Long-term exposure to wildfire smoke associated with higher risk of death

As wildfires blaze across the U.S., public health researchers are growing increasingly concerned about the health impact of fine particles in the smoke – particles that are so small they can penetrate small cracks in buildings and be inhaled deep into the lungs.

In a study published by PNAS, researchers from the Yale School of Public Health (YSPH) investigated whether long-term exposure to fine inhalable particles in wildfire smoke –known as PM2.5 because they are generally 2.5 micrometers, and smaller, in diameter – was associated with increases in causes of death. The study looked at a wide range of deaths in the contiguous United States including deaths related to non-accidental causes, cardiovascular problems, digestive issues, endocrine complications, mental health problems, and chronic kidney disease.

The researchers analyzed and compared monthly data regarding wildfire smoke and recorded levels of PM2.5, along with mortality data from all 3,108 counties in the contiguous U.S. from 2007 to 2020. PM2.5 is used as an indicator of exposure because it is a major component of smoke and can deeply penetrate the respiratory system. In recent years, wildfires contributed to up to 25% of total PM2.5 concentrations across the U.S. and up to half in some Western regions.

Pollutants from fires can travel great distances and have the potential to affect human health thousands of kilometers away. The Smokehouse Creek Fire in February of 2024, for instance, burned an estimated 1.1 million acres in Texas, yet smoke from the fire was reported over Alaska, most of Canada, and the continental U.S, and moved as far north and east as the Hudson Bay and Quebec.

The YSPH study found that PM2.5 in smoke contributed to approximately 11,415 non-accidental deaths per year in the contiguous U.S. per year. 4,512 were attributed to cardiovascular disease.

The study’s findings highlight the need for a comprehensive approach to mitigate the compound health risks of wildfire smoke and heat, said senior author Dr. Kai Chen, PhD, associate professor of epidemiology (Environmental Health Sciences) and co-faculty director of the Yale Center on Climate Change and Health. “For example, public health responses can include clear communication and education about the health risks of compound events of smoke and heat exposure. In addition, during wildland fire events, special attention should be given to populations vulnerable to heat, such as the elderly and individuals with pre-existing health conditions.”

Of the deaths associated with long-term exposure to PM2.5, approximately 72.5% were from cardiovascular diseases, mental disorders, endocrine diseases, and digestive diseases, according to the study. The data also revealed greater increases in mortality rates among older adults exposed to PM2.5 than has been reported in previous studies.

PM2.5 can be deposited in the kidneys, contributing to kidney diseases, the researchers said. Endocrine disease deaths associated with PM2.5 exposure were ranked second highest in the study with about 1,142 deaths per year. Chronic kidney disease deaths were estimated to be 320 per year. PM2.5 exposure has also been associated with insulin resistance, which may lead to diabetes and other endocrine diseases, the researchers said.

“These fine particles can penetrate the respiratory tract and bloodstream, triggering oxidative stress and inflammation, which can impair lung and vascular function,” said Dr. Yiqun Ma, PhD ‘24, first author and now a postdoctoral fellow at Scripps Institution of Oceanography.. “Additionally, PM2.5 can enter the gastrointestinal tract, causing imbalances in the intestinal microecology.”

Mental health is also affected through oxidative stress, neuroinflammation, and dysregulation. This has been linked to psychological diseases and aggravation of physical health conditions, which could worsen mental health.

Overall, the findings further clarify the significant public health threat of wildfire smoke, and the health risks it presents for individuals residing near the fires as well as those residing downwind of the wildfire. The health threat has been further exacerbated by climate change, which extended the annual wildfire season from July to September to June to November.

The study was supported by the National Heart, Lung and Blood Institute of the National Institutes of Health, and the Institution for Social and Policy Studies at Yale University. YSPH contributors are first author Dr. Yiqun Ma, PhD ‘24, and Chen, the senior last author.

“Interdisciplinary collaboration is critical in conducting this research,” said Chen. “For example, environmental epidemiologists designed and performed the formal analysis, experts in air pollution modelling contributed to the exposure assessment of PM2.5, researchers in biostatistics helped improved our statistical models, and experts with clinical medicine backgrounds contributed to the interpretation of the results.”

Contributing to this study was Assistant Professor Dr. Emma Zang, PhD, from the department of sociology at Yale University and the department of biostatistics at YSPH; Distinguished Professor Dr. Yang Liu, PhD, from the Rollins School of Public Health at Emory University; Assistant Research Scientist Dr. Jing Wei, PhD, from the Earth System Science Interdisciplinary Center at the University of Maryland; and Assistant Professor Yuan Lu. Also contributing were Dr. Harlan Krumholz, MD, Harold H. Hines Jr. Professor of Medicine at Yale School of Medicine and professor of public health at YSPH, and Dr. Michelle L. Bell, PhD, Mary E. Pinchot Professor of the Yale School of the Environment.