Wildfires threaten more than land and homes. The smoke they produce contains fine particles (PM2.5) that can poison the air for hundreds of miles. Air pollution from the 2016 Fort McMurray fire in northern Alberta, Canada sent people in Michigan to the hospital with respiratory illnesses.

As wildfires increase in frequency and severity due to climate change, more and more communities are at risk of prolonged exposure to harmful levels of smoke.

Harvard University researchers, in collaboration with colleagues at Yale University, have created a watch list of hundreds of counties in the western United States at the highest risk of exposure to dangerous levels of pollution from wildfires in the coming decades.

Among those counties, heavily populated counties such as San Francisco County, CA, King County, WA, Alameda County, CA, and Contra Costa County, CA are estimated to face the highest level of risk of wildfire smoke exposure in the coming decades.

The research is described in the journal Climatic Change.

"It hasn't been well understood which populations will be most affected by the threat of air pollution from wildfires induced by climate change," said Loretta J. Mickley, Senior Research Fellow at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and coauthor of the paper. "If we can better predict, down to a county level, who will be most affected, the U.S. Forest Service can prioritize efforts to reduce wildfire risk, such as setting prescribed fires, to clear out dry underbrush."

To identify the highest-risk areas, the team used a fire prediction model and advanced atmospheric modeling to separate pollution caused by wildfires from other pollution sources and track the likely movement of smoke. The team coined a new term, 'smoke wave,' to describe two or more consecutive days of unhealthy levels of PM2.5 from fires.

The study found that across the western U.S., climate change will likely cause smoke waves to be longer, more intense, and more frequent. Of the 561 counties studied, 312 are expected to have more intense smoke waves in the next 30 years.

The team found that between 2004 and 2009, about 57 million people in the western U.S. experienced a smoke wave. Between 2046 and 2051, the team estimated more than 82 million people will likely to be affected by smoke waves, mostly in Northern California, Western Oregon and the Great Plains, where fire fuel is plentiful.

The team estimated that about 13 million more children and seniors - who are at higher risk for respiratory illness - will be affected by smoke waves compared with the present day.

"In the coming decades, we will be seeing the significant human health consequences from these extreme events in a changing climate," said Jia Coco Liu, a recent Ph.D. graduate at the Yale School of Forestry and Environmental Studies and first author of the paper.

But it's not just the future that worries health officials.

"Climate change is a public health crisis and it's happening right now," said Francesca Dominici, Professor of Biostatistics and Senior Associate Dean for Research at the Harvard T.H. Chan School of Public Health and coauthor of the paper.

"Asthmatic kids are going to the hospital today in California because of the smoke from wildfires. If we can figure out who is most at risk, we can start thinking about smoke evacuations and early alert systems for hospitals and local primary care physicians."