Unequal burdens: air quality, built environments, and asthma prevalence in the Greater Philadelphia Metroplex

Air quality disparities disproportionately affect low-income and historically marginalised communities and remain a key environmental justice concern in urban communities. Prior studies have linked neighbourhood land use, transportation infrastructure, and redlining legacies to unequal exposure to pollution and respiratory outcomes. Yet few integrate both historical and contemporary urban factors to explain current asthma disparities. This study bridges that gap by examining the intersection of asthma prevalence, air quality, and neighbourhood characteristics across 766 census tracts in the Greater Philadelphia Metropolitan Area. Using two-stage least squares regression, we instrument diesel particulate matter (PM), nitrogen dioxide (NO₂), and toxic air pollutants with built-environment variables such as industrial land share, green space, and road proximity. Results show that industrial land increases, while green space reduces, and pollutant exposure increases. In turn, higher predicted exposure, especially to diesel PM, is associated with elevated asthma prevalence. Tracts with larger shares of Black residents and adults without a high school diploma experience the greatest burdens. By linking structural land use patterns and social inequities to health outcomes, this study advances understanding of urban environmental injustice and offers insights for zoning reform and equitable infrastructure investment.