I play a 3rd-level Air Genasi Monk. My party will be fighting a Green Dragon soon.

Can a Green Dragon's Poison Breath affect me if I'm not breathing due to the Air Genasi's Unending Breath trait?

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    \$\begingroup\$ Howdy howdy! Welcome to our stack! Please take the tour to learn more about how we operate and you also visit the help center for more info. \$\endgroup\$ – NautArch Oct 8 '20 at 16:08

No, holding your breath won't save you

The language of the ancient green dragon's Poison Breath is (MM, p. 284; emphasis mine):

The dragon exhales poisonous gas in a 90­-foot cone. Each creature in that area must make a DC 22 Constitution saving throw, [...]

All it says is that there is a poisonous gas, and anyone in its cone is affected. If holding your breath were an option to save you from the breath weapon, it would provide that caveat.

For a counter-example, please note the language in the description of the dust of sneezing and choking (DMG, p. 166-167; emphasis mine):

When you use an action to throw a handful of the dust into the air, you and each creature that needs to breathe within 30 feet of you must succeed on a DC 15 Constitution saving throw or become unable to breathe, while sneezing uncontrollably.

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    \$\begingroup\$ As far as flavor goes, it's worth noting that inhalation is merely one way a gas can be harmful, e.g. mustard gas causes burns on skin. \$\endgroup\$ – ChumpNicholson Oct 8 '20 at 19:07
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    \$\begingroup\$ @ChumpNicholson I was debating about adding other poison gas types, but it seemed like a rabbit hole I didn't need to drop into. \$\endgroup\$ – NautArch Oct 8 '20 at 19:38
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    \$\begingroup\$ @sharur: Good question, but AFAIK mustard gas is still causing a biological reaction (en.wikipedia.org/wiki/…), not something that would etch metal, and the biological reaction isn't just acidity (or caustic alkaline, which D&D would class under acid damage I'm sure). So yes it makes sense that poison immunity would still apply, not acid immunity, e.g. for a warforged in mustard gas. Wiki does say it's also corrosive, though, separate from poisonous. But in terms of damage to creatures, mostly via biological mechanisms. \$\endgroup\$ – Peter Cordes Oct 9 '20 at 8:10
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    \$\begingroup\$ @sharur: The term chemical burns does get used, but that wiki article clarifies that that can be either from strong acid / base (D&D acid damage) or cytotoxicity, which being biological is pretty clearly D&D poison damage. The Mustard Gas article is pretty clearly talking about cytotoxicity, so yes, poison damage. Good point about contact poisons that can be absorbed through skin, though. \$\endgroup\$ – Peter Cordes Oct 9 '20 at 8:15
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    \$\begingroup\$ Old school green dragons actually breathed a cloud of chlorine gas, which is both toxic and corrosive. \$\endgroup\$ – barbecue Oct 9 '20 at 14:31

At best, advantage.

The poison gas doesn't require you to breath. However, as a DM, feel free to grant the Air Genasi advantage.

Note that poison gas quite often can harm people who don't breath; mustard gas, for example, causes the skin to blister (and not from acid).

Contact poisons exist. This being a dragon... we can assume that poison is poisonous in every way.0

However, your lungs are much less well defended against stuff like poison (they are more permeable; human skin really is a layer of armor), so not having to breathe would be an advantage. Which is sort of what advantage is for; when the circumstances would be more to your advantage than normal, you give advantage on the check.

As a player, you can ask for advantage, but the DM is under no obligation to give it to you. Asking for immunity would be a bit much.

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    \$\begingroup\$ Couldn't everyone always get advantage then by holding their breath? \$\endgroup\$ – NautArch Oct 9 '20 at 16:56
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    \$\begingroup\$ @NautArch I'd argue that is what the saving throw would be about (to some degree). You are engaged in vigorous combat, stopping yourself from breathing is hard and (honestly) would be crippling, unless you don't need to breathe. "Holding your breath" in 5e doesn't imply you are impaired while you do (nor does it say you work at full capacity), but in the real world you really are. \$\endgroup\$ – Yakk Oct 9 '20 at 17:35
  • \$\begingroup\$ That doesn't sound like something you should get advantage for then. \$\endgroup\$ – NautArch Oct 9 '20 at 17:44
  • \$\begingroup\$ The DMG section on poisons even goes as far as to say that "Holding one’s breath is ineffective against inhaled poisons, as they affect nasal membranes, tear ducts, and other parts of the body", though admittedly in that context it is talking about purchasable poisons available to the players, not all poison generically. \$\endgroup\$ – Carcer Oct 11 '20 at 9:05

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