Again, once you get where you are going and step out of your car, you are breathing unfiltered air.
Unless you are wearing a SARS mask everywhere you go like a Chinese woman, you're negating any health benefits of avoiding "natural" air in the limited time spent in your car.
There are plenty of other health concerns in this world to avoid already, I can't justify obsessing about breathing the atmosphere. I don't want to live like that. Sometimes stressing about things is more unhealthy than the actual things themselves.
This of course is my own narrow perspective.