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In London, England, on Thursday, Dec. 5, 1952, the visibility was no more than five meters. This was due to a high-pressure air mass over the Thames River Valley mixed with the sudden cold air that came in from the west.
This fog was due to other factors as well, including a culmination of coal furnaces, smoke, soot, sulfur dioxide from cars, industrial plants, and buses. London had also recently replaced an electric tram system with steam locomotives and diesel-fueled buses.
That fog mixed with pollutant smoke and created a thick smog. Between Dec. 4-8, an estimated 4,000-12,000 people died.