You can drop lots of hints, a perception check to notice the flame of their torches has diminished and is threatening to be extinguished. They start making con saves every 2 minutes, if they fail they get dizzy, subsequent failed saves cause headaches, then vomiting. Give them heal or wisdom checks for a chance to determine what’s happening. After 3 failed checks you use the rules for holding your breath and party members with the lowest constitution begin to lose consciousness. Seems fair