I've got an ongoing run where the players are currently at the lowest level of an underground research facility. The hallways, which they have yet to enter, are currently flooded with Halon gas (or equivalent), and the air within will be completely devoid of oxygen as a result.
Does anyone know of any existing rules for asphyxiation, and holding one's breath? I'm hoping they get the hints I've laid out that the corridors might not be as hospitable as they'd expect, but they might forge on without careful evaluation of the situation...