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