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On July 26, 1943, Los Angeles was blanketed by a thick gas that stung people's eyes and blocked out the Sun. Panicked residents believed their city had been attacked using chemical warfare. But the cloud wasn't an act of war. It was smog. So what is this thick gray haze actually made of? And why does it affect some cities and not others? Kim Preshoff details the science behind smog.

But those measures did little to curb the smog the first time. Toxic air cloaked Beijing until strong winds blew the pollutants south. The main source of foul air in the capital comes from coal-burning factories in surrounding provinces, not from activity in the city itself, and cities in those provinces did not take the same drastic emergency measures that Beijing did. Many parts of northern China, including the provinces of Hebei, Henan and Shanxi, regularly have air quality that is worse than that of Beijing.