It's really dangerous to be exposed to space without wearing a spacesuit.
In fact, NASA has provided relevant information on this: http://imagine.gsfc.nasa.gov/docs/ask_ astro/answers/970603.html
These results have been confirmed by animal experiments.
Here is a general description of the conclusion
1. Low temperature
In the shadow area, the temperature in space is really very, very low, even close to absolute zero. But in fact, because there are very few molecules in space, there is very little heat that can be carried away by these molecules. From a thermal point of view, vacuum is more like a good insulation, like a super down jacket. This is the least of many threats to outer space.
2. High temperature
If, unfortunately, it is facing the sun, then the strong radiation without atmospheric filtration will cause serious sunburn in a short time, which can be said to be one of the most serious threats. Within 10 seconds, sunburn can be mild and reversible. However, if the stay time is a little longer, the intense ultraviolet radiation will produce a serious and almost irreversible burn to the human body.
3. Hypoxia
It is well known that there is no oxygen in outer space. Exposure to space can take about 15 seconds to move (roughly equivalent to the time required for oxygen to reach the brain from the lungs). However, beyond this time, the lack of oxygen will bring about a decline in mobility, to the loss of consciousness, which will make self-help extremely difficult.
4. Low voltage
Low pressure is undoubtedly the most obvious threat. The body fluids and saliva will start to boil, and the skin will begin to swell and ache. If you hold your breath, the high pressure in your lungs can damage your lungs. Low pressure can also cause the water in the venous blood to vaporize, making the venous blood pressure rise, and even exceed the arterial blood pressure, leading to blood circulation stop. The gas will be discharged from the lungs, rapidly expand and absorb heat at the mouth and nose, which will make the temperature drop quickly.
After decompression, if the pressure can be rapidly increased in a short time (1 minute - 90 seconds), then life will not be seriously threatened. But if you can't boost the pressure in time, you can only say "goodbye, my friend.".