Suffocation Damage is a special type of damage that occus when normal breathing is cut off. Every turn a character spends without air, they take Fatigue Damage equal to the rank of whatever is causing them to suffocate (a poison gas, being choked, having had their throat slit, etc.). If they are in an environment with no air, they simply take 1 Fortitude Damage per turn per rank of "oxygenlessness" (with 4 being absolutely none). As soon as oxygen is returned to the character they stop taking this damage.
If a character falls unconscious, they can be given CPR to start breathing again, which requires a successful First Aid roll. Otherwise, they will continue losing Fortitude until they die.