Meteorologists struggled to explain “Smokeageddon,” the acrid campfire haze that settled over the East Coast and Midwest last month, and with good reason: It was unprecedented. 

Before this summer, code purple and code maroon air quality alerts, the two most dire categories, were almost unknown in the eastern United States. The American Lung Association, which collects air data, recorded exactly 10 code purple air days in the East or Midwest between 2000 and 2021, each reported by a single county. In those 21 years, no Eastern county reported a code maroon day. 

Over three weeks in June, dozens of cities logged purple and maroon alerts, triggered by wildfire smoke that drifted south from Canada. 

Wildfire smoke has menaced the West in recent years, an era of rising temperatures and erratic rainfall, fed by global warming. Yet, until this summer, the smoke had mostly spared the East. 

Jenna Krall, an assistant professor and air quality expert at George Mason University in northern Virginia, was days away from giving birth when the toxic air arrived. Air quality monitors around the region registered code purple on June 8, the worst air anyone there had breathed in years.  

Krall studies the air, and she had watched the air quality readings rise in the days before the worst of the smoke blew in.  

“I looked out my front window,” she said. “And day after day, it became harder just to see down the street.” 

Chrysan Cronin, associate professor of public health at Muhlenberg College in Allentown, Pa., watched the smoke settle in the Lehigh Valley. On June 7, the region logged a daily air quality index of 309, a code maroon. 

“I would take my dog out. I would let her stay out two minutes: ‘Pee. Poop,’” she said. “It looked foggy out, until you stepped outside.” 

Charles Driscoll, an environmental scholar at Syracuse University, watched the foul cloud roll into Upstate New York. The air quality index peaked at over 400, a level hazardous to any human. “You could taste the wood smoke,” he said.  

After two days of code purple air up and down the Eastern Seaboard, the smoke thinned and drifted away. Rattled meteorologists warned that hazardous air might return.  

Three weeks later, it did. A second wave of toxic smoke blanketed the Midwest on June 27, 28 and 29, setting off a new round of code purple readings in Cleveland, Chicago, Indianapolis, Milwaukee and Pittsburgh. 

Neil Donahue, a chemistry professor at Carnegie Mellon University, greeted the haze with sad irony. Pittsburgh was once the Smoky City, its dramatic features blurred by perpetual soot from steel mills and coal plants.  

The air quality in Pittsburgh has improved dramatically over the decades. Thus, in a historical sense, the city’s code purple air of late June was “as bad as it was every now and then in 2000, and practically every day in 1974,” Donahue said. 

Wildfires and their attendant smoke threaten to undo decades of progress in fighting air pollution in America. Amendments to the federal Clean Air Act in 1990 set emission standards that gradually thinned the smudge of smog that had hung over New York, Chicago and Los Angeles for generations.  

And then, as the planet warmed, a new source of dangerous air surged in. 

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Particle pollution, mostly from wildfires, triggered hundreds of code purple days in Western communities between 2018 and 2021, according to the American Lung Association. Code maroon days, almost unknown in the recent past, multiplied in 2020 and 2021. 

But nearly all of those air quality events played out in the West. The reason, experts say, is that Western forests tend to dry out in the arid summer heat, while Eastern forests, especially up north, remain comparatively damp. 

“In the East, people say the forests are asbestos forests, because it’s wet,” Driscoll said, alluding to the flame-resistant fiber. 

That maxim remains true, but climate change has brought odd and unpredictable weather shifts, with occasionally dramatic results.  

Remember the unseasonably mild winter of 2022-23, and the remarkably dry spring that followed? Those seasons yielded too little snow, and then too little rain, to keep the forests damp in eastern Canada. They dried out, and then they burned.  

When meteorologists labored to explain the smoke events of last month, the go-to theory was that fire season had started early and fierce, burning record numbers of acres.  

But that is only part of the story. Eastern Canada isn’t supposed to have a fire season, or, at least, not one that sends plumes of smoke drifting over New York. 

In recent years, large-scale forest fires were rare in the region of Canada that sits above the eastern United States.  

Global warming is changing that.  

“Even though we think this is a wet area, and it is indeed a wet area, we’re experiencing periodic droughts,” Driscoll said. Less snow is falling, which means less accumulated snowpack. Warmer temperatures melt the snow earlier in the spring. The remaining water evaporates in the heat. 

That is more or less what happened this spring. Once the eastern forests were ablaze, the stage was set for Smokeageddon. And in early June, a delivery vehicle arrived: a pair of weather systems that pushed smoky air south from Canada to America’s densely populated Eastern Seaboard. 

A high-pressure system hung west of the fires, spinning clockwise. A low pressure system loomed to the east, spinning counterclockwise. The systems combined to shoot the smoke south, much like the wheels in a baseball pitching machine. 

“That’s essentially what you had, except instead of baseballs, it was smoke,” said Peter DeCarlo, an associate professor and air quality expert at Johns Hopkins University.  

“Those weather systems set up all the time, but usually there’s not a fire behind them.”  

The code purple warnings indicated high levels of fine particles, each with a diameter of 2.5 microns or less — “many times smaller than the width of a hair,” Donahue said. The particles can travel long distances and linger in the air for days, defying gravity’s pull.  

They’re small enough to travel freely through the human body, deep into the lungs and then into the bloodstream, wreaking havoc on the cardiovascular system. 

“There’s no safe dose of fine particulate matter,” Driscoll said. Citizens of Syracuse seemed to know that. Many stayed indoors or wore the KN95 masks they had stockpiled in the pandemic.  

In the Lehigh Valley, by contrast, Cronin watched many townspeople go about their business as normal. She saw road workers toiling in the code maroon haze and thought, “It’s like they just smoked three packs of cigarettes.” 

The second wave of Smokeageddon, at month’s end, proved the first was no fluke. It rode on a high-pressure system parked over the Midwest, its clockwise motion again pushing smoke south.  

During both air quality crises, meteorologists warned the public that the once-unthinkable episodes were bound to happen again. 

When, exactly, is anyone’s guess. 

“This year is an incredible outlier,” with its sparse winter snowfall and dry spring, DeCarlo said. 

But all of the trends that made the smoke storms possible, from earlier springs to hotter summers to milder winters, are likely to continue. 

“I don’t think we’ll see it next year, but I think the conditions are all moving in that direction at a slow, measured pace,” Driscoll said. 

“I don’t think it’s a singular event anymore,” Cronin said. “I think we’re past that.” 

–Updated at 6:23 a.m.