when I first saw this question on SciShow, I was like, hmm OK, this is interesting.. maybe you will freeze or what?
Just ask anybody, anybody who does not study physics and they will say to you that person exposed to vacuum of space is going either to: explode, freeze or their blood is going to boil. Those people are right in only one thing and that is that you will die horribly. And that is why I like this question so much, it is full of misconceptions!
So last time I talked about vacuum and how it is not really empty but that is not important right now. We will talk about vacuum of interstellar space. This space is not absolute vacuum, there are still hydrogen and other atoms flying around but not much of them.
So lets get to first answer that somebody would maybe say. Person is going to explode because the the pressure in vacuum wants to equalize so you are going to be ripped apart in drastic death. I mean, imagine the person ripped to pieces and all the blood floating in very small balls all around, this simply too awesome.
So why wont it happen? Well the force of the vacuum is not strong enough. You would probably feel some pain as your tissue would be stretched a little bit, but humans are not so fragile so they wont explode.
At first I thought that person would freeze.. but watch out this is probably the greatest misconception about temperature.
What is that space between two parts of window? Well you have probably heard that it is very good vacuum. Why? Well you need to keep your house isolated so what happens is that the heat or cold air from outside will only not get inside because there is nothing that could transfer this heat!
Temperature is only the movement of particles and when there is none there is none temperature. This means that you would be perfectly isolated and that means to feel pretty warm!
Also people say that your blood would boil. Well first, why does not it boil when you are in space suit? Because the pressure inside space suit is separated from the pressure outside. This is also true for your blood circuit, it is separated from the vacuum of space. BUT what is not separated is your saliva in your mouth which would boil right away.
When we are talking about what is closed system and what is not, well your lungs are connected to the outside world, all of their oxygen would get out of you immediately. Whole circuit of blood would run “backwards” because that blood running through your lungs would lose all the rest of the oxygen instead of receiving it.
So the thing which would kill you would be the lack of oxygen. You would pass out in 15 seconds and in two minutes you would be dead.
Meanwhile you would get awesome amount of radiation and really cool sunburns which would cause probably even better cancer.
Once during some test for astronaut, the space suit was not working properly and the tester was exposed to artificial vacuum. He passed out, and his last thought was that his saliva was actually boiling, that must be very strange!