Leaders have long thought that air pollution crossing lines from neighboring countries posed a significant challenge. But how much of a challenge and what to do about it has remained an open question. A new study sheds some light by measuring the toll of transboundary air pollution on people’s health, as well as the important role strong policies can play in improving health outcomes throughout the region.

“Using recent advances in atmospheric science that allowed us to measure the trajectory of fine particulate pollution—the deadliest form of air pollution—we discovered that countries are vastly impacted by the pollution caused by their neighbors,” says Koichiro Ito, an associate professor at the University of Chicago Harris School of Public Policy. “The good news is the benefits of strong policies to reduce pollution also extend to neighboring states. This underscores the importance of treating air pollution as a regional and even global challenge that requires cooperation, and not just a problem to confront at a local level.”

Ito and his co-authors Rao Kotamarthi from the U.S. Department of Energy’s Argonne National Lab, and Seonmin Will Heo, a PhD student at UC Santa Barbara who began work with Ito in 2020 as a Harris Public Policy graduate student, study the case of South Korea. Northwest South Korea experiences strong westerly winds from China during the fall and winter. The researchers found that these winds lead to significantly higher levels of pollution compared to southeast South Korea. The northwest and southeast experience similar pollution levels during the spring and summer—when there is far less westerly wind from China.

This pollution has a direct impact on people’s health in South Korea, leading to a 0.6 percent increase in mortality for every one microgram per cubic meter increase in transboundary particulate matter pollution (PM2.5 ). That translates to an additional 31.2 deaths for every million people annually. The youngest members of society—babies who are a year old or younger—are the most vulnerable, with the mortality rate increasing by 2.1 percent for every one microgram per cubic meter increase in transboundary particulate pollution from China.

Other health indicators also increased during the time studied, 2013 to 2017. The researchers found that a one microgram per cubic meter increase in particulate pollution from China leads to about 18 additional ER visits for every million people a year for asthma and 176 additional ER visits for every million people for rhinitis (nasal inflammation)—a 0.5 percent and 3.4 percent increase, respectively.

“Pollution coming from China makes people in South Korea sicker and die at higher rates—that much is clear,” says Ito. “But China’s immense success in reducing pollution in recent years provided the perfect case study to see that the benefits of pollution policies also travel across boundaries.”

After China began its “war on pollution” in 2014, which came with a series of strong clean air rules, pollution significantly declined nationwide. Ito and his coauthors find that China had a 14.07 microgram per cubic meter drop in particulate pollution and South Korea had a 9.63 microgram per cubic meter drop in transboundary particulate pollution from China from 2015 to 2019. This drop led to fewer deaths from transboundary air pollution—about 300 fewer deaths for every million people a year—which they calculate saved South Korea $2.62 billion per year.

Ito and his coauthors did find evidence that China may have strategically reduced more air pollution in Chinese cities where most air pollution stays inside the country borders and reduced less in cities where most air pollution blows outside of the country. This implies the spillover benefits of clean air rules could be larger for neighboring countries, and therefore, further regional or global cooperation can be valuable to address air pollution.

Ito and Kotamarthi thank the Global Energy Challenge conference sponsored by the Energy Policy Institute at the University of Chicago (EPIC) and the Argonne National Laboratory for seeding their collaboration and work in exploring this topic.