More than half of all air-quality-related early deaths in the United States are a result of emissions originating outside of the state in which those deaths occur, MIT researchers report in a paper in the journal Nature.

The study focuses on the period between 2005 and 2018 and tracks combustion emissions of various polluting compounds from various sectors, looking at every state in the contiguous United States, from season to season and year to year.

In general, the researchers find that when air pollution is generated in one state, half of that pollution is lofted into the air and carried by winds across state boundaries, to affect the health quality of out-of-state residents and increase their risk of early death.

Electric power generation is the greatest contributor to out-of-state pollution-related deaths, the findings suggest. In 2005, for example, deaths caused by sulfur dioxide emitted by power plant smokestacks occurred in another state in more than 75% of cases.

Encouragingly, the researchers found that since 2005, early deaths associated with air pollution have gone down significantly. They documented a decrease of 30% in 2018 compared to 2005, equivalent to about 30,000 avoided early deaths, or people who did not die early as a result of pollution. In addition, the fraction of deaths that occur due to emissions in other states is falling—from 53% in 2005 to 41% in 2018.

Perhaps surprisingly, this reduction in cross-state pollution also appears to be related to electric power generation: In recent years, regulations such as the Environmental Protection Agency’s Clean Air Act and other changes have helped to significantly curb emissions from this sector across the country.

The researchers caution, however, that today, emissions from other sectors are increasingly contributing to harmful cross-state pollution.

Regulators in the US have done a pretty good job of hitting the most important thing first, which is power generation, by reducing sulfur dioxide emissions drastically, and there’s been a huge improvement, as we see in the results. Now it’s looking like other emissions sectors are becoming important. To make further progress, we should start focusing on road transportation and commercial and residential emissions.

—study leader Steven Barrett, an associate professor of aeronautics and astronautics at MIT

The research was a collaboration between MIT’s Laboratory for Aviation and the Environment and the MIT Joint Program on the Science and Policy of Global Change.

Scientists have long known that pollution observes no boundaries, one of the prime examples being acid rain. In the United States, tracking how pollution from one state affects another state has historically been tricky and computationally difficult, Barrett says. For each of the 48 contiguous states, researchers would have to track emissions to and from the rest of the 47 states.

But now there are modern computational tools that enable you to do these assessments in a much more efficient way. That wasn’t really possible before.

—Steven Barrett

Barrett and his colleagues developed such tools, drawing on fundamental work by Daven Henze at the University of Colorado at Boulder, to track how every state in the contiguous US affects pollution and health outcomes in every other state. They looked at multiple species of pollutants, such as sulfur dioxide, ozone, and fine particulates, from various emissions sectors, including electric power generation, road transportation, marine, rail, and aviation, and commercial and residential sources, at intervals of every hour of the year.

They first obtained emissions data from each of seven sectors for the years 2005, 2011, and 2018. They then used the GEOS-Chem atmospheric chemistry transport model to track where these emissions ended up, from season to season and year to year, based on wind patterns and a pollutant’s chemical reactions to the atmosphere.

Finally, they used an epidemiologically-derived model to relate a population’s pollutant exposure and risk of early death.

We have this multidimensional matrix that characterizes the impact of a state’s emissions of a given economic sector of a given pollutant at a given time, on any other state’s health outcomes. We can figure out, for example, how much NO_x emissions from road transportation in Arizona in July affects human health in Texas, and we can do those calculations instantly.

—Steven Barrett

Early-death source–receptor matrices for 2011. a, Source–receptor matrix showing total early deaths per year for 48 × 48 states (right), and its breakdown into PM_2.5- and ozone-attributable impacts (left). b, Source–receptor early-death attribution to emission sectors (top) and emission species (bottom) that lead to the formation of PM_2.5 and/or ozone. States are grouped into US Bureau of Economic Analysis regions and ordered west (left) to east (right). Boxed percentages represent the fraction of impacts that occur out of the state that caused the corresponding emissions. Obtaining the summarized matrices shown using conventional approaches would require 433-year-long simulations. Dedoussi et al.

The researchers also found that emissions traveling out of state could affect the health of residents beyond immediate, neighboring states.

It’s not necessarily just the adjacent state, but states over 1,000 miles away that can be affected. Different kinds of emissions have a different kind of range.

—Steven Barrett

For example, electric power generation has the greatest range, as power plants can loft pollutants far into the atmosphere, allowing them to travel over long distances. In contrast, commercial and residential sectors generally emit pollutants that chemically do not last as long in the atmosphere.

In general, the researchers found that out-of-state air pollution was associated with more than half of all pollution-related early deaths in the US from 2005 to 2018.

In terms of the impact on individual states, the team found that many of the northern Midwest states such as Wyoming and North Dakota are “net exporters” of pollution-related health impacts, partly because the populations there are relatively low and the emissions these states generate are carried away by winds to other states.

Those states that “import” health impacts tend to lie along the East Coast, in the path of the US winds that sweep eastward.

New York in particular is what the researchers call “the biggest importer of air pollution deaths”; 60% of air pollution-related early deaths are from out-of-state emissions.

This research was supported, in part, by the US Environmental Protection Agency, the MIT Martin Family Fellowship for Sustainability, the George and Marie Vergottis Fellowship at MIT, and the VoLo Foundation.

Resources

• Dedoussi, I.C., Eastham, S.D., Monier, E. et al. (2020) “Premature mortality related to United States cross-state air pollution.” Nature 578, 261–265 doi: 10.1038/s41586-020-1983-8