Suffocation

Creatures need to breathe, and running out of air can be lethal in just minutes.

Holding your breath: A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum 30 seconds). A creature with CON 16 (+3) can hold its breath for 4 minutes.

Running out of air: When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum 1 round). At the start of its next turn after those rounds expire, it drops to 0 hit points and is dying. It cannot regain hit points or be stabilized until it can breathe again.

Common situations:

• Underwater without water breathing or a swim speed with gills
• Trapped in a sealed room with limited air
• Inside a cloudkill or similar gas effect (though these typically deal damage rather than suffocation)
• Buried alive in rubble or sand
• Strangled or choked by a creature or effect

Strenuous activity: Holding your breath while fighting, swimming, or running may reduce the duration. Some DMs rule that each round of combat or strenuous activity consumes 1 minute of held breath.

Spells and features that help:

• Water breathing: breathe underwater for 24 hours (ritual)
• Air bubble: creates a bubble of breathable air
• Triton ancestry: breathe water naturally
• Items like cloak of the manta ray or cap of water breathing