That doesn't really answer my question but I was able to find some more info.
These claims appear to be based on studies in which PM2.5 levels have been correlated (by statistical models) with increased risks of heart disease as well as respiratory problems.
I haven't been able to find anything describing exact mechanisms.
But Pope et al [1] concluded that "a decrease in the concentration of PM2.5 of 10 μg per cubic meter is associated with an increase in life expectancy of 0.77 year."
For comparison, the EPA's clean air standard for PM2.5 is 12 μg per cubic meter.
These claims appear to be based on studies in which PM2.5 levels have been correlated (by statistical models) with increased risks of heart disease as well as respiratory problems.
I haven't been able to find anything describing exact mechanisms.
But Pope et al [1] concluded that "a decrease in the concentration of PM2.5 of 10 μg per cubic meter is associated with an increase in life expectancy of 0.77 year."
For comparison, the EPA's clean air standard for PM2.5 is 12 μg per cubic meter.
1: https://www.nejm.org/doi/full/10.1056/NEJMe0809178
2: https://www.epa.gov/criteria-air-pollutants/naaqs-table