Factories continue to pump smoke out in the air. When the people of big cities like Los Angeles look into the sky they don't see blue, they see a dark, puke gray. You could also say it is water pollution. Many of these same factories regularly pour chemical wastes into lakes, rivers and harbors. And you could even say it is light pollution.
Big cities are full of bright lights.
Yet most of these light sources send just as much light into space. That is not only a waste of light but a waste of money. So it definitely falls under all of these categories, but even those are not quite covering it. You see, the above definitions are the effects. This pollution is the cause.