A new nationwide study finds that the U.S. made little progress from 2000 to 2010 in reducing relative disparities between people of color and whites in exposure to harmful air pollution emitted by cars, trucks and other combustion sources.
The groundbreaking study led by University of Washington researchers estimated exposure to outdoor concentrations of a transportation-related pollutant -- nitrogen dioxide (NO2) -- in both 2000 and 2010, based on neighborhoods where people live. It found disparities in NO2 exposure were larger by race and ethnicity than by income, age or education, and that those inequities persisted across the decade.
While absolute differences in exposure to the air pollutant dropped noticeably during that time period for all populations, the relative difference -- or the gap between pollution levels to which white people and people of color were exposed -- narrowed only a little.
The study will be published Sept. 14 Environmental Health Perspectives. The researchers developed a first-of-its-kind model that combines satellite and regulatory measurements with land use data to predict pollution at a neighborhood level throughout the United States.
The positive news is that across the U.S., average exposure to NO2 for all races and income levels dropped from 2000 to 2010. Measured in parts per billion (ppb), estimated average annual NO2 exposure decreased from 17.6 to 10.7 ppb for nonwhite populations, and from 12.6 to 7.8 ppb for white populations.
Yet people of color were consistently exposed to more air pollution than their white non-Hispanic counterparts during the decade. Considering relative differences, nonwhites experienced 40 percent higher exposures than whites in 2000; in 2010, that gap shrunk only slightly, to 37 percent. Furthermore, in 2000, concentrations of NO2 in neighborhoods with the highest proportion of nonwhite residents were 2.5 times higher than in neighborhoods with the lowest proportion of nonwhite residents. In 2010, that value increased slightly, to 2.7 times higher.
The study concludes that if people of color had breathed the lower NO2 levels experienced by whites in 2010, it would have prevented an estimated 5,000 premature deaths from heart disease among the nonwhite group.
"The finding that shocks us is that when it comes to how much NO2 a person breathes, it's still race that matters," said senior author Julian Marshall, UW professor of civil and environmental engineering.
"At any income level -- low to medium to high -- there's a persistent gap by race, which is completely indefensible. It says a lot about how segregated neighborhoods still are and how things are segregated," Marshall said.
NO2 comes from sources such as vehicle exhaust, power plants and off-road equipment and is one of six important "criteria air pollutants" monitored by the Environmental Protection Agency. It has been linked to asthma symptoms, increased susceptibility to respiratory problems and heart disease.
The research team, which began their work at the University of Minnesota, previously analyzed NO2 concentrations for the year 2006 by race, income and other demographic factors identified in the U.S. Census. The team's air pollution model, which combines existing EPA air quality monitoring and satellite data with detailed land use information, allows them to accurately predict pollution concentrations across the country at the U.S. Census block level -- information previously unavailable at that scale.
In this first longitudinal study of its kind, researchers wanted to examine how much progress was made in addressing inequities in NO2 exposure over a decade. They compared environmental injustice metrics in 2000 and 2010 on a national basis and by region, state, county and urban areas.
On the whole, researchers said, policies to reduce NO2 air pollution are working. But the finding that exposure differences are larger by race and ethnicity than by income, age or education was equally true in 2010 as in 2000.
"Everyone benefited from clean air regulations and less pollution; that's the good news," said lead author and UW civil and environmental engineering doctoral student Lara Clark. "But the fact that there is a pervasive gap in exposure to NO2 by race -- and that the relative gap was more or less preserved over a decade -- is the bad news."
The UW study did not explore the underlying reasons for that gap, but its findings are consistent with previous research. Both racial minorities and low-income households are disproportionately likely to live near a major road where transportation-related pollution is typically highest. U.S. cities, in general, also tend to be more segregated by race and ethnicity than by income.
The UW team did conclude that the narrowing of the racial gap in NO2 exposures was driven more by improving air quality than by demographic changes over the 10-year period.
"That suggests that air pollution is coming down faster than cities are becoming less segregated," Marshall said.
Next steps for the research team include looking at how changes in demographics, industry and urban form at the city level affect NO2 exposure, and developing similar models for other EPA criteria air pollutants.
The study was co-authored by Dylan B. Millet, professor in the University of Minnesota's Department of Soil, Water and Climate.
The research was funded by the National Science Foundation and the Environmental Protection Agency.