This 2014 episode covers why when the Great London Smog descended in December of 1952, nobody initially realized anything unusual was going on. At its largest, it extended 30 kilometers around London, and it killed thousands of people.

Robert Moses and the Shaping of New York, Part 2
42:12

Robert Moses and the Shaping of New York, Part 1
41:59

SYMHC Classics: Eradication of Smallpox
43:25