Sitting in a Shenzhen traffic jam on a bad air day, surrounded by buses and vans belching smoke, it might seem simple to pinpoint the source of the smog through which buildings just a block away are barely visible.
But, the experts say, it is not so simple.
In fact, the suspended respirable particulates - scientific jargon for the tiny bits of soot and other matter that are a key ingredient of smog - you are inhaling may have been generated days or weeks earlier and many kilometres away.
Amid the debate on the reasons for Hong Kong's deteriorating air quality there is one factor most people agree on: discovering the source is notoriously difficult.
'The problem is that air pollution has no boundary. It is just like we are all living inside a glass box and the pollutants are accumulating over our heads and eventually intermingling without possible traces of their origins,' said Ho Kin-chung, of the Open University's environment programme.
According to another scientist, the Pearl River Estuary may be a vast pollution sink, where pollutants from around the delta gather before spreading across the region.