[reidacdc]

Some discussion down-thread about this also, somebody runs the numbers for the road itself.

If the emissions numbers are right, it can't be matter that is being consumed or discarded, so it has to be matter that is being returned to the system and "emitted" again. Crap on the road itself meets this requirement -- a car drives by, throws up a bit of rubber off the tires, a bit of asphalt off the road, and a lot of junk from the road surface, and almost all of this stuff falls back on to the road to be thrown up again by the next car.

The article doesn't say how the measurements were done, so it's possible they sampled a small fraction of the air and measured the density, so having the junk available to the sensor doesn't necessarily rule out the recirculation idea.

But if the junk is mostly recirculating, then maybe the health threat is not as bad as if it's being emitted off the road and being lofted into the air with a long dwell-time?

Lots of things are possible and consistent with what the article states, evidently more research is needed.