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

Major improvements in air quality since 1990, observed through reductions in fine particulate matter (PM), have been associated with reduced cardiovascular mortality rates (CMR). However, it is not well understood whether the health benefit attributed to PM reductions has been similar across strata of socioeconomic deprivation (SED). Using mixed effect regression models, we estimated the PM-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 (μg/m), and an index of county SED based on socioeconomic factors from the 1990 US Census. The contribution of PM reductions to decreased CMR varied by SED strata and over time. Yearly differences resulted from varying rates of PM reduction and because of the non-linear relationship between CMR and PM concentration. In early years, PM-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 reduction in these counties. However, in later years, PM-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. We identified that CMR reductions related to air quality improvements have become more similar over time between socioeconomic strata.