Anecdata I was living very close to a busy highway, and my young daughter was wheazing when she had a cold, we had to give her medicine via a nebulizer. When we moved out to the country more, far away from any major roads, these issues all disappeared within a couple months.
As long as the country doesn’t involve large scale farms because the glyphosate sprayed on them causes serious issues.
A friend of mine does research on air pollution from cars - the TL;DR is you should not live within a third of a mile of a highway. The falloff is pretty sharp, but you don't want to be in that zone.
Doctors used to prescribe a "change of air" in the 18-19th centuries. Maybe they still should.