Ever been somewhere where coal is a real issue? I lived in SLC for years and, particularly thanks to the mountains creating a nasty inversion in the winter, there were times when the air was downright toxic. I recall doing a project about it in high school and finding a lot of data related to increased health issues when it got particularly bad.

Of course SLC is an extreme example, most cities never see inversions like that, but it is certainly the case there that probably the largest contributor, and far and away the main source of power, is coal emissions.