Air pollution is easily apparent if you just look around. Oh, there are places where clean air is a local treasure, but in other areas air pollution is apparent even if you are just looking around. In this area it is so prevalent that it is easy to spot. And, in those clean air locales, are you sure?
Of course you will see more if you know what you are looking at.
Air pollution can be from a singular source or it can be the result of multiple sources.
The introduction image is one of my Zazzle products.