Industrial fallout to me suggests that it originates from industrial pollution, and falls out of the sky. The pollution that comes from the exhaust of factories is lifted out with an updraft, and once it escapes that updraft, it begins to fall to the ground in the surrounding areas. Any bit of wind would be able to move these very fine particles a substantial distance.
The Lake Michigan carferry was a prime example. When I used to work at the Manitowoc Marina, I would see the carferry come in every day at 10:00, and sit there until 2:00, with its coal-fired engine running the entire time. The black smoke coming from the exhaust travelled for miles over the city, right toward my house. I'm convinced that all the black dust that I would find on the Maxima was coming from the carferry's engine exhaust.
