Sometimes smog can be trapped over cities for days.
History is littered with examples of the hundreds of thousands who have perished in such a terrible way. The deadliest sort of cloud is called the nuee ardentes at temperatures of up to 450 celsius, and 30,000 people died on Martinique in 1902 when Mt Pelee collapsed explosively.
Perhaps you think that the white line you see behind a jet plane is smoke. Actually, it is not smoke, but water vapour.