Pretty much all large cities are dirty. People like to stereotypically dunk on Paris but to be frank it's got nothing on London or New York. There's just something inherently filthy that comes with large concentrations of human beings. The only exception I've seen first hand is Tokyo. I guess if you're not used to large cities it must be shocking.

"There was a certain something about the air in [Ankh-Morpork]. You got the feeling that it was air that had seen life. You couldn't help noting with every breath that thousands of other people were very close to you and nearly all of them had armpits." -Terry Pratchett, Mort