In 1952, a fog swept into London, England, just as fogs had always done, but this one proved to be deadly. After five days, the fog had hospitalized more than 150,000 people and the death toll rose to 12,000. An uncounted number of animals in London also died. This lead to all sorts of speculation, but scientists said it was simple air pollution. England passed its Clean Air Act and while London remains foggy, there have been no poisonous fogs rolling over the city.
Now Chinese cities have poisonous and researchers are studying the 1952 London fog. Read what they discovered: http://earthsky.org/human-world/team-solves-mystery-of-londons-1952-killer-fog.