I used to live on a busy parkway in Brooklyn and the grime that would collect on our 4th floor window sill was gnarly. I'm guessing each floor downward would ha

I used to live on a busy parkway in Brooklyn and the grime that would collect on our 4th floor window sill was gnarly. I'm guessing each floor downward would have been worse. I'm also guessing that simply walking around the city and being in the subway is also pretty bad. Separating the tracks from the platform with doors would probably help with that.