Airmann
Firstly, airliners have been flying for years on end with bleed from the engines leaking all sorts of oily residue into the cabin. There has been no major epidemic among passengers or crew.
An epidemic is not required to highlight that the risk. There was recently a coroner's report published in the UK, in relation to the death of a BA F/O which was apparently fume related. The coroner stated that the aviation industry is living in blissful ignorance and is doing very little to tackle this issue.
Walking around Beijing is perhaps not the best thing for ones health but being exposed to the toxic fumes that one may breath on board, is comparing apples and oranges. I have nearly been made incapacited on landing due to oil related fumes on board. It was an incredibly unpleasant feeling and one that I clearly remember years on. I doubt the same debilitating effects that I encountered, could be achieved from a short walk around a polluted city.