All cities and urban/suburban areas have "dirty" and "clean" neighborhoods. Think your city is cleaner than city X or Y? Take a trip through some of the areas away from Downtown and the tourist areas sometime; you'd be surprised just how bad some parts of your city really are (and which ones, at that!). Heck, if you want to see some really "dirty" cities, take a trip to the Middle East and visit some of the cities in the Persian Gulf; although some of the governments there have tried to keep things clean, there's still a lot of trash in the streets (in the downtown and waterfront areas, not just in the slums) and the air quality... well, let's just say that you might not do too well if you're asthmatic (and we ain't talkin' about dust in the air, although that can be a problem on the Arabian Peninsula). Cities and filth go hand in hand; that's the normal result of having a lot of people toghether in a relatively small area and a throwaway society, and no city has been able to completely eliminate that problem.