Neighborhoods that were subject to redlining in the 1930s tend to have higher levels of air pollution many decades later, a new study has found.

The paper's authors looked at air quality data from 202 U.S. cities and found a strong correlation between pollution levels in 2010 and the historical patterns of redlining. Their study was published this week in the journal Environmental Science & Technology Letters.

Redlining was the discriminatory mortgage appraisal practice used by the federal government after the Great Depression, drawing lines around Black and immigrant areas that denoted them as risky sites for mortgages. Neighborhoods were classified from "A" or best, to "D" for hazardous, and colored in red.

"Neighborhoods in those cities for which maps were drawn by the Home Owners' Loan Corporation, those neighborhoods that were redlined have worse air quality compared to those that were graded either A or B," explains Rachel Morello-Frosch, one of the paper's authors and a professor at the University of California at Berkeley School of Public Health and the Department of Environmental Science Policy and Management.

Researchers looked at the levels of two pollutants: nitrogen dioxide (a gas associated with vehicular exhaust and industrial facilities) and tiny particles known as PM 2.5. Both are regulated by the Clean Air Act.

"We see a really clear association between how these maps were drawn in the '30s and the air pollution disparities today," Joshua Apte, one of the paper's authors and an assistant professor of environmental engineering and environmental health sciences at UC Berkeley, tells NPR. "And that's not surprising, but it is very striking."

Redlining's legacy is troubling and persistent

This study, by researchers at UC Berkeley and the University of Washington, is significant because it analyzes modern urban air pollution and historical redlining at the national level. But many other studies have found other deleterious effects of redlining. As NPR has previously reported, historically redlined neighborhoods are hotter in temperature and residents experience a range of health disparities.

The places assigned low grades tended to be the places where minorities and immigrants lived. Morello-Frosch says the language on the maps was often pointedly racist, with "comments about the hopeless 'heterogeneity' of the neighborhood, which led to a lower grade."

Maps also reflected environmental factors — factors intertwined with power and race. For instance, a neighborhood would receive a lower grade if it already had industrial uses there.

The federal maps and grades then compounded the problem, allowing for more hazardous facilities that spew emissions to locate in the same neighborhoods. Now, people of color at every income level in the U.S. are exposed to higher-than-average levels of air pollution.

The researchers found that historical redlining isn't the only factor affecting air quality: Race matters too. Even within the same grade on a historical map, people of color were experiencing greater air pollution in 2010.

"So it's not like [redlining] is the only thing that's driving the racial and ethnic disparities," Apte says. "In fact, there are pretty big disparities such that white people experience cleaner air pollution or people of color experience more air pollution, regardless of what grade people lived in."

There could be several reasons for that. White people living in a formerly redlined area might live in a part of the neighborhood that is further from the highway, for instance, and air pollution within a neighborhood can vary a lot. The places that are more polluted within a redlined area may have more people of color living in them.

The findings highlight the legacies of structural racism

The health impacts of air pollution are serious: The air we breathe kills thousands of Americans every year. Among the effects of nitrogen dioxide pollution are childhood asthma, heart attacks, strokes and lung cancer.

The difference between the air quality in neighborhoods is significant — and noticeable. In terms of nitrogen dioxide measured in the cities in this study, "the D-grade neighborhoods on average experience 50% greater pollution than the A-grades. And in some cities, it's more than double," Apte says.

"We know that these air pollutants ... have adverse health consequences and that these legacies of structural racism will have consequences — and are having consequences for community health," Morello-Frosch says.

That's why, she says, targeted approaches are needed to regulate the emission sources of these pollutants — and the solution can't be just improving the average overall air quality.

"We need to go beyond simply regional approaches to addressing poor air quality, to also address the persistence of inequities and exposures to air pollution by race," Morello-Frosch says. "Because while we can improve air quality for everybody within a region and decrease levels of air pollutants for everybody, very often by not taking a targeted approach, that equity gap will still persist, and some people will still not have as great of public health benefits of air quality regulation as others.

"Future strategies really, really, really need to take a targeted approach to narrow that racialized air quality gap that we found in our study," she says.

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