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The U.S. is making progress in reducing the public health consequences of air pollutants and fine particulate matter. But these improvements have not benefited everyone equally, according to a new study published on Wednesday in the journal Environmental Health Perspective.

Researchers found an overall drop from 2010 and 2019 in death and disease linked to nitrogen dioxide (NO2), an air pollutant produced from fossil fuel burning, and fine particulate matter (PM2.5) pollution. Deaths attributable to PM2.5 decreased by nearly a third, going from 69,000 to 49,500, and new pediatric cases of asthma attributable to NO2 dipped almost 40%, from 191,000 to 115,000.

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But the relative disparity in mortality and morbidity caused by air pollution among non-Hispanic whites and other ethnic and racial groups increased. For instance, the disparity in pediatric asthma linked to air pollution between the least and the most white communities increased by nearly 20%. Similarly, racial disparities in deaths attributable to fine particulate matter grew by 16%.

“That was a result that we were pretty surprised by,” said Gaige Kerr, a researcher in the department of environmental and occupational health at the George Washington University and the paper’s lead author.

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While asthma cases generally declined across the U.S., they increased in the South, researchers found, especially in areas that tend to have lower income communities with more ethnic and racial diversity. These hardest-hit areas also tend to be closer to sources of air pollution.

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“These disparities have existed for over 100 years. It’s coming up on 100 years of redlining and its impact,” said Cesunica Ivey, an assistant professor of civil and environmental engineering at the University of California, Berkeley and the principal investigator of the university’s Air Quality Modeling and Exposure Lab. “But the footprint is still there.”

More attention to air quality means heavy polluting industries are often bound to remain within or even relocate to poorer areas, worsening the health conditions of their inhabitants. “The issue is the clustering, the boxing in of those populations near these [pollution] sources,” said Ivey, who was not involved in the study.

The paper’s findings also point to possible solutions, including regulating emissions and air pollution on a more local basis. “The way we currently regulate ambient air pollution in the U.S. is agnostic of the starting point — all neighborhoods or all jurisdictions need to achieve [a specific] threshold. This doesn’t account for the fact that there are certain neighborhoods that are chock-full of refineries and highways,” said Kerr. “A better approach based on our results, and on a ton of other research, is that we should have more place-based approaches that account for the fact that some communities do not have a leg up when it comes to their starting point.”

Related: As respiratory diseases rise, EPA tightens air quality standards

Ivey, too, recommends interventions based on a similar principle. “We could potentially introduce a new amendment to the Clean Air Act that takes into account what I call cumulative impact, which represents the historical, long-term disproportionate impact from nearby air pollution sources,” she said.

Such an amendment could enforce stricter air pollution parameters in at-risk areas, as well as impose certain demands on the industries that are located there, such as requiring that companies pay residents to renovate their homes to reduce indoor air pollution. This “location-based approach would pretty much be guided by the maps of disadvantaged communities around the country,” she said.

But while targeting air pollution is an essential step to reduce the morbidity and mortality it causes, Kerr cautioned that these outcomes also reflect other underlying inequalities that need to be addressed to erase disparities.

“We could, in theory, eliminate air pollution,” he said. “But because of poor nutrition, poor access to health care, you know, poor housing stock, we could still see disparities in these same diseases or health endpoints.”