Vacuum and Explosive Decompression
The common misconception of freezing in space is badly inaccurate - there is no air in space to absorb heat from a body, and as such a person exposed to vacuum will die of suffocation long before his body becomes an ice block. Additionally, there is the problem of explosive decompression. Unless you exhale immediately when exposed to vacuum, the air pressure contained in your lungs will literally explode.
Anytime a creature is exposed to vacuum, he must make an immediate DC15 will save to exhale, or have his lungs explode. If he passes, he follows the suffocation rules (with half the normal duration for holding his breath) as normal until he can find a source of breathable air.
If they fail, then their lungs explode and they may function for 1 round, after which they fall unconscious. . A DC30 heal check made upon the player within 10 minutes will stabilize and revive the character, and keep him alive until a suitable lung transplant can be made. Medical grade life support gear is be available to stabilize someone. (such as that found in a hospital, or a starship's medical bay)