The first time I took my overly evolved American nostrils to Europe, I braced myself for gagging smells in Paris in particular (that's just someghitn all Americans are raised to expect).  Well, on that trip and half a dozen subsequent ones, I have yet to have an upleasant olfactory experience in Paris.

But actually I don't think London is that bad -- not compared to Scandinavia and Germany. Those are the countries where, imho, being in a confined space is pure Nose Purgatory. (And really, this is not criticism, but rather a recognition that Americans, with our mandatory morning showers and full-body deodorants, are the odd ones out.)