My dad often said that we didn't appreciate how clean the air was (in the 60's) from when he grew up. Indianapolis mostly heated with coal until oil-burners (and later gas and electric furnaces) came on the market. He said some days you could hardly see downtown, the coal smoke was so thick.
That's really true of where I grew up. We weren't far from mountains and we had lots of temperature inversions that held smoke in and low to the ground. By the 60s things were getting better but I can recall dramatic improvement late in that decade and onward. Nothing I'm aware of now compares to those days pollution-wise.
I'm glad for the improvements but ironically there's a FR thread today saying energy costs in CA are creating huge demand for fire wood -- and there's a lot of concern about smoke pollution in the L.A. basin. Bottom line, people will do whatever's needed to stay warm. If energy's priced out of sight people will revert right back to campfire technology.