Normally, America’s smokiest cities lie out west. But Canada’s unusually intense 2023 wildfire season smothered American cities in smoke farther east than is usual, according to new research to be presented on at AGU’s 2024 Annual Meeting.

Smoke from forest fires impacts millions of people every year. It can travel thousands of miles from its origin, creating a layer of haze and worsening respiratory conditions such as asthma.

Now, researchers have developed a new algorithm using satellite imagery and particulate matter (PM2.5) levels to quickly see which parts of the country experienced the most smoke over the last five years. Cities in Oregon, Nevada, Washington and other Western states had the highest smoke levels on average. But 2023 saw metropolitan areas as far east as Baltimore flooded with unhealthy smoke levels, and cities in Wisconsin and Minnesota bore the brunt of the incoming smoke from Canadian forest fires.

Knowing where smoke strikes can help cities prepare for health issues and understand just how much of their PM2.5 air pollution comes down to wildfires, said Dan Jaffe, an atmospheric chemist at the University of Washington who led the research.

Jaffe will present his research on 10 December at AGU’s 2024 Annual Meeting in Washington, D.C., where more than 26,000 scientists will gather to discuss the latest Earth and space science research.

Smoke City, USA

Wildfires are growing in size and frequency because of climate change, and as people move from urban areas to woodlands and other more fire-prone areas, the population exposed to smoke is growing. Current methods for measuring impacts of wildfire often focus on “smoke days,” where smoke is above ‘normal’ levels. Those methods are labor intensive, and data analysis can take up to a year to complete.

To get a faster turnaround time, Jaffe and colleagues created an algorithm that could quickly sorted through two sets of data collected between 2019 and 2023: satellite images of smoke and PM2.5 concentrations collected at ground level throughout the United States. The researchers trained an algorithm to recognize smoke days from these two data sets. They then added health data from hospitals in those cities to see whether smoke had an impact on emergency room visits during especially hazy days.

The algorithm revealed that three cities in western Oregon — Medford, Grants Pass and Bend — were the smokiest cities on average over five years. In Medford, the smokiest city, daily levels of PM2.5 averaged 4.2 µg/m3 over one year. Smoke doesn’t impact towns every day. But averaged out over a whole year, the smokiest cities in each Western state saw daily averages of 3.4 µg/m3 in Gardenville-Rancho in Nevada; 2.7 µg/m3 in Bishop, California; Yakima, at 2.5 µg/m3 in Washington; and 2.3 µg/m3 in Fairbanks-College, Alaska. The smokiest city with more than a million people is the Sacramento metropolitan area, with an annual daily PM2.5 averaging 2.0 µg/m3.

The EPA recommends that annual exposure to PM2.5 should not exceed 9.0 ug/m3. That means that in the smokiest parts of the country, average daily exposures are “a very significant fraction’ of the EPA’s annual cut off, Jaffe said.

A strange wildfire season

States outside the West also saw a rise in emergency room visits due to smoke. The researchers estimate that around one-third of all PM2.5-related emergency room visits in Detroit during 2023 were due to smoke. Those visits were likely caused by smoke from Canadian fires pouring in over the border.

“2023 was this strange year where the Canadian forests were just torched like crazy, and the Midwest got hit extremely hard,” Jaffe said.

Within the study period, 2023 saw the highest increase in emergency room visits related to wildfire smoke. The entire country saw an extra 16,000 emergency room visits during “smoke days” in 2023 compared to previous years. In Bend, Oregon, a city that outside of the fire season has relatively clean air, around 60% of asthma-related visits in 2023 were related to smoke.

Because of the Canadian fires, North Dakota, Minnesota and Wisconsin joined the list of smokiest spots. Even Baltimore’s daily PM2.5 average reached 2.0 µg/m3 in 2023, compared to an average of 0.7 µg/m3 during the other four years.

A repeat of 2023 isn’t likely to occur immediately, Jaffe said. But wildfire patterns are changing in North America. That comes with health implications for millions of Americans, with a growing body of research suggesting that smoke exposure can have long-term impacts on human health.

“For cities and towns, I think it’s important to be planning ahead and thinking about what’s a normal year, and what’s an extreme year,” Jaffe said.