Satellite observations of NO2 indicate legacy impacts of redlining in US Midwestern cities

Redlining was a practice of financial discrimination in the mid-20th century in which banks refused loans or increased interest rates based on the grade of an applicant's neighborhood as designated by the federally sponsored Home Owner's Lending Commission (HOLC). The HOLC primarily graded neighborhoods from "A" (best) to "D" (hazardous) based on characteristics including the racial demographics and economic status of the residents, with neighborhoods with higher percentages of non-white and/or recent immigrant residents given lower grades; this and similar discriminatory practices can be traced to modern-day economic and environmental inequalities between neighborhoods. The legacy of redlining and related housing discrimination on modern-day urban air quality, which presents a significant threat to public health, remains an important issue in addressing environmental injustice in U.S. cities. In our study, we used remotely sensed estimates of the air pollutant nitrogen dioxide (NO2) collected with the TROPOMI satellite sensor, and shapefiles of redlined neighborhoods, to determine whether air quality varies among historic HOLC grades in 11 U.S. Midwestern metropolitan areas. This approach allowed us to test these tools for within-city analysis of NO2 for which high spatial and temporal resolution measurements are not often available, despite their importance for monitoring impacts on human health. We found that NO2 levels were as much as 16% higher in neighborhoods that were graded "D" compared to those graded "A" (as in Chicago), with the mean difference across all cities an increase of 7.3% +/- 5.9%. These results present evidence of persistent modern-day inequality in urban air quality associated with historic discriminatory policies and should be used as an argument for government action improving air quality in neighborhoods that were poorly graded by the HOLC.