I lived in some places with pretty bad air. The San Francisco Bay Area was where I spent my childhood and youth. I'm old enough, that I was breathing air polluted with gasoline lead, for many years. I remember many smoggy days in the Bay Area. Then, when in the Navy, I was stationed on a ship that was homeported in San Diego. More pollution.
The worst was the 11 years living in Sacramento with its filthy air in both summer and winter. In the summer, field burning nearby sent thick smoke into the city. Winter brought inversions that trapped wood smoke close to the ground. It's not so bad anymore. Living in Eugene and Salem, Oregon, the air quality is fairly good. Now, all that I worry about is the possible radon contamination emanating from the ground!