Trends in air pollution emissions in the contiguous United States from 1970 to 2010: an environmental justice analysis

Background: Air pollution emissions have substantially decreased in the US since the 1970 Clean Air Act. We examined racial and economic disparities in emissions reductions from specific pollution sources. Methods: We evaluated racial/ethnic and economic disparities in emissions reductions from six pollution sectors [industry (SO2), energy (SO2), agriculture (NH3), commercial (NOx), residential (organic carbon), and on-road transport (NOx)] across the contiguous US from 1970–2010. We used the CEDGBD-MAP emissions inventory to estimate the average relative emissions change from one decennial to the following and its link to demographics (percent White, Black, Asian, American Indian Hispanic, family income, property value, poverty, and unemployment) at the county level. We constructed hierarchical models with the relative emissions change as the dependent variable and demographics as independent variables. We modeled each pollutant and demographic variable separately, adjusting for potential confounding, and tested for nonlinearities. Results: On average, national emissions decreased from 1970 to 2010 except in the agriculture and the residential sectors. The largest reductions across the study period were in industry and energy SO2 emissions (-66% and -89%, respectively) and on-road transport NOx (-59%). Although average emissions decreased nationally, trends varied across counties. We found racial disparities in industry, energy, and agriculture emissions. An increase in % American Indian, Hispanic, or Asian was associated with increases in industry SO2, agriculture NH3, and energy NOx. We also found economic disparities: an increase in median family income was associated with decreases in emissions from all sectors except agriculture, and increases in unemployment or poverty were associated with increases in industry and energy SO2 emissions. Conclusion: Although air pollution emissions, on average, decreased since 1970, the trends in emissions varied across counties. On average, low-income and communities of color have experienced smaller improvements or a relative increase in emissions despite the nationwide downward trends.