Historical Redlining Is Associated with Present-Day Air Pollution Disparities in U.S. Cities

Abstract

Communities of color in the United States are systematically exposed to higher levels of air pollution. We explore here how redlining, a discriminatory mortgage appraisal practice from the 1930s by the federal Home Owners' Loan Corporation (HOLC), relates to present-day intraurban air pollution disparities in 202 U.S. cities. In each city, we integrated three sources of data: (1) detailed HOLC security maps of investment risk grades [A ("best"), B, C, and D ("hazardous", i.e., redlined)], (2) year-2010 estimates of NO₂ and PM_2.5 air pollution levels, and (3) demographic information from the 2010 U.S. census. We find that pollution levels have a consistent and nearly monotonic association with HOLC grade, with especially pronounced (>50%) increments in NO₂ levels between the most (grade A) and least (grade D) preferentially graded neighborhoods. On a national basis, intraurban disparities for NO₂ and PM_2.5 are substantially larger by historical HOLC grade than they are by race and ethnicity. However, within each HOLC grade, racial and ethnic air pollution exposure disparities persist, indicating that redlining was only one of the many racially discriminatory policies that impacted communities. Our findings illustrate how redlining, a nearly 80-year-old racially discriminatory policy, continues to shape systemic environmental exposure disparities in the United States.

Figure 1

Population-weighted distributions of NO₂ and PM_2.5 levels within HOLC-mapped areas at the census block level. Bars represent 25th and 75th percentiles. Medians are indicated with horizontal lines, and means by the dot marker; the overall mean is indicated by the dotted line. Unadjusted national distributions are presented for (a) NO₂ and (b) PM_2.5. Adjusted distributions (c and d) report the national distributions of intraurban differences for census blocks within a given HOLC grade relative to the PWM level within each city. In each panel, pollution level distributions are reported by both HOLC grade (left cluster) and race/ethnicity (right cluster). Vertical lines between these clusters reflect the pollution range of the group means: the difference in the population-weighted mean between groups A and D (left line) and between the highest-exposed and lowest-exposed racial/ethnic group. Panels c and d illustrate how intraurban disparities are consistently higher by historical HOLC grade than by race/ethnicity.

Figure 2

Population-weighted mean annual intraurban PWM levels by HOLC grade and race/ethnicity for (a) NO₂ and (b) PM_2.5. All race/ethnicity groups demonstrate monotonic increases by HOLC grade. Disparities by HOLC grade were larger than those associated with differences between racial/ethnic groups (100% higher for NO₂ and 50% higher for PM_2.5).