Air Pollution Levels Have Improved, but Not for Everyone

Disparities in air pollution exposure between racial and ethnic groups are getting worse, according to a new study.

From 2000 to 2016, the average concentration of fine particulate matter (PM_2.5) pollution in the U.S. decreased by 40%, from 12.6 µg/m^-3 to 7.5 µg/m^-3, according to Abdulrahman Jbaily, PhD, of Harvard T. H. Chan School of Public Health, and colleagues, reporting in Nature.

However, after modeling PM_2.5 air pollution and racial demographics by zip code, they found that the improvements were much more apparent in disproportionately white communities compared with disproportionately Black communities.

Findings show that “PM_2.5 reductions between 2000 and 2016 have not benefited all areas of the U.S. equally, and consequently resulted in an increase in relative disparities in exposure to air pollution,” the study authors wrote.

In 2016, the average amount of pollution people of color experienced hovered between 8 µg/m^-3 and 10 µg/m^-3, while the average amount of pollution white people were exposed to was around 7 µg/m^-3. Compared with the white population, average PM_2.5 concentration exposures were 13.7% higher in the Black population and 36.3% higher for Native American populations in that year.

“One thing that the scientific literature is quite clear about is that what we’ve been trying so far hasn’t been working, so it needs to be something different. Probably the most important thing is to center the voices and priorities of members of the affected communities in any planning process for how to proceed,” commented Christopher Tessum, PhD, environmental engineering researcher at the University of Illinois Urbana-Champaign.

There are multiple ways to define disparity, Tessum noted. Jbaily and colleagues had opted for relative disparity, comparing the proportion of exposures above three different policy thresholds — the Environmental Protection Agency (EPA) threshold (12 µg/m^-3), the World Health Organization threshold (10 µg/m^-3), and a recommended lower threshold (8 µg/m^-3) — between groups over time.

A very similar study published last month examined absolute disparity, or the actual differences between exposure levels to air pollution, and found that disparities between different racial and ethnic groups had actually declined.

“Both definitions of disparity have merit,” said Tessum, who was not involved with either study. “Looking at exposures above a given regulatory threshold [is] perhaps most useful to inform policy, but looking at overall concentrations is useful because many studies have found that PM_2.5 is harmful even at levels below current standards.”

Long-term exposure to air pollution has been tied to a range of negative health outcomes, from increased hospital visits due to asthma to higher levels of premature death, according to the CDC. There are a number of studies that have documented how people of color are consistently exposed to more air pollution and have worse health outcomes as a result, the study authors said.

PM_2.5 pollutants, particles that are 2.5 micrometers or smaller, pose the greatest risk since they can get into the lungs and even the bloodstream, according to the EPA. One major source of PM_2.5 pollution, nitrogen oxides from vehicle emissions, was tied to nearly 2 million new cases of childhood asthma in 2019.

For the study, Jbaily’s group used air pollution and population estimates from 2000-2016 modeled with zip code regions in the U.S., with population data taken from the U.S. Census Bureau for 2000-2010 and from the American Community Survey for 2011-2016. Zip code regions were ranked from least to most dense in terms of every racial and ethnic group and in terms of median income. Then, air pollution concentrations in each zip code region above the policy thresholds were found and compared.

Notably, the study found that as the proportion of Black or Hispanic residents grew over time in a given area, the average concentration of PM_2.5 air pollution also increased. Regions that were more than 85% Black or Hispanic saw particularly steep inclines in air pollution levels, the study authors said.

On the other hand, as regions became increasingly white, the concentration of pollution decreased, and regions that were more than 70% white experienced disproportionately lower amounts of air pollution.

Differences in air pollution exposure levels between groups with different income levels were small. In 2016, areas with low median income had 4% higher air pollution levels compared to areas with high median income, the investigators reported.

Study authors noted that they used moving averages for the years between 2000 and 2010 in which U.S. Census data was not available. They also acknowledged that their use of zip codes does not account for variations in pollution levels or income within a given region.