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Aquatic Rules / Surviving Underwater

Suffocation and Drowning

Source Starfinder #36: Professional Courtesy pg. 47
Unless a creature has a way to breathe underwater (or doesn’t need to breathe), it must hold its breath or risk drowning. A creature can hold its breath for a number of rounds equal to twice its Constitution score. If the creature takes a standard or full action, the remaining duration that the creature can hold its breath is reduced by 1 round. After these rounds have elapsed, the creature must attempt a Constitution check (DC = 10 + 1 per previous check) each round to continue holding its breath. When the creature fails one of these Constitution checks, it begins to suffocate. On the first round of suffocation, it is reduced to 0 Hit Points and becomes unconscious (but stable). The following round, it is no longer stable and gains the dying condition. On the third round, the creature suffocates and dies.

An unconscious character must begin attempting Constitution checks immediately upon losing air access (or upon becoming unconscious, if the character was conscious when they lost access to air). Once the character fails one of these checks, they are immediately reduced to 0 Hit Points and gain the dying condition. On the following round, they suffocate and die.

Certain effects, such as androids’ constructed racial trait, the life bubble spell, and the water breathing universal creature rule allow creatures to mitigate or ignore this danger.