Racial inequity biases algorithms, skews Covid-19 death rates, and exacerbates the digital divide. Your race is even a good predictor of what’s in the air you breathe—and now, a new study shows that those pollution concentrations are also tied to how segregated your community is.
Using five years’ worth of data, a team led by scientists at Colorado State University confirmed a long-suspected link between ambient air quality and racial residential segregation. In a recent paper published in Nature Communications, they show that people in highly segregated counties in the United States are exposed to more fine particulate matter, airborne particles that are less than 2.5 micrometers in diameter (or PM 2.5 for short). What’s more, the makeup of that pollution contains higher toxic metal concentrations than what is found in well-integrated areas. The results are congruent with a growing number of studies showing that people of color are exposed to heightened levels of pollution compared to their white counterparts, crucial information that could be used to push for more equitable air protection policies.
“It’s really an insult-to-injury type of finding,” says John Volckens, a public health engineer who co-authored the study. “Not only do we see more air pollution in these communities, it contains a nastier mix of bad actors.” It’s the first time researchers have analyzed variations in the specific composition of PM 2.5, which itself is a mixture of natural and anthropogenic ingredients, including wildfire smoke, soot, mineral dust, vehicle emissions, and trace metals. “And we’re not talking about essential metals, like potassium or calcium, that you need in your diet,” Volckens says. “We’re talking about things like lead and chromium”—substances that can do a number on your health.
Environmental researchers fixate on PM 2.5 because these particles are small enough to be inhaled and travel through the respiratory tract. Trace metals make up only a tiny fraction of PM 2.5, but are especially concerning because of their known or suspected health effects. Lead, for example, has been linked to increasing blood pressure, nervous system damage, and loss of cognitive function. Some forms of chromium harm the liver and cause respiratory problems. Nickel attacks the lungs and kidneys. And all of these are carcinogenic. Excessive exposure to essential nutrients can also be dangerous: High levels of manganese can be neurotoxic, and too much iron and copper increases the risk of cardiovascular disease.
The scientists leading the study focused on nine metals, classifying them into those typically associated with natural sources (iron, titanium, and manganese) and anthropogenic emissions (copper, zinc, nickel, chromium, lead, and vanadium). The boundary between those categories is fuzzy—iron, for instance, can come from mineral dust and also from burning fossil fuels—but the researchers found them useful as general groupings for the analysis.
Trace metals can travel away from their sources, but they tend to stay densely concentrated near where they were produced. They don’t chemically degrade into something else, so scientists can easily correlate the presence of these toxins in the air with what people living in the area below are likely to inhale. These particles only stick around for a couple of weeks before bumping into a raindrop or other nearby objects and getting filtered out of the atmosphere. But without getting rid of their sources, places saddled with high-emitting industries like metal factories and power plants will likely suffer poor air quality for generations to come.
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To prove the link between residential segregation and exposure disparities, the team pulled air quality data from the Federal Land Manager Environmental Database for about 250 counties. They used statistics collected by the US Census Bureau to assign each county a segregation score between 0 and 1, based on how closely the racial makeup of the individual census tracts matched the county’s overall demographics. A score less than 0.3 was defined as well integrated, 0.3 to 0.6 as moderately segregated, and above 0.6 as highly segregated.
Take a county where 30 percent of its residents are Black, for example. If all of the census tracts also have a 30 percent Black population, the county would be considered well integrated and receive a score of 0. That means the demographics of the individual tracts are equal to those of the county at large.
But if that same county has some tracts where the Black population is 80 percent, and others where it’s only 5 percent, it would be considered segregated. Then the county’s score would be closer to 1, depending on how extreme that segregation was.
As a first round, the researchers categorized counties by the level of their segregation between Black and white residents. For each 10 percent increase in the segregation score—say, a county that scored 0.6 compared to another that only scored 0.5—residents were exposed to 5 percent more total PM 2.5, and 9 to 16 percent higher trace metal concentrations. People in a county with a score of 0.7 would inhale twice that: 10 percent more total PM 2.5 and 18 to 32 percent more trace metals than concentrations found in a county that scored 0.5.
Trends in exposure also differed between primary sources of these metals: Highly segregated counties have up to 20 times more anthropogenic metals in their air. In urban areas, a 10 percent increase in the segregation score led to a 5 percent increase in lead, a 10 percent increase in chromium, and an 11 percent increase in zinc. The team observed similar trends when repeating the analysis for segregation between Hispanic, Asian, or Native American populations and white residents.
“This shows that not all air pollution is the same,” says John Kodros, an atmospheric scientist who led the study at Colorado State University. But the results, he says, weren’t exactly surprising. “Environmental injustice in this country is something that’s been known for a while, not specifically in terms of metal concentrations and racial segregation, but certainly in terms of exposure disparities.” What did catch Kodros off guard, he says, is how wide that gap actually is. In the worst cases, anthropogenic metal concentrations were a couple thousand percent higher in segregated counties than well-integrated areas. Northeastern coastal regions and a cluster of counties in the Midwest (near the Ohio River Valley) showed notably elevated concentrations. And for every metal—except one—this discrepancy persisted over the past decade.
The exception to that rule was vanadium, a metal commonly found in marine fuel oil. In 2010, California and the North American Emissions Control Area began limiting the amount of sulfur in this oil. As ships began using bunker fuel with less vanadium, concentrations of the metal in coastal cities dropped. Kodros says their findings showed that vanadium exposure disparities between segregated and integrated counties also went down. He and Volckens view this diminishing disparity as a kind of silver lining: “We found a lot of bad news, but we found some good, too,” Volckens says. “And that gives us hope that we can reverse this problem.”
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Sacoby Wilson, an environmental health scientist at the University of Maryland College Park, thinks the study is an important step toward a more equitable future. “This brings our exposure science forward,” says Wilson, who was not involved in the work. He wants the analysis to be reproduced on hyperlocal scales, to see how toxic metal concentrations vary, for example, across census tracts within a single county. This would help pinpoint exposure differences based on proximity to structures, like landfills or incinerators, that are spewing these toxins into the atmosphere. It could also confirm suspicions that communities of color within highly segregated counties are the ones bearing the brunt of the most noxious fumes.
But the resources for that don’t exist—yet. The Environmental Protection Agency maintains monitors designed to study metallic components of PM 2.5, but they’re expensive, so this network is sparse; only a few hundred of the devices exist across the nation. That means many regions don’t have monitors at all. Most of the counties included in the study only had one.
Volckens has plans in motion to democratize air quality monitoring by making affordable sensors that are wearable, or can be used with an app on your smartphone. And on November 3, the EPA announced a $53 million grant to fund clean air citizen science initiatives, which Wilson believes could put these monitors in the hands of community green teams themselves. “Data is power,” he says. “So getting better data should lead to better policy.”
Scientists have long known that race is a driver of environmental injustice. These results reveal a new layer, one the researchers hope can be used as a tool to actually combat disparities and make people’s lives healthier. “This study is just more gasoline on the fire. Environmental racism is a real problem in this country, and it has been for decades,” Volckens says. “The technology to clean up the air already exists. We just have to start doing a better job of it.”