Farewell frost!
October 30, 2024
In the United States, premature death associated with exposure to nitrogen dioxide pollution — a toxic gas emitted primarily by burning fossil fuels in cars, trucks and power plants — is more likely to impact people of color compared to the white population, a new Northwestern University study has found.
The study is the first to estimate premature deaths attributed to nitrogen dioxide exposure and associated disparities across the contiguous U.S.
By combining air pollution concentrations from high-resolution models, relative risks from epidemiological studies and census mortality data, the researchers pinpointed where and who nitrogen dioxide pollution affects most. Ultimately, they found that approximately 171,000 premature deaths per year are linked to nitrogen dioxide exposure, and a disproportionate number of those deaths occur in marginalized communities. In predominantly Black census tracts, for example, premature deaths related to nitrogen dioxide exposure are 47% higher than the national average, the study authors found.
The study provides further confirmation that historically marginalized communities shoulder disproportionate health burdens related to poor air quality.
The study was published today in the Environmental Science & Technology Letters. Although previous studies have confirmed the unjust burden of nitrogen dioxide exposure on marginalized communities, Northwestern’s study is the first to estimate the mortality burden (from all natural causes excluding accidental deaths) attributed to nitrogen dioxide exposure across the continental U.S.
In predominantly Black census tracts, premature deaths related to nitrogen dioxide exposure are 47% higher than the national average.
“When looking at who is affected most by nitrogen dioxide, we look at not just air pollution exposure, but also susceptibility of the population,” said Northwestern’s Sara Camilleri, who led the study. “We quantify this by looking at the underlying baseline mortality, which represents susceptibility. In this context, susceptibility could be related to higher occurrences of underlying conditions or lesser access to health care. The Black population, in particular, experiences both high susceptibility and higher nitrogen dioxide concentrations.”
“Simultaneous advances in epidemiology and air-quality research have allowed us to more confidently quantify the impacts of nitrogen dioxide exposure on health outcomes,” said Northwestern’s Daniel Horton, the study’s senior author. “Our results indicate that policies aimed at reducing nitrogen dioxide emissions could reduce long-standing environmental injustices and motivate targeted adoption of more stringent standards to protect public health.”
Horton is an assistant professor of Earth and planetary sciences at Northwestern’s Weinberg College of Arts and Sciences, where he directs the Climate Change Research Group. Camilleri is a postdoctoral scholar in Horton’s laboratory.
Combining pollution models with census data
Particularly common in high-traffic and industrial areas, nitrogen dioxide exposure is linked to a number of health complications, including asthma, respiratory infections and even death. Although the U.S. Environmental Protection Agency has strengthened regulations aimed at reducing nitrogen dioxide concentrations, premature deaths associated with exposure remain.
“Over recent decades, we’ve seen a declining trend in nitrogen dioxide emissions,” Camilleri said. “But recent epidemiological studies have shown that nitrogen dioxide is detrimental to health even at low concentrations. Therefore, low concentrations still have an impact, and disparities in air pollution exposure and associated health impacts still persist across the U.S.”
To better understand the most affected populations, Camilleri, Horton and their co-investigators turned to a nationwide land use regression model (LURM) to estimate nitrogen dioxide concentrations. The LURM incorporates surface nitrogen dioxide observations and remotely sensed satellite observations along with land use and roadway information to predict the concentration of nitrogen dioxide pollution.
“Land use regression models estimate pollutant concentrations by associating land use with observed pollutant concentrations,” Horton said. “We know that a roadway has higher nitrogen dioxide than a park, for example. Surface observations help inform that relationship, and satellite data can further constrain it. These associations allow us to estimate nitrogen dioxide concentrations across the contiguous United States.”
Profound impact on Black communities
By combining land use, monitor and satellite data, the researchers estimated nitrogen dioxide concentrations across areas as small as one kilometer throughout the contiguous U.S. Then, to characterize the residents living within these areas, the researchers used population and demographic data from the American Community Survey and mortality rates at the census-tract level derived by Industrial Economics, Inc.
At a national level, the researchers found areas with the largest estimated nitrogen dioxide-attributable mortality rates are 29% Black, 18% Hispanic or Latinx, 5% Asian and 45% white. Considering that the racial and ethnic makeup of total U.S. population is 12% Black, 18% Hispanic or Latinx, 5% Asian and 61% white, the disproportionate impact on Black communities is profound.
Nationwide urban hotspots
Because internal combustion engine-based transportation is one of the greatest sources of nitrogen dioxide emissions, peak concentrations of the pollutant accumulate along highways and major road networks. A large portion of people living close to these hotspots are people of color, who generally have higher-than-average susceptibilities, the authors write.
Filled with dense highways and industrial activity, urban areas experience the highest rates of nitrogen dioxide-related deaths. The larger Detroit area in Michigan, specifically, experiences the highest premature death rate in the country with 1.6 times more nitrogen dioxide-attributable mortalities than the U.S. average. The researchers uncovered a similar pattern in the Chicago and New York City metropolitan areas, where mortality rates are 1.3 and 1.4 times higher than the national average.
The disproportionate burden on marginalized communities is amplified in these areas. For example, Black people make up just 21% of the population in the larger Chicago and Detroit metropolitan areas, yet about 60% of the people affected by nitrogen dioxide-related mortality in these areas are Black.
Exploring potential solutions
To address the unjust impacts of nitrogen dioxide pollution, Camilleri and Horton suggest the adoption of policies that incentivize vehicle electrification and the removal of high-emitting combustion-engine vehicles from roadways. Just last month, their team published a study finding that electrifying 30% of on-road heavy-duty vehicles could save hundreds of lives per year — largely benefitting disadvantaged communities.
“Based on our previous work, we have shown that shifting to cleaner transportation options can have large implications for reducing inequitable transportation-related health burdens,” Camilleri said. “The shift to electric vehicles is definitely one solution that is worth incentivizing from an air quality, public health, climate change and economic perspective.”
The study, “All-cause nitrogen dioxide-attributable mortality burden and associated racial and ethnic disparities in the U.S.,” was a collaborative effort among Northwestern and George Washington University researchers and was supported by the National Science Foundation and NASA.
