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Microscopic airborne particles known as PM 2.5 contribute to 100,000 premature deaths in the United States each year. A new University of Notre Dame study finds that 40 percent of these deaths can be attributed to pollution that crosses state lines, highlighting the impact of the problem and pinpointing which states are responsible.

The study, published in , provides a tool to quantify responsibility for PM 2.5 pollution, a type of fine particulate matter. It comes amid renewed debate over pollution regulations in the United States. In January, the Environmental Protection Agency said it will stop calculating the聽 鈥� including lives saved and health care costs avoided 鈥� from air pollution rules targeting ozone and PM 2.5.

The research was co-authored by , assistant professor in the and , assistant professor of environment, peace and global affairs in the , along with , a doctoral student in the .

鈥淥ur analysis gives states an evidence-based way to demonstrate when cross-state pollution exceeds safe thresholds and threatens public health,鈥� Marcantonio said. 鈥淩ather than relying solely on an economic evaluation, policymakers can use clear data on health risks to guide decisions that protect vulnerable residents and communities.鈥�

The new study translates the concept of an airshed 鈥� a geographical area where air moves together as a single unit, similar to hydrology鈥檚 concept of a watershed 鈥� into a practical framework that regulators around the world can use.

鈥淥ur study introduces a simpler, data-driven framework that policymakers and regulators can readily adapt,鈥� Crippa said. 鈥淭his is an important improvement upon previous studies, which have had limited scalability and adaptability.鈥�

Findings underscore pollution鈥檚 unequal impact

The study highlights the inequality of pollution in the United States. Researchers found that:

• The percentage of premature deaths attributed to cross-state PM聽2.5 pollution has remained constant at approximately 40 percent since 1998, even though air pollution has fallen by 35 percent in the same time period.

• More than half of U.S. states are net exporters of air pollution; roughly a third are net zero contributors, meaning they are neither importers nor exporters; and the remainder, less than a fifth, are net importers.

• Florida stands out as the largest exporter of cross-state pollution (affecting Georgia and the Carolinas), followed by Vermont and Iowa.

• Exporters include a number of states across the Upper Midwest, the Mid-Atlantic and the Southeast 鈥� including Wisconsin, Michigan, Indiana, Ohio, Kentucky, West Virginia, Virginia, Tennessee, Mississippi, Alabama and South Carolina 鈥� as well as California, Oregon and Washington on the West Coast.

Instead of relying primarily on complex chemical transport models, the authors drew on the concept of an airshed to design a simpler, data-driven approach. They highlighted concentrations of PM 2.5 that exceed established safety thresholds and mapped how these particulates, pushed by prevailing winds, cross state lines, harming communities in which they did not originate.

The research builds upon, which has documented that major air polluters are more likely to be located near a state鈥檚 downwind borders than other types of polluters. This dynamic allows states to economically benefit from industry while avoiding the health issues that come with it.

鈥淥ur findings underscore the unequal distribution of both health risk and responsibility across state lines,鈥� Marcantonio said.

Informing policy to address pollution

Crippa and Marcantonio want their work to inform policy both in the United States, where legal challenges are the main barriers to regulation, and in other global contexts, where a lack of resources is the main barrier.

鈥淯ltimately, we want to provide an evidence-based approach to protecting public health and supporting human dignity,鈥� Marcantonio said.

In the United States, air pollution regulation faces legal and political challenges. Traditionally, it has been decentralized and the Clean Air Act鈥檚 鈥淕ood Neighbor鈥� provision has been the strongest regulatory tool. In 2024, the Supreme Court paused an updated Environmental Protection Agency plan aimed at strengthening that provision and addressing shortcomings in state plans.

In lower-resource contexts outside the United States, regulators often have limited access to complex, resource-intensive air quality models. But they face a similar public health problem: pollution that crosses political boundaries and harms other communities.

Whether the regulatory challenges are legal or technological, Crippa and Marcantonio said their work can provide a more timely and complete picture of pollution鈥檚 impact, furnishing policymakers with data that can help them protect vulnerable downwind communities.

The researchers are extending their work globally through a multi-year project co-led with Danielle Wood, director of the , focused on how pollution that originates in mega-cities affects other communities.

Funding for that work and the cross-state study was provided through a additional support for the cross-state research came from Notre Dame鈥檚 and the .

This work aligns with Notre Dame鈥檚 , a University-wide effort to build a sustainable future where people and nature flourish together.

鈥淥ur research can inform policy and help regulators prioritize permitting, monitoring and enforcement activities,鈥� Marcantonio said. 鈥淯ltimately, we want to provide an evidence-based approach to protecting public health and supporting human dignity.鈥�

Crippa is the principal investigator for the , part of Notre Dame鈥檚 , and Liao is also affiliated with the group. Marcantonio is affiliated with the and the in Notre Dame鈥檚 ; with the and; and with the Keough 91视频鈥檚 and .

Originally published by Josh Stowe at on Jan. 28.

Contact: Tracy DeStazio, associate director of media relations, 574-631-9958 or tdestazi@nd.edu