SIAM News Blog

Quantifying the Disproportionate Exposure of Minority Populations to Fine Particulate Air Pollution

By Lina Sorg

The concept of environmental justice—a social movement that addresses poor and marginalized communities’ exposure to negative environmental consequences—has existed for quite some time. Numerous studies have indicated that people of color are disproportionately exposed to hazardous emissions of particulate matter 2.5 (PM2.5) from incinerators, traffic, and industrial sources like powerplants. For example, a 1983 study by Robert Bullard revealed that all five of the garbage incinerators in Houston, Texas, were located in either Black or Hispanic neighborhoods [1].

During a minisymposium at the 2022 SIAM Conference on Mathematics of Planet Earth, which is currently taking place in Pittsburgh, Pa., in conjunction with the 2022 SIAM Annual Meeting, Christopher Tessum of the University of Illinois Urbana-Champaign used mathematical modeling to investigate the disparities of racial and ethnic minorities’ exposure to fine particulate air pollution.

Tessum utilized a reduced complexity air quality model called InMAP (Intervention Model for Air Pollution) that he and his collaborators developed to track the distribution of impacts among different groups of people. “It has high resolution to study cities specifically while still looking at the whole country,” Tessum said. “This is important because a lot of inequities and exposure happen among different neighborhoods in the same city. You really need a model that can resolve those differences in order to see the inequities.”

Using InMAP, Tessum examined the 5,000 source types of emissions in the U.S. Environmental Protection Agency’s National Emissions Inventory (NEI). PM2.5 source types range from gas-powered passenger vehicles to coal powerplants. By plotting the individual air pollution impacts of each emission source, Tessum found that 40 percent of the pollution that affects White people stems from sources that expose them to PM2.5 at a disproportionate rate; in contrast, 75 percent of the pollution that affects people of color emanates from sources that disproportionately expose them to pollutants. “The vast majority of the pollution is coming from source types that disproportionately expose people of color,” Tessum said. “It’s not just transportation, it’s not just powerplants or industry, it’s really everything.”

Tessum next condensed the 5,000 original NEI sources into 14 key sources. The resulting charts reveal that only a few source groups disproportionately expose pollutants to White populations, which benefit from the remaining groups. The inverse is true for people of color, who face negative consequences from heavy-duty diesel vehicles, off-road highway vehicles and construction equipment, and light duty gas vehicles. In fact, the only sources to which people of color are not disproportionately exposed are agriculture and coal-electric utilities. These sources are more regional and present in parts of the U.S. with smaller populations of Hispanic individuals. Black people, however, are disproportionately exposed to all source groups.

After subdividing this exposure by individual state, Tessum confirmed that the pattern holds for almost all of the 48 contiguous states and nearly 490 urban areas. The issue exists in rural areas as well, though to a lesser extent. Tessum also looked at income. “All of the income bins for people of color are disproportionately exposed on average for the majority of sources,” he said. “It’s not just a couple of cities, it’s not just a couple of states, it’s not just rural areas or urban areas, its everywhere. It’s also not just a matter of income.”

Figure 1 PM2.5 concentrations due to emissions from each emitter group (far left), as well as relationships among PM2.5 health impacts as attributed to emitters (second from left), end uses (second from right), and end users (far right). Figure courtesy of [2].

Tessum then sought to confirm that people of color are not in fact engaging in higher amounts of polluting activities. He did so by connecting the emission and air pollution exposure models to a model of the economy (see Figure 1). After mapping the exact location of air pollution for all 14 source groups, Tessum translates that information into the number of people who are killed by pollutants in each source type. “We have 130,000 people who are killed every year from PM2.5,” he said. “It’s the largest environmental cause of death.” 

Next, Tessum connected this information to the economic emitters that drive PM2.5 emissions and cause pollution. For example, coal electricity plants exist to meet economic demand for electricity. Tessum tracked the origins of these demands, which include electricity, goods, service, shelter, transportation, and exports. Finally, he identified the end users—other countries, the government, and people, the majority of whom are white—who utilize the resulting services. .

Figure 2. Average PM2.5 exposure that racial-ethnic minority groups both experience and cause. Christopher Tessum divides the data into Black, Hispanic, and White populations. Figure courtesy of [2].

Tessum normalized this data to yield the average per capita for three groups: Black, Hispanic, and White/other (see Figure 2). Each group has two bars on the chart that represent (i) the amount of pollution that the group generates through its end uses and economic activities, and (ii) the amount of pollution to which people are exposed based on their addresses. “You can see that Black people are causing a lot less pollution per person,” Tessum said. “It’s the same thing for Hispanic people.” Unfortunately, the opposite is true for the White/other group. Taking the percent difference between the two bars for each group yields what Tessum calls pollution inequity. This calculation reveals that Black and Hispanic populations are respectively exposed to roughly 56 and 63 percent more pollution than that of their economic activities. 

To conclude, Tessum suggested that the process of finding a solution is more important than the technology behind the solution itself. Direct participation from the affected communities can also help avoid unjust outcomes.

[1] Bullard, R.D. (1983). Solid waste sites and the black Houston community. Sociol. Inquir., 53(2-3), 273-288.
[2] Tessum, C.W., Apte, J.S., Goodkind, A.L., Muller, N.Z., Mullins, K.A., Paolella, D.A., …, Hill, J.D. (2019). Inequity in consumption of goods and services adds to racial-ethnic disparities in air pollution exposure. Proc. Ntnl. Acad. Sci., 116(13), 6001-6006.

Further Reading
Tessum, C.W., Paolella, D.A., Chambliss, S.E., Apte, J.A., Hill, J.D., & Marshall, J.D. (2021). PM2.5 polluters disproportionately and systemically affect people of color in the United States. Sci. Advan., 7(18).

Lina Sorg is the managing editor of SIAM News.
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