Dirty density: air quality and the density of American cities

We study whether urban density affects the exposure of city dwellers to ambient air pollution using satellite-derived measures of air quality for the contiguous United States. For identification, we rely on an instrumental variable strategy, which induces exogenous variation in density without affecting pollution directly. For this purpose, we use three variables measuring geological characteristics as instruments for density: earthquake risks, soil drainage capacity and the presence of aquifers. We find a positive and statistically significant pollution-density elasticity of 0.13. We also assess the health implications of our findings and find that doubling density in an average city increases annual mortality costs by as much as $630 per capita. Our results suggest that, despite the common claim that denser cities tend to be more environmentally friendly, air pollution exposure is higher in denser cities. This in turn highlights the possible trade-off between reducing global greenhouse gas emissions and preserving environmental quality within cities.