Laura-
You are on the right track but you need to widen your net just a little. Yes, ultra fine particulates are a major part of the problem. But it isn't just manganese. These particulates are so incredibly tiny that they can ride the conveyor systems that are inside the individual neurons. Not only can they enter via the olfactory bulb as Braack suggested, they can be swallowed and enter via the stomach wall, his other suggested route. Being decidedly foreign, they trigger inflammation on both sides of the BBB among those already "primed" by earlier immune challenge and the microglia do the rest.And in such an environment, stress further adds to the inflammatory pressures.

Coal is one source of particulates and the coal smoke of London during the Industrial Revolution is still remembered.

Two other literary references that are most likely to be PD are from China circa 800 BC and the Indus Valley circa 4000 BC. The former is still remembered for the huge bronze castings produced and the latter for the mastery of copper smelting. Both those required sooty charcoal fires.