Abstract

Racial-ethnic minorities in the United States are exposed to disproportionately high levels of ambient fine particulate air pollution (PM_2.5), the largest environmental cause of human mortality. However, it is unknown which emission sources drive this disparity and whether differences exist by emission sector, geography, or demographics. Quantifying the PM_2.5 exposure caused by each emitter type, we show that nearly all major emission categories—consistently across states, urban and rural areas, income levels, and exposure levels—contribute to the systemic PM_2.5 exposure disparity experienced by people of color. We identify the most inequitable emission source types by state and city, thereby highlighting potential opportunities for addressing this persistent environmental inequity.

INTRODUCTION

Ambient fine particulate matter air pollution (PM_2.5) is responsible for 85,000 to 200,000 excess deaths per year in the United States (1, 2), with health effects observed even at concentrations below the current national standard of 12 μg m⁻³ (3–5). Racial-ethnic and socioeconomic disparities in air pollution exposure in the United States are well documented (6–10) and have persisted despite overall decreases in PM_2.5 pollution (11–13).
Most evidence of exposure disparity relies on measured or empirically modeled ambient concentrations or on assessment of proximity to industrial or roadway emission sources (6, 10, 12–20). From the existing evidence, however, it is not possible to determine the relative contributions of different source types to racial-ethnic disparity in exposure to PM_2.5. Here, we model anthropogenic sources of PM_2.5 exposure resolved by race and ethnicity and show that nearly all major emission source sectors disproportionately affect people of color (POC).
We estimate exposure impacts for each emission source type on five racial-ethnic groups based on the U.S. Census: White (62% of the population), Black (12%), Hispanic (17%), Asian (5%), and POC (38%; see Materials and Methods for details). As a proxy for exposure to PM_2.5, we calculate population-weighted average ambient PM_2.5 concentrations for each race-ethnicity based on census-designated residential location.
We examine exposure disparity—the population-weighted concentration difference between each racial-ethnic group and the population average—in relative (percent) and absolute (μg m⁻³) terms. Sources with the highest relative disparity may yield the largest disparity mitigation per unit mass of emission reduction, whereas sources with the highest absolute disparity may have the greatest potential for overall disparity reduction.

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