Mithcha said:
They must've done more than simply hold their breath surely. Stood in a particular spot, carried some sort of bag, did a voodoo dance, something.
This is an example of a mazuku, a place where CO2 emissions from magma accumulate in low-lying areas. The name comes from East Africa, where there are many mazuku filled with the bones of animals who died in them, as well as those of the scavengers who went in after first victims.
Mammoth Mountain in California also does this. There's a place called Horseshoe Lake there, and it looks and feels like Mordor annexed it recently. All the trees are dead and there is next to no animal life because of the CO2. People have nearly died there because of it.
Lake Nyos, also in Africa, is a lake that occupied a mazuku, and occasionally releases massive amounts of CO2 all at once like a bursting can of soda. One of these eruptions was large enough that the CO2 flowed downhill and displaced the oxygen in a village, killing most of the people there without them realizing what was happening.