Suffocation occurs when a character cannot access breathable air because it is either non-existent, or has been replaced by something else (like smoke). This is also what you use for drowning.
Once a character is out of air, they will take 1 dice of physical damage for every Grace Period x 6 seconds they go without it.
Holding Your Breath
A character can hold their breath for Grace Period x6 seconds before a Medium WP Gut Check is required. They must make a Gut Check at each subsequent interval, at +1 difficulty. Failure means the character breathes in in spite of themselves, and/or starts to take suffocation damage.
Of course, a character needs a second of notice in order to be able to hold their breath - otherwise, the damage clock starts immediately.