Sure, if I want to dive down into the underwater cave or explore a mineshaft with unbreathable air, I can announce that my character holds his or her breath. I would be fine for 1 + CON modifier minutes. And holding my breath is not an action, I can just do it.
But what if the transition from safe environment to hazardous environment happens all of a sudden? For example: a) I am drunk and fall off a ship; b) I get teleported to a plane with no breathable atmosphere; c) I get polymorphed into a beast that can only breathe underwater; etc.
In that case, the character wasn’t really holding his or her breath in that moment, because it all happened so fast. Does that mean he or she is out of air immediately and will fall to 0 hit points after a number of rounds equal to his or her CON modifier? But at any given point, you would have at least some oxygen in your lungs (and blood), even without holding your breath. Is this one of these question where the answer is “There is no rule, ask your GM?”. Because I am the GM and all these soliloquies are bothersome ^^.
If I spontaneously find myself in an environment I can’t breathe in, am I automatically holding my breath?
Best Answer
You're correct -- there are no real rules about this, so it's up to the DM, but if you want some additional guidance:
I'd suggest that if you think there's a moment between realizing what's happening and becoming unable to breathe, allow a saving throw (probably Dexterity) to grab a breath and hold it before the condition that prevents breathing is in place. If they succeed the save, they get a breath and can act under the "holding your breath" rules; if they fail, they get caught out and go directly to "suffocating".
So somebody falling out of a boat might need to make only a relatively easy DC 12 save, while a darkmantle dropping on your head might be a little harder (or a save opposed by the darkmantle's attack, possibly). Getting teleported could be even harder, possibly using the caster's spell DC, or might just be too fast to save against at all -- that's up to your judgement.
As an aside, polymorph is an unusual case where most of the time it's to the target's benefit not to hold their breath, because the spell breaks if the new form dies, and suffocating is one of the faster ways to have that happen regardless of HP damage.