Air pollution is one of the world’s most lethal killers, contributing to millions of premature deaths each year. It contributes to a host of long-term health issues, but the effects of air pollution are not felt equally, especially in the U.S.

Two recent studies published in the American Journal of Public Health and Environmental Science & Technology Letters reveal stark racial disparities in air pollution exposure, finding that certain historically marginalized communities suffer disproportionately from toxic pollutants in the air. The findings show that despite the overall declining patterns of air pollution in the U.S., certain communities are being left behind.

What’s new — The first study compares the differences in air pollution between counties with significant American Indian populations and those without.

The researchers specifically looked at PM2.5 air pollution, a type of fine particulate matter that can linger for days or weeks in the air, and contribute to the smog shrouding the skylines of Los Angeles and other cities. It is arguably the greatest environmental risk factor for human mortality, contributing to cardiovascular and other diseases.

Using statistical models, researchers examined PM2.5 air pollution between 2000 and 2018, and the disparities between American Indian and non-American Indian communities were evident.

Even though American Indian communities experienced less air pollution than other communities in 2000, those benefits had effectively been reversed over the course of the past two decades. By 2018, counties with significant American Indian populations experienced a greater concentration of air pollution per cubic meter than compared to non-American Indian counties, even though air pollution overall declined in the U.S. due to more stringent environmental regulations.

The findings suggest “that many tribal communities, particularly in more rural areas, may not have benefited as much from increased air pollution regulation as the rest of the country,” Maggie Li, lead author on the study and a Ph.D. candidate in the Department of Environmental Health Sciences at Columbia Mailman School School of Public Health, tells Inverse.

The second study examines air pollution levels of both PM2.5 and the shorter-lived local pollutant nitrogen dioxide (NO2) — typically produced from emissions from vehicles and power generators — in historically redlined areas in more than 200 American cities. The study analyzes data from 2010, based on demographic information from the U.S. Census conducted that year.

For context: In the 1930s, the federal Home Owners’ Loan Corporation (HOLC) drew maps that graded neighborhoods on a scale from most desirable (A) to the most hazardous or “redlined” (D), which resulted in widespread discrimination against communities of color. Neighborhoods with higher Black and low-income populations often received D grades. People of color were denied federal loans and often explicitly prohibited from purchasing homes in better-rated neighborhoods due to racist homeownership covenants. The Fair Housing Act banned the practice of redlining in 1968, but its effects persisted across generations.

“This practice isolated communities of color, restricting their ability to build wealth through homeownership and informed later local government land-use decisions that placed hazardous industries in and near D neighborhoods,” write the researchers.

In other words, local governments often placed higher polluting industrial facilities near or in redlined areas — predominantly communities of color — contributing to greater air pollution compared to other neighborhoods, even in the present day.

The researchers found that the effects of the HOLC policies linger today in the form of air pollution disparities between historically redlined areas and other neighborhoods.

The “national statistics show that redlining is strongly associated with NO2 and more weakly but detectably associated with PM2.5,” according to the paper.

The researchers add that “present-day disparities in U.S. urban pollution levels reflect a legacy of structural racism in federal policy-making […] apparent in maps drawn more than 80 years ago.”

Why these studies matter — Both studies point to communities whose higher-than-average air pollution exposures have been overlooked in larger trends of air pollution decline — yet another form of environmental injustice against communities of color.

“While air quality has improved in the United States over the past several decades, people of color (POC), particularly Black and Hispanic Americans, are still exposed to higher-than-average levels of air pollution,” write the researchers in the redlining study.

For example, air pollution monitoring is often conducted in cities, providing little information on air pollution levels in rural communities. As the researchers write in the first study:

Little is known, however, about the extent of particulate air pollution exposure and its potential health effects among rural Native American communities, as most US studies of air pollution have been conducted in urban settings.

But the studies also highlight how intentionally racist policies over time have contributed to the disproportionate air pollution burden in these communities of color.

The second study illustrates “how redlining, a nearly 80-year-old racially discriminatory policy, continues to shape systemic environmental exposure disparities in the United States,” according to the paper.

The findings demonstrate how longstanding racial disparities impact communities of color today in the most basic form of environmental injustice: air pollution.

What’s next — The two studies’ findings are sobering, but linking this data to further public health and policy research may be the next step in addressing the impacts of air pollution on communities of color.

The researchers write in the first study that the racial disparities in PM2.5 air pollution between American Indian and non-American Indian counties highlight “the need to strengthen air pollution regulations and prevention implementation in tribal territories and areas where AI populations live.”

Since PM2.5 is often linked to heart disease, the researchers also hope to see “future investigations of health impacts associated with air pollution in [American Indian] populations.”

The researchers of the report on redlining believe their work will help policymakers address and find solutions to racial disparities in air pollution exposure, though it will take time to fix problems that are the result of longstanding environmental injustice and systemic racism.

“Fully addressing exposure inequities will require transformations sustained across generations,” the researchers conclude.