An Evil Wind

Wiki:

In geology, a mazuku is a pocket of oxygen-poor air that can be lethal to any human or animal life inside. The term comes from Swahili and means "evil wind." Mazukus are created when an odorless and invisible gas such as carbon dioxide accumulates in pockets low to the ground. CO₂ is heavier than air (oxygen and nitrogen) which causes it to stay close to the ground, and is also undetectable by human olfactory or most visual conditions. (Other volcanic gases such as sulfur dioxide have pungent odors and sometimes a yellowish/off white haze associated with them). Gases that form mazukus themselves simply displace the local oxygen supply, essentially creating the outdoor hazard one would expect in a confined space depleted of breathable air.