How do you determine when a character or creature is able to hold their breath? It's pretty easy when a character knowingly jumps into water or buries their head in the sand, but say they are unexpectedly plunged underwater, or an enemy throws down an alchemical item that turns out to be a dead air vial. Do the characters get to hold their breath, or do they immediately begin suffocating? (Meaning everyone in the area of that dead air vial will all be dead in 3 rounds--for a mere 200gp!)
The rules for holding your breath are under the Swim skill. They are repeated almost exactly in the section on Suffocation. There's also a reference under the race guide to a racial trait Hold Breath that lets you hold your breath longer than usual. There's a monster ability that lets you hold it even longer. There's a fairly useless rogue talent that just adds two rounds. There's a human alternative racial trait, Heart of the Sea that lets them double the amount of time they can hold breath.
The alchemist has an archetype (Internal Alchemist) that lets him hold his breath for minutes rather than rounds and, with preparation, for hours called Breath Mastery. This is the only place I've found where any sort of preparation for holding your breath is mentioned.
The swim skill rules say that if you're underwater, you MUST hold your breath, but that does not mean that you can't hold your breath if you aren't underwater. They don't say anything about any sort of action needed to prepare to hold your breath.
Therefore, by RAW, you can hold your breath any time you want. What you're looking to simulate is when you can't get a good lung-full of air before starting to hold your breath. There is currently no rule that I know of for that.
So how simulationist do you want your house-rule to be ? You could institute a reflex save to unexpectedly hold your breath (failure means you're treated as if you'd start making con checks to hold your breath right away). Or you could just say that if you have to hold your breath unexpectedly, you only get your CON in rounds (instead of twice). Or you could roll randomly to see what percent of your 2xCON you get. Again, it boils down to how you want it to be.
Thanks for all the links. I will definitely go through those myself when I have some more time.
I am less interested in possible house rules, and more interested on what, if anything, the RAW says on the matter. I'm trying to gauge just how powerful suffocation-type attacks really are.
I am asking as both a GM and as a player I guess.
For me, the only time you can't hold your breath is when something specifically says so (because otherwise it's very powerful). The spell Suffocation includes the line that if you fail the initial save air is forcibly extracted from your lungs. Which is the main point of the issue, the way I see it is, only if the air is removed from your lungs should you be unable to hold your breath. Otherwise, I would allow players to hold their breath as a non-action (not even free because free actions require it to be your turn). Otherwise you could use items like you mentioned to incredibly devestating effect if your surprised your target.