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 Environmental Research Letters , 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 economic value of health benefits — including lives saved and health care costs avoided — from air pollution rules targeting ozone and PM 2.5.
The research was co-authored by Paola Crippa , assistant professor in the Department of Civil and Environmental Engineering and Earth Sciences and Richard (Drew) Marcantonio , assistant professor of environment, peace and global affairs in the Keough School of Global Affairs , along with Wenxu Liao , a doctoral student in the Department of Civil and Environmental Engineering and Earth Sciences .
"Our analysis gives states an evidence-based way to demonstrate when cross-state pollution exceeds safe thresholds and threatens public health," Marcantonio said. "Rather than relying solely on an economic evaluation, policymakers can use clear data on health risks to guide decisions that protect vulnerable residents and communities."
The new study translates the concept of an airshed — a geographical area where air moves together as a single unit, similar to hydrology's concept of a watershed — into a practical framework that regulators around the world can use.
"Our study introduces a simpler, data-driven framework that policymakers and regulators can readily adapt," Crippa said. "This is an important improvement upon previous studies, which have had limited scalability and adaptability."
Findings underscore pollution's 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 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 — including Wisconsin, Michigan, Indiana, Ohio, Kentucky, West Virginia, Virginia, Tennessee, Mississippi, Alabama and South Carolina — 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 previous work , which has documented that major air polluters are more likely to be located near a state's 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.
"Our findings underscore the unequal distribution of both health risk and responsibility across state lines," 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.
In the United States, air pollution regulation faces legal and political challenges. Traditionally, it has been decentralized and the Clean Air Act's "Good Neighbor" 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's 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 Notre Dame Global Adaptation Initiative , 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 Notre Dame Strategic Framework Grant; additional support for the cross-state research came from Notre Dame's Environmental Change Initiative and the Lucy Family Institute for Data & Society .
This work aligns with Notre Dame's Just Transformations to Sustainability Initiative , a University-wide effort to build a sustainable future where people and nature flourish together.
"Our research can inform policy and help regulators prioritize permitting, monitoring and enforcement activities," Marcantonio said. "Ultimately, we want to provide an evidence-based approach to protecting public health and supporting human dignity."
Crippa is the principal investigator for the Atmospheric Modeling Group , part of Notre Dame's College of Engineering , and Liao is also affiliated with the group. Marcantonio is affiliated with the Business Ethics and Society Program and the Deloitte Center for Ethical Leadership in Notre Dame's Mendoza College of Business ; with the Environmental Change Initiative and Lucy Family Institute for Data & Society ; and with the Keough School's Kellogg Institute for International Studies and Kroc Institute for International Peace Studies .
How PM 2.5 harms public health
- Each year, PM 2.5 contributes to 4 million deaths globally.
- PM 2.5 affects the lungs, heart and brain, contributing to cancer, COPD, stroke and cognitive decline.
- It disproportionately affects vulnerable communities worldwide.
- Pregnant women, children, seniors and people with asthma, diabetes, obesity or kidney disease face higher risks.
