The Great Smog of 1952, aka the "Killer Fog," was a pollution-befouled miasma that smothered to death some 4,000 Londoners from Dec. 5 to Dec. 9.
Washington, D.C. and Baltimore, Md. tied for the No. 3 spot of the smoggiest large metropolitan areas in 2010, according to an Environment America report.
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