That isn’t fog in these images of Pittsburgh taken in 1940. That’s what the city looked like all the time before coal burning regulations.
For more than a century, Pittsburgh was marked as a smoky city. In 1941 an effective smoke control ordinance was passed in the city of Pittsburgh regulating the burning of coal, but the onset of World War II delayed the enactment of the legislation until 1946.
Visit the Thinx site