Physical activity makes you breath more polluted air though.
https://www.researchgate.net/publication/297712056_Can_air_p...
I don’t understand your point here. If everyone makes the effort to walk/bike more there will be a reduction in pollution.
I believe it's worse here in Europe than in the US as we have a large proportion of diesel cars (though where I live now, most pollution comes from the harbour) and during bad pollution events you don't just smell it but you feel the particle-polluted air aggressing your respiratory system.
The reality for most people in a city is that to exercise they inevitably first need to escape 5 miles worth if clogged up car arteries.