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Wednesday Was the Worst Day for Wildfire Pollution in U.S. History

The finding by Stanford researchers confirms the danger — and lack of precedent — for the East Coast’s ongoing wildfire smoke crisis.

The Chrysler Building.
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Wednesday, June 7, was by far the worst day for wildfire smoke exposure in American history, subjecting more people to greater amounts of soot and ash than any previous day on record, according to a new and rapid analysis conducted by Stanford researchers.

The finding confirms the danger — and lack of precedent — for the ongoing wildfire smoke crisis, which has smothered the northern and eastern United States in an acrid haze. Since Monday, a strong low-pressure system has fanned dozens of wildfires in northern Quebec, then blown the resulting smoke into North America’s most densely populated corridor, covering New York City, Philadelphia, Washington, D.C., and Toronto.

Even as the smoke moved in, it made history. Tuesday became the country’s third-worst day for wildfire smoke on record, according to research from the same Stanford team. Only two days in September 2020 — when vast wildfires sheathed California, Idaho, and the Pacific Northwest in smoke — had exposed more Americans to higher intensities of toxic particulate matter.

Wednesday smashed those records. By the late afternoon, New York and New Jersey’s sky had turned a pungent orange, and their air quality fell to “very unhealthy” or even “hazardous” levels for all groups, the worst category. The rest of the Northeast and Mid-Atlantic faced similar or only slightly better conditions, and wildfire smoke at least somewhat afflicted virtually everywhere between Minneapolis, Atlanta, and Boston.

Stanford chart.Stanford.

The vast geographic sweep of the smoke — and the fact that it was so intense in the country’s biggest city — obliterated the old record. The country’s previous worst day for smoke exposure was September 13, 2020, when nearly 35 million Americans were exposed to more than 50 micrograms per cubic meter of microscopic soot and ash. That is five times worse than what the World Health Organization considers safe.

But on Wednesday, roughly 62 million Americans — nearly one in five — were exposed to such high levels of particulate matter.

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  • It was “just a massive, awful event, with highly populated areas getting hit with unprecedented levels of pollution,” Marshall Burke, an economist and professor at the Stanford Doerr School of Sustainability, said on Twitter. He leads the school’s Environmental Change and Human Outcomes Lab, which conducted the research.

    The study encompassed data from 2006 to 2023. The researchers studied which days were worst on a population- and exposure-adjusted basis. In other words, on which days in history were the most Americans subjected to the most intense amounts of wildfire smoke?

    Because the U.S. population has grown over time, and because wildfires have enlarged over the past few decades, it is unlikely that any days prior to 2006 saw so many Americans exposed to so much smoke. Most of the record days in the data set have happened since 2018.

    With the new record in place, Tuesday falls to the fourth-worst day for wildfire smoke on record.

    Speaking earlier this week, Burke said that what made this event so distinct from previous ones is that the wildfire smoke hit the country’s largest and most populated cities. “It’s mainly due to the East Coast having so many people. It’s New York, Boston, D.C., Detroit. Out West, our cities are just smaller,” he told me.

    Read more about the wildfire smoke engulfing the East Coast:

    Wednesday Was the Worst Day for Wildfire Pollution in U.S. History

    The 5 Big Questions About the 2023 Wildfire Smoke Crisis

    Nowhere Is ‘Climate Proof’

    When There’s Smoke, Getting Indoors Isn’t Enough

    How Many People Will This Smoke Kill?

    Robinson Meyer

    Robinson is the founding executive editor of Heatmap. He was previously a staff writer at The Atlantic, where he covered climate change, energy, and technology. Read More

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