I was about to write the same thing. I lived in Paris and now in London. Often residential streets are disgusting and smelly because people just leave their trash bags there out in the street. Talking about stones, you can easily see the dark layer of pollution on bricks as well.

Talking about the Loire valley, I lived in Tours also and It's true that this is very difficult to clean, if I'm correct the best technique is to use a laser but it's slow and expensive.