The western United States has endured a sharp increase in two kinds of pollutants related to wildfire smoke since the beginning of the century, a new analysis suggests. 

Scientists examined records of fine particulate matter and ozone from 2000 to 2020, and discovered that events in which people were simultaneously exposed to extreme concentrations of both pollutants have become more frequent, cover larger areas, and last longer. Furthermore, the researchers found a connection between days in which high levels of ozone and fine particulate matter co-occurred and areas burned by wildfires. They estimate that the maximum extent of land subjected to these noxious air conditions on a given day has more than doubled over the past two decades.

“We are expecting these trends to continue given climate change, so the best we can do at this point is to increase awareness and take steps to protect ourselves from worsening air pollution,” says Dmitri Kalashnikov, a doctoral student in environmental science at Washington State University Vancouver. He and his colleagues reported the findings on January 5 in Science Advances

Wildfires emit a mixture of hazardous compounds that contribute to cardiovascular and respiratory illnesses when people breathe them in, including fine particulate matter or “PM2.5.” These tiny particles suspended in the air are less than 2.5 micrometers in size—a small fraction of the width of a human hair. Wildfires also release some ozone, Kalashnikov says, as well as compounds that react with other pollutants to produce ozone. Studies indicate that ozone and fine particulate matter are disproportionately harmful when people are exposed to both pollutants at the same time, he adds.

In the past, levels of fine particulate matter across the western states tended to peak in winter. “You have these stagnant air patterns, and air gets trapped in the valleys and just sits there for days, which builds air pollution,” Kalashnikov says. “But wildfires have changed that equation.”

Climate change is worsening summer wildfire activity. Meanwhile, the hot, sunny conditions that persist during fire season also fuel the chemical reactions that increase ozone. “It used to be that ozone would be high in summer and PM2.5 would be high in winter,” Kalashnikov says. “It’s now changed to where both are high during the summer thanks to wildfire smoke.”

To investigate these trends, he and his colleagues examined daily air pollution records from a region that spanned from eastern Colorado to the west coast. The team divided the area into a grid with squares, or cells, measuring 1-degree latitude by 1-degree longitude. For every grid cell, the researchers identified the 37 days with the highest concentrations of ozone and of fine particulate matter annually. “Each year I looked at what are the top 10 percent days of PM2.5 and what are the top 10 percent days of ozone,” Kalashnikov says. “I was interested in seeing when and how often high levels of these two pollutants coincide.” 

Over the past two decades, the team found, periods when extreme levels of fine particulate matter and ozone co-occurred have significantly increased over much of the western U.S. between July and September. As a result, the researchers calculated, people’s exposure to poor air quality increased by about 25 million person-days per year (a measure that refers to the population of a given location multiplied by the number of days it faced extreme levels of ozone and fine particulate matter).

[Related: Colorado just saw its most destructive wildfire ever—in the middle of winter]

To understand the weather patterns that might drive this pollution, the researchers zeroed in on phenomena called atmospheric ridges or heat domes. During a heat dome—like the one that engulfed the Pacific northwest last summer—a block of hot air sits over an area for days on end. This leads to hot, dry conditions that allow wildfires to spark and spread, and allows air pollutants to build up. In the past four decades, Kalashnikov says, these weather patterns have become more common and persistent during the summer in the western states.

Finally, the researchers examined wildfire activity and air temperatures in the week leading up to occasions when ozone and fine particulate matter reached extreme concentrations. They identified a “robust correlation” between the extent of air pollution events and the amount of land that burned in the western United States and southwestern Canada in the preceding seven days, Kalashnikov says. Similarly, he says, widespread hot weather was correlated with widespread air pollution. 

“It’s this perfect storm of things which are all increasing to produce more air pollution and more population exposure to air pollution,” Kalashnikov says. “We can link these changes to increases in the kinds of weather patterns that cause this pollution…and also more severe wildfire years which produce more smoke.”

Questions for future studies include how these air pollution trends might continue to play out under climate change and how variables such as the type of land a wildfire sweeps over influence the kinds and amount of air pollution unleashed, Kalashnikov says.

One limitation of the findings is that the researchers had to rely on a network of stations to monitor air quality that is “relatively sparse” in some parts of the western United States, Kalashnikov and his team acknowledged in the paper. Increasing the number of monitoring stations is vital to get more accurate and detailed estimates, they wrote. 

Understanding the relationship between fine particulate matter and ozone levels “is tricky,” says Daniel Jaffe, an environmental chemist at the University of Washington who wasn’t involved in the research. He and his colleagues have previously found that the most extreme ozone and fine particulate matter concentrations tend to occur on different days. 

“Exactly why is it that we get these co-occurrences of [fine particulate matter] and ozone on some days but not others, and what is the interplay between wildfire smoke and urban air quality?” Jaffe says. Still, he says, the new paper “is definitely a step forward.”

The worsening air pollution described in the new report isn’t going away anytime soon, he adds, highlighting the importance of protecting indoor air quality and using prescribed burns and other techniques to manage forests so they’re less vulnerable to massive fires. “All of us in the West have to think about how we’re going to live with wildfires,” Jaffe says.

One crucial step is increasing people’s awareness of how pervasive and unhealthy air pollution from wildfire smoke can be, Kalashnikov says. “If there was more awareness, then it might lead to better policy solutions, maybe better protection for vulnerable communities [such as] outdoor workers and farm workers.”