Yep, definitely London.
I remember my flatmate and I running an errand which took us out to some part of Greater London, far from the city centre. We stepped off the tube and nearly gasped, with the same immediate reaction - “Oh my God, I can breathe!”
Trying to wear hard contact lenses there was an agony.
And yes, your snot is indeed a peculiar greyish-black.
I’ve really only lived in two cities-Columbus, Ohio, and Seattle, WA. Since Seattle is one of the cleanest cities in the U.S., obviously Columbus is the dirtiest I’ve lived in. But I didn’t think it was actually dirtier than average overall though.
If we include places I’ve visited, then Mexico City wins, hands down. I thought Acapulco was bad after I almost stepped on a dead rat on the beach, but in MC you can’t go more than 50 feet anywhere in town without smelling sewage. And the air is so bad I gave up wearing my contacts after just a few hours. I’ll bet if I had stayed there any longer I would’ve contracted asthma.
Los Angeles is, without a doubt, the dirtiest city I’ve ever lived in. I had to quit going to the beach because the air was brown, and that makes it kinda hard to breathe. It’s just ugly, dirty and soulless.
Robin
Whoa, don’t let a little muck stop ya, just watch where you step, is all. Everyone should visit Gray Paree at least once.
Wow. What a wonderful, disgusting thread 
I feel much better about Pasadena now, although I can definitely sympathize with msrobyn.
pantom: I’ve never been to NYC, although my wife has seen mice rooting around the subways in Toronto. She also lived in an apartment building in Toronto where the cockroaches got all the way to the sixteenth floor. (Pretty tame example, I know…Toronto’s not bad when you get right down to it.)
Fletch: I have been to Boston, however briefly, and I actually thought it was one of the nicest cities I’ve seen. Although I have read “Zodiac”, so I can begin to guess where you’re coming from.
SterlingNorth: My wife has also lived in Windsor, and would probably agree with you completely. She couldn’t leave Windsor fast enough when the time came.
And to all the people from New Orleans: wow again. I wouldn’t have guessed.
So New Orleans seems to be leading in votes, with London second and New York and Paris fighting for third…
tecnicly I diddnt live there, but when I went to India for mission work last October, I stayed in Marcapur (forget how to spell it) and that place…egh. I diddnt like it at all. it was the nastiest of all the towns I stayed at.