Hi:
 
I have been out of the country looking at Apis dorsata in Malaysia, so am
just catching up on the discussions.  As per lead in propolis, honey, or
bees.  Yes, vehicle exhausts are one source, and yes, we have documented
it in bees near highways.  The Germans did some work on lead years ago.
As per levels of lead in propolis peaking because it is toxic to bees, I
doubt that will happen since lead doesn't normally kill quickly, but is
accumlutive.  So, I'd guess the bees could pull in some whopping levels
of lead over the short term.  Lead paint on beehives is certainly a good
source of this contaminant, as is vehicle exhaust (leaded gasoline),
various heavy industries (such as lead and copper smelters), battery
plants, etc.
 
As per bees in cities, the plus is a diverse and often abundant floral
resource.  The negative is all the chemicals, ranging from traffic to
household chemicals to industries.  My studies suggest bees in the
industrial areas and bees in the pricey residential areas are subjected
to the greatest chemical insults.  The first is obvious, the second may
not be so.  My guess, "rich folks" pay to have their pests routinely
killed (the weed and feed approach).  All those lawn chemicals extract a
heavy toll on bees - and probably the people and their pets.
 
None of this has been based on a hard study, but we have bits and pieces
of evidence that suggest this is true.
 
Cheers
 
Jerry