I recall when I was in my teens I would go backpacking in the wilderness for a week at a time, far away from any vestige of civilization. My friends and I would park the 3 yr old Mustang with a few other cars at the jumping off point, which was very isolated. Hiking back to the car a week later I could smell the gasoline and other obnoxious industrial odors the few cars were giving off from about 100 yards - well before I could see the cars. But within 15 minutes we all reaclimated and could no longer detect the odor. 2 hrs later we were back in a town where auto fumes and the gasoline smell were everywhere. It was just part of the background. Back then gasoline had lead.
We all thrived and survived. Even in those circumstances life expectancy was increasing. It is nicer to have clean air. But 20% of the population dying from dirty air? Not a chance. In every one of those places with dirty air life expectancy is going up.