The contribution of improved air quality to reduced cardiovascular mortality: Declines in socioeconomic differences...

Major improvements in air quality since 1990, observed through reductions in fine particulate matter (PM_2.5), have been associated with reduced cardiovascular mortality rates (CMR). However, it is not well understood whether the health benefit attributed to PM_2.5 reductions has been similar across strata of socioeconomic deprivation (SED). Using mixed effect regression models, we estimated the PM_2.5-related change in the CMR across 2,132 US counties in five SED strata between 1990 and 2010. The analysis included annual county CMR (deaths/100,000 person-year), annual county PM_2.5 (microg/m³), and an index of county SED based on socioeconomic factors from the 1990 US Census. The contribution of PM_2.5 reductions to decreased CMR varied by SED strata and over time. Yearly differences resulted from varying rates of PM_2.5 reduction and because of the non-linear relationship between CMR and PM_2.5 concentration. In early years, PM_2.5-related CMR reductions were smallest in the most deprived counties compared to all other counties (range: 0.4-0.6 vs 0.7-1.6 fewer deaths/100,000 person-year), due to slower rates of PM_2.5 reduction in these counties. However, in later years, PM_2.5-related CMR reductions were highest counties with moderate to high deprivation, compared to counties with the least deprivation (range: 1.0-2.2 vs 0.5-0.9 fewer deaths/100,000 person-year) due to larger CMR reductions per decrease in PM_2.5. We identified that CMR reductions related to air quality improvements have become more similar over time between socioeconomic strata.