Normally exposure to vacuum means you get one round of no deleterious effects, and 1d20 hit points of damage thereafter—the big random swing of damage reflecting that you're probably going to freak out and die, but there's a chance you might keep it together long enough to open the airlock.
On the G+ discussion about vacc suits, he proposed giving an additional round for each rank of Culture/Spacer, to reflect that you're accustomed with the dangers of vacuum, and know how to keep calm and survive a little bit better.
I like it!
Chris
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