History: Great Smog Of '52, London, England, United Kingdom

Most of the deaths were caused by respiratory tract infections from hypoxia and as a result of mechanical obstruction of the air passages by pus arising from lung infections caused by the smog. The lung infections were mainly bronchopneumonia or acute purulent bronchitis superimposed upon chronic bronchitis.
More recent research suggests that the number of fatalities was considerably greater, at about 12,000.