Introduction
In 2019, fine particulate matter [P[M.sub.2.5], particulate matter (PM) with aerodynamic diameter of [less than or equal to] 2.5 [micro]m] air pollution was responsible for approximately 6.4 million and 50,000 premature deaths worldwide and in the United States, respectively. (1) Distinct health effects of short-term (i.e., hours-to-days) P[M.sub.2.5] exposures are well-documented. (2) Although studies have consistently found disparities in long-term P[M.sub.2.5] exposures for people of color (POC) in the United States, (3-6) racial/ethnic disparities in short-term P[M.sub.2.5] exposures nationwide have not been investigated. We aimed to advance knowledge by estimating short-term P[M.sub.2.5] exposure disparities for racial/ethnic groups nationwide and within the U.S. Environmental Protection Agency (U.S. EPA) regions.
Methods
We used 2012-2016 American Community Survey estimates of census tract composition for the four largest racial/ethnic groups in the United States: a) non-Hispanic/Latina/x/o (NH) White, b) Hispanic/Latina/x/o of any race(s), c) NH Black, and d) NH Asian. We also analyzed e) a "POC" group [i.e., people of Hispanic/Latina/x/o ethnicity and/or non-White race(s)].
To estimate short-term P[M.sub.2.5] exposures for contiguous U.S. census tracts (n = 71,913), we used the high-performing U.S. EPA downscaler model. (7,8) We calculated three exposure estimates of the counts of days during the period 2012-2016 wherein the census tract-level 24-h P[M.sub.2.5] concentration equaled or exceeded a) 15 [micro]g/[m.sup.3] [the current 24-h mean World Health Organization (WHO) guideline (9)], b) 25 [micro]g/[m.sup.3] (2006-2021 24-h mean WHO guideline (9)), and c) 35 [micro]g/[m.sup.3] [24-h mean National Ambient Air Quality Standard (NAAQS) (10)]. Following methods of Lui et al.,6 we then calculated the population-weighted mean counts of days during the period 2012-2016 that the P[M.sub.2.5] concentration was [greater than or equal to]15, 25, and 35 [micro]g/[m.sup.3] for each racial/ethnic group and the total population, nationally and by U.S. EPA region.
Nationwide, we examined two absolute exposure disparity metrics. First, following methods of Lui et al., (6) we derived the percent difference relative to the total population-weighted mean exposure (PWME) (Table 1, "Total Population" column) for the racial/ ethnic groups with the highest and lowest PWME for each threshold {[(highest PWME -lowest PWME)/total PWME X 100%]}. Second, we calculated the percent difference relative to the total PWME for POC PWME minus NH White PWME. We also calculated two relative PWME disparity metrics for each threshold: a) highest PWME divided by lowest PWME, and b) POC PWME divided by NH White PWME.
For the U.S. EPA regions, we examined two absolute exposure disparity metrics at each P[M.sub.2.5] threshold, following methods of Lui et al. (6) First, we calculated the normalized regional disparity as the U.S. EPA regional PWME in each racial/ethnic group minus the U.S. EPA regional total PWME, divided by the national total PWME. Thus, we analyzed 30 exposure thresholdU.S. EPA region combinations (3 thresholds X 10 regions) and 120 exposure threshold-EPA...