For instance, the deepest red shade in Figure .A signifies that a 1% rise in the non-white population share correlates with an increase in NOX emission density exceeding 1,700 g/km 2 . Conversely, the darkest blue shade in Figure .D indicates that a $1 increment in median household income is associated with a reduction in NOX emission density by at least 2.3 g/km 2 . This translates to a decrease of at least 2,300 g/km 2 for every $1,000 rise in median household income. Counties with statistically insignificant slope estimates (p > 0.05, based on a 95% confidence interval) are hatched in Figure and excluded from the analysis. For the corresponding figure on PM2.5 emissions, refer to Figure S.4.