. For example, in 1965, a technician who was inside a vacuum chamber at the Johnson Space Center in Houston accidentally depressurized his space suit when he broke a hose. When it comes to exposure to the interstellar medium, you might survive with the right help, but it's probably not to your liking. If someone, like me, has wondered what would happen to a human who is in outer space without a space suit, there are many different possible outcomes.
A NASA test subject who survived an accident in 1965 in which he was exposed to conditions close to a vacuum felt the saliva on his tongue start to boil before losing consciousness after 14 seconds. With a push from a mutinous lieutenant out of the lock or a fierce tear in a space suit, the average victim of a film is guaranteed to die quickly and silently, albeit with fewer body parts that explode than the screenwriters would have you believe.