You might have never heard of the Great Smog of 1952, or you may have been introduced to it by an episode of The Crown on Netflix, like I was. While London is notoriously foggy, it was infinitely worse than usual on December 5, 1952, when a combination of fog and air pollution reduced visibility to only a few feet.
The smog smothered the city for five days. Transportation came to a virtual standstill. Flights were grounded and trains cancelled. Only the Underground was running. Ambulance services were affected, leaving people to find their own way to hospitals. The fog even seeped indoors though windows and doors. Plays and concerts were cancelled because the audience were unable to see the stage.
Remarkably, there was no panic as Londoners were accustomed to fog. But the death toll increased. Most of the victims were the very young and the elderly with pre-existing respiratory problems. Estimates of how many people died during that period vary from 6,000 to as high as 12,000.
The smog was produced by a combination of weather, geography, and several sources of air pollution. Read about the factors that produced the smog and what's been done about them at Amusing Planet.