A multi-decade decline in the concentrations of fine particles (PM2.5) has benefited public health greatly by reducing the incidence of premature death and illnesses. A large body of literature has examined the change over time in this burden among the US population. However, less is known about these changes have affected subgroups most vulnerable and susceptible to poor air quality. In this study, we applied the results of an air quality prediction model that estimated changes in PM2.5 between 1980 and 2010 across all counties in the contiguous US. For the year 1980, we defined groups of vulnerable and susceptible populations according to attributes including county-level educational attainment, income, urbanicity, and all-cause death rate. Using a health impact function, we quantified PM2.5-attributable premature deaths for 1980 and 2010 among the overall US population as well as these subgroups. We then used the Atkinson index to explore the inequality in the absolute level and change in PM2.5 mortality among and between these subgroups. Between 1980 and 2010, total PM2.5 mortality risk declined for all groups. Risk inequality fell across all population groups and the risk experienced among each vulnerable and susceptible group became more equitably distributed. These results suggest that falling ambient PM2.5 concentrations over a 30-year period yield significant benefits among populations most susceptible and vulnerable to poor air quality. Furthermore, our results imply that policies including the Clean Air Act were effective at reducing health disparities.