It seems that whenever we set clean air standards, they are not tight enough: epidemiology shows substantial harm at well below whatever level we have set. In the USA it's currently 12 micrograms per cubic metre for 2.5 micron particles (PM2.5) and 70 parts per billion (ppb) for daily ozone. Here's the study which says those levels are too high. It's a massive study - must have used the computing power of at least 2 iPhones. The entire Medicare population of over 60m people tracked for 12 years, and exposure data modelled across the entire continental USA.
And as usual disadvantaged people are most affected:
'the study showed that black men and persons eligible to receive Medicaid had a much higher risk of death associated with exposure to air pollution than other subgroups. These findings suggest that lowering the annual NAAQS may produce important public health benefits overall, especially among self-identified racial minorities and people with low income'
It's difficult to know why that should be, and the authors don't speculate. But perhaps lungs already damaged by adversity are more susceptible. (Or maybe it's not the lungs - maybe I should have written 'bodies already damaged by...')