in general gas density rises in proportion to its molecular weight for the same temperature and pressure
the following assumes same temperature and pressure (except for the hot wood fire smoke which is compared to normal ambient air temperature)
hot carbon monoxide, like smoke, coming from a stove or heater will initially rise hence smoke alarms and CO alarms should generally be mounted near a ceiling for the earliest warning, later both will mix with air as they cool to air temperatures and smoke may even sink
underground coal miners are at major risk from:
pools of carbon dioxide (“black damp”) as it forms from human respiration, the rotting of timber supports, or the slow oxidation of coal
carbon monoxide poisoning via either:
methane asphyxiation “firedamp” from methane trapped within coal seams and rock strata that bleed out when mined
hydrogen sulphide “stinkdamp” produced by the decomposition of iron pyrites in the presence of moisture or organic breakdown
NB. fog tends to lie close to ground not because of intrinsic gas density but because it is colder and thus more dense