"In the United States, it is widely accepted that economically disadvantaged and minority populations share a disproportionate burden of air pollution exposure and risk (26,27). A growing body of evidence demonstrates that minority populations and persons of lower socioeconomic status experience higher residential exposure to traffic and traffic-related air pollution than nonminorities and persons of higher socioeconomic status (5,28–31). Two recent studies have confirmed that these racial/ethnic and socioeconomic disparities also exist on a national scale (32,33)."
Setting aside race, to me it seems like "economically disadvantaged populations share a disproportionate burden of traffic-related air pollution exposure" makes this pretty clear. <shrug>
Even into the 1960s wealthy black people were prevented from buying and living in the 'good' parts of even northern US cities.
[1]: https://www.theguardian.com/us-news/2019/apr/04/new-york-sou...
Density doesn't equal exposure.
I suppose that there likely are small numbers of people living far outside of cities and away from traffic, like up in the mountains of Appalachia or living off the grid in tents or in communes deep in forests, who are poorer than people you'd typically find in urban areas, but it's still the poor in the US who are dealing with the worst of the harms traffic causes compared to the better off/wealthy. They're also the least able to mitigate those harms or deal with the fallout caused by them.