For about ten full seconds– a long time to be loitering in space without protection– an average human would be rather uncomfortable, but they would still have their wits about them. Depending on the nature of the decompression, this may give a victim sufficient time to take measures to save their own life. But this period of “useful consciousness” would wane as the effects of brain asphyxiation begin to set in. In the absence of air pressure the gas exchange of the lungs works in reverse, dumping oxygen out of the blood and accelerating the oxygen-starved state known as hypoxia. After about ten seconds a victim will experience loss of vision and impaired judgement, and the cooling effect of evaporation will lower the temperature in the victim’s mouth and nose to near-freezing. Unconsciousness and convulsions would follow several seconds later, and a blue discoloration of the skin called cyanosis would become evident.
At this point the victim would be floating in a blue, bloated, unresponsive stupor, but their brain would remain undamaged and their heart would continue to beat. If pressurized oxygen is administered within about one and a half minutes, a person in such a state is likely make a complete recovery with only minor injuries, though the hypoxia-induced blindness may not pass for some time. Without intervention in those first ninety seconds, the blood pressure would fall sufficiently that the blood itself would begin to boil, and the heart would stop beating. There are no recorded instances of successful resuscitation beyond that threshold.