A character may hold his breath for a number of rounds equal to twice his Constitution score. At the start of each round thereafter, he must make a Fortitude save (DC 10 + 1 per previous save). With failure, his vitality points drop to 0 (if he’s a player or special character), or he falls unconsciousAn unconscious character is helpless and may take no actions. A character loses this condition after 2d4 full hours of sleep. (otherwise). At the start of his Initiative Count during the following round, the character must start breathing if he’s able. If not, his wound points drop to 0 (if he’s a player or special character) and the character begins to die regardless of his type.

Some gear provides a dedicated air supply that staves off suffocation in airless environments.

Spycraft 2.0
Characters - Skills - Feats - Gear - Combat - Dramatic Conflict