A character can hold their breath for 12 seconds (2 rounds in combat) per Endurance Bonus they have. This duration is doubled if the character does nothing but hold their breath (no action or movement allowed). For every round beyond the character’s Endurance Bonus holding their breath, the character gains a temporary level of fatigue until they die or regain access to fresh air, after which the temporary fatigue is removed at a rate of 1 per round.