The Great Smog of 1952 in London

When a thick fog engulfed London from December 5 to December 9, 1952, it mixed with black smoke emitted from homes and factories to create a deadly smog. This smog killed approximately 12,000 people and shocked the world into starting the environmental movement.
http://history1900s.about.com/od/1950s/qt/greatsmog.htm
https://en.wikipedia.org/wiki/Great_Smog_of_London