In early December 1952, a great mass of cold air moved off the English Channel, draped itself over London like an icy comforter and then simply stayed put.

Trying to keep warm, Londoners piled extra coal into their fireplaces, sending plumes of black, sooty smoke into the air that mixed with clouds of exhaust from factories and coal-burning power plants. But instead of rising into the atmosphere and dispersing, the smoke stayed close to the ground, trapped by the cold air above.

Over the next five days, a city already famous for its smog experienced the worst air pollution it had ever seen. A thick haze hovered over the streets, penetrating homes and offices. Public transportation nearly ground to a halt, and at night the visibility was so poor that some parts of London became unnavigable. Indoor concerts were canceled because the audiences could not see the stage.

And then the smog lifted and the problems were over. Or so it seemed.

''There was no sense of drama or emergency,'' said Dr. David V. Bates, who at the time had just started work at St. Bartholomew's Hospital in London. ''It was only when the registrar general published the mortality figures three weeks later that everybody realized that there had, in fact, been a major disaster.''