Define “air quality” as a single metric is crude at best, but arguably dishonest. The nature of pollution and the pollutants themselves have changed have they not?
Edit: To illustrate the point, assume that getting regular physical is more important now than 100 years ago, due to on average more sedentary contemporary lifestyles. Assume heavy air pollution decreases outdoor activity, which seems plausible if not certain. How long would it take for a body of scientific literature to develop to document the problem? Given measurement problems and the difficulty of doing simulation heavy research, we might never be able to prove such a linkage exists, even if it did.
So, you're telling me the air quality is getting worse despite this trend not being observed, yet? There is a logical fallacy somewhere. Furthermore, the link between lung cancer and cigarette smoking was well observed already in the 50s but the tobacco industry naturally downplayed the risks.
BTW: It is an established fact that air quality in the cities of 19th century was way worse -- mainly due to the fact that people cooked and heated their places with open fires and coal. For some insight on the UK: https://theconversation.com/air-pollution-in-victorian-era-b...