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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?

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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.

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    \$\begingroup\$ I am not sure I would allow my players to suicide when Polymorphed unless they had a method to be absolutely sure that is what happened to them. Most of the time RP wise it would be a huge panic to be turned into a fish and start suffocating! Good answer though. \$\endgroup\$ – SeriousBri Feb 6 at 8:00
  • \$\begingroup\$ I don't disagree, necessarily, though I'd say getting turned into a fish should be a strange enough situation that I'm not sure your instinct would be to take a big breath and hold it right before the spell takes hold. Mostly I was just pointing out that "you immediately start suffocating" is actually not a bad outcome in that particular situation. \$\endgroup\$ – Darth Pseudonym Feb 6 at 12:37
  • \$\begingroup\$ In the real world: Even with "empty" lungs, you don't suffocate immediately. You have blood in your blood stream. By volume blood is ~20% oxygen (STP); lungs are 6 liters (and also ~20% oxygen) and blood is about 5 liters. My point is, lungs help, but if you exhale immediately and go under water you don't immediately suffocate. As the game places no requirements on breathing (or any action really) prior to holding your breath, and your rulings/suggestions above don't line up with real life, I'd suggest fixing them. Average con, put head under water, KO in 6 seconds? Just wrong. \$\endgroup\$ – Yakk Feb 7 at 14:03
  • \$\begingroup\$ Nor is it realistic that you can put your face in water, breathe out, and have a full minute of useful activity before you start getting in trouble. Your average untrained person might last a full minute underwater with a deep inhale first. But also, keep in mind that if you're really calculating rounds underwater, it means there's a threat involved that you're actively dealing with. It isn't the "being very relaxed and not moving" thing you do to see how long you can hold you breath. Go underwater, breathe out, and try to swim, see how long that lasts... it's not long. \$\endgroup\$ – Darth Pseudonym Feb 7 at 14:45
  • \$\begingroup\$ Anyway the point is "what will be fun" not "what will be realistic". If you always get to start with "holding my breath", even your most incompetent character gets a full 11 rounds underwater, which is long enough for most combat situations to be resolved. In that case, you might as well never even think about how long your breath lasts. My ruling is based the idea that suffocation should at least occasionally be a threat. \$\endgroup\$ – Darth Pseudonym Feb 7 at 14:48
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It's up to the DM

There are no rules about that, so [un]fortunately it is up to the DM to decide what happens. And that's a good thing! Allowing the DM to craft their story, environment, and events are what roleplaying is all about. You can set the stage for what happens and how dangerous it is. That freedom is a feature, not a bug :)

When deciding, I would recommend staying consistent so that your players understand how things work and will work in the future.

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As other answers have indicated, it's up to the GM, but here's some guidance on how the GM can decide...

Note that for a real-life human, "falling into water" and "falling into unbreathable air" are very different situations, due to the mammalian diving reflex. A person falling into water (even at the end of an exhale) will reflexively hold their breath, saving at least some oxygen... whereas a person falling into, say, pure carbon dioxide will probably exhale and go straight to suffocating unless they consciously know what is happening and choose not to exhale.

For that reason, I would rule that any remotely human-like player automatically holds their breath when surprised by a liquid, but goes straight to suffocating when surprised by an unbreathable gas.

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Yes

The rules use the word “can”, not “may” so this is a capability you always have, not one you choose to engage.

For suffocation, the amount of air in your lungs is (almost) irrelevant - what matters is the amount of oxygen in your blood. For a healthy person, like an adventurer, the blood is 99 to 100% saturated with oxygen at all times so there is a lot of oxygen available and when you can’t breathe stress hormones reduce blood flow to non-essential body parts reserving the bulk of this oxygen for the brain. It is notable in this context that free divers’ pre-dive breathing techniques are intended to remove as much air from the lungs as possible.

Therefore, for game purposes there is no need to make a distinction.

Parenthetically, a suffocation challenge in the game is unlikely to be part of combat - it takes a minimum of 7 rounds to reach unconsciousness and I can count on one hand the number of combats I’ve been in that last that long.

I would also like to note that there are circumstances where a person doesn’t know, consciously or unconsciously that they should hold their breath. Being immersed in a fluid or exposed to noxious gasses does. However, insufficient oxygen or many toxic gasses does not.

Our breathing is triggered by a buildup of CO2, not a deficiency of oxygen; if we are successfully expelling the CO2 we will not notice the lack of oxygen until we pass out unless we are trained to notice the cognitive signs like fighter pilots are.

Similarly, many toxic gases are odourless, for example CO and H2S. H2S or rotten egg gas can only be detected at low concentrations; at toxic levels (which are quite low) it is completely odourless. Of course, these should use the poison rules, not the suffocation rules.

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    \$\begingroup\$ "low concentrations" looks like a typo to me; did you mean "high concentrations"? \$\endgroup\$ – Brilliand Feb 6 at 22:08
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A possible simple solution:

You're either breathing in or out (maybe a 50/50 chance of either) at the time of transfer.

Out: If out, you have a second or two to react to the new environment. The GM describes the environment, emphasising the air situation ("You're in a room full of thick black smoke") and either it's up to you to tell him you hold your breath, or maybe if the GM sees fit, you get some sort of Wis or Dex check to do it before your next inhalation. If you fail, proceed to "In".

In: You're not holding your breath (unless you're in a vacuum), you're coughing up whatever you've accidentally breathed in. If it's poisonous, you've probably been affected by the poison. Maybe another sort of Dex or Wis check to avoid taking another breath (harder than you think after you've just been through a coughing fit - I'd leave that to the GM's opinion)

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