If you could see long term PM2.5 averages and how they vary, we'd approach as a national crisis.

https://www.sciencedirect.com/science/article/pii/S266601722... (this groups methods can be substantially improved).

Having done some additional follow on work in the space -- the results definitely do not follow socioeconomic boundaries as one might expect.

Roads are a huge contributor.

Roads being a huge factor also plays into socioeconomic factors though, at least in some places. Take New York City for example, where the off-ramps for highways were purposefully planned to let traffic out in larger numbers in impoverished areas to keep the noise and pollution minimal for the more affluent burrows.