Suffocating

You can hold your breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds). For example, a creature with a Constitution of 14 can hold its breath for 3 minutes.

If you attack or cast any spells on a given turn, then the remaining time that you can hold your breath for is reduced by 1 round. You also lose 1 round worth of air each time that you are critically hit. If you speak (including casting spells with verbal components or activating items with command components) you lose all remaining air.

After this period of time, you must make a DC 10 Constitution (Athletics) check in order to continue holding your breath. The check must be repeated each round and the DC increases by 5. When you fail one of these Constitution checks, you fall unconscious and start suffocating. You can’t recover from being unconscious and must attempt a DC 20 Constitution defence check at the end of each of your turns. On a failure, you take 1d10 damage. On a critical failure, you die. On each check after the first, the DC increases by 5 and the damage by 1d10; these increases are cumulative. Once your access to air is restored, you stop suffocating and are no longer unconscious (unless you’re at 0 stamina).