Changes in Transportation-Related Air Pollution Exposures by Race-Ethnicity and Socioeconomic Status: Outdoor Nitrogen Dioxide in the United States in 2000 and 2010

Abstract

Background: Disparities in exposure to air pollution by race-ethnicity and by socioeconomic status have been documented in the United States, but the impacts of declining transportation-related air pollutant emissions on disparities in exposure have not been studied in detail.

Objective: This study was designed to estimate changes over time (2000 to 2010) in disparities in exposure to outdoor concentrations of a transportation-related air pollutant, nitrogen dioxide (NO2), in the United States.

Methods: We combined annual average NO2 concentration estimates from a temporal land use regression model with Census demographic data to estimate outdoor exposures by race-ethnicity, socioeconomic characteristics (income, age, education), and by location (region, state, county, urban area) for the contiguous United States in 2000 and 2010.

Results: Estimated annual average NO2 concentrations decreased from 2000 to 2010 for all of the race-ethnicity and socioeconomic status groups, including a decrease from 17.6 ppb to 10.7 ppb (-6.9 ppb) in nonwhite [non-(white alone, non-Hispanic)] populations, and 12.6 ppb to 7.8 ppb (-4.7 ppb) in white (white alone, non-Hispanic) populations. In 2000 and 2010, disparities in NO2 concentrations were larger by race-ethnicity than by income. Although the national nonwhite-white mean NO2 concentration disparity decreased from a difference of 5.0 ppb in 2000 to 2.9 ppb in 2010, estimated mean NO2 concentrations remained 37% higher for nonwhites than whites in 2010 (40% higher in 2000), and nonwhites were 2.5 times more likely than whites to live in a block group with an average NO2 concentration above the WHO annual guideline in 2010 (3.0 times more likely in 2000).

Conclusions: Findings suggest that absolute NO2 exposure disparities by race-ethnicity decreased from 2000 to 2010, but relative NO2 exposure disparities persisted, with higher NO2 concentrations for nonwhites than whites in 2010. https://doi.org/10.1289/EHP959.

Figure 1.

Estimated ${\text{NO}}_2$ concentration (ppb) by race-ethnicity, household income quintile, educational attainment, and age group, for year 2000 and year 2010. Box-and-whiskers indicate the 90th, 75th, 50th, 25th, and 10th percentile concentrations, and circles indicate population-weighted mean concentration. Race-ethnicity groups shown above are the six largest groups (Table 1 includes remaining race-ethnicity groups). Income groups are quintiles on a national basis for year-2000 households (38% of total population in year 2000). Educational attainment is reported for population over 25 y (65% of total population in year 2000).

Figure 2.

Estimated mean ${\text{NO}}_2$ concentration versus percent nonwhite population for block groups in year 2000 and year 2010. Each point represents the mean ${\text{NO}}_2$ concentration for 1% of the 210,000 block groups in the United States, binned by percent nonwhite residents. (The first point represents the 1% of block groups with the lowest percent nonwhite population, and the last point represents the 1% of block groups with the highest percent nonwhite population.)

Figure 3.

Estimated environmental injustice metric (absolute difference in population-weighted mean ${\text{NO}}_2$ concentration (ppb) between nonwhites and whites) (a) in year 2000, (b) in year 2010, and, (c) change over time (year 2010–year 2000) for United States (1) regions ($n=9$), (2) states ($n=49$ [including District of Columbia]), (3) counties ($n=\mathrm{3,109}$), and (4) urban areas ($n=481$). For maps in columns (a) and (b), red indicates that annual mean ${\text{NO}}_2$ concentrations are higher for nonwhites than whites, blue indicates that annual mean ${\text{NO}}_2$ concentrations are higher for whites than nonwhites, and white indicates that annual mean ${\text{NO}}_2$ concentrations are equal for nonwhites and whites. For maps in column (c), red indicates that the absolute difference in annual mean ${\text{NO}}_2$ concentration between nonwhites and whites increased over time, blue indicates that the absolute difference decreased over time, and white indicates no change in the absolute difference over time. For maps in row (4), circle icons are located at the centroid of the urban area. For all plots, the box-and-whiskers indicate 90th, 75th, 50th, 25th, and 10th percentiles, and circles indicate maximum and minimum. Map boundary data are from the National Historical Geographic Information System (MPC 2011).