Seeing a photograph of streetlights on during the day in Pittsburgh prompted me to investigate how bad air pollution really was back then. Voila: a 1979 journal article by Cliff Davidson notes that typical Total Suspended Particulates (a now-deprecated measure) in Chicago in 1913 were 0.3-2.0 mg/m3, with a maximum of 9.3. Cleveland in 1915 reported an average of 14.3 mg/m3, and Pittsburgh was estimated at <7 mg/m3. All of those are many, many times higher than the current EU limit, which is 0.15 mg/m3 – about 99% lower than Cleveland a century ago.
The Baltimore Museum of Art currently has an interesting exhibit of art from the turn of the last century, with a bit of added context: how gosh-darn smoke-filled the skies were back then. “Smog was visually stimulating and helped Monet see his urban environment in new ways.” It translates hazy scenes of London and Paris into air quality scores that would be nightmarish for present-day Americans, or challenging even for current residents of developing-world cities.
west north 
🚌 I grew up in Pittsburgh in the 1960s and 1970s and I remember that air. The nightly TV news weather report would always rate the day’s air quality as “unacceptable.” It was common knowledge, tacitly assumed, that this was an unavoidable feature of the steel industry that the city’s economy depended on.And then parking lot attendants went on strike, a lot of people took the bus to work instead of driving for the duration, and the air was acceptable for the first time in my life. It was a formative lesson for me.