The U.S. cities with the worst air quality in 2023 — mostly in the Midwest — had particle pollution levels two to three times higher than what the World Health Organization (WHO) recommends, according to a new report.

The big picture: It's not coincidental that the cities with the worst air quality last year were in the Upper Midwest and Mid-Atlantic, as those regions were smothered in smoke from Canada's unprecedented wildfire season.

• After decades of progress, air quality in the U.S. is expected to steadily deteriorate over the coming decades as climate change causes more large wildfires, heat waves and drought.
• Most U.S. cities had levels one to two times higher than WHO guidelines, while pockets of the West met the guidelines, according to the report.

Why it matters: Air pollution is the world's leading environmental health threat, causing 6.7 million premature deaths every year, according to WHO estimates.

How it works: The report is from IQAir, a Swiss air quality technology company that maintains a global air quality system that continuously measures contaminants in thousands of cities across the world.

• The report examines the average level of fine particles less than 2.5 microns (PM2.5) in diameter — a particularly dangerous form of air pollution because the solid and liquid specks can penetrate deep into lung passageways.
• Inhalation of PM2.5 can exacerbate pre-existing lung and heart conditions. Evidence suggests it can also have neurological effects.
• Much of the fine particulate matter pollution in the U.S. and around the world is a byproduct of fossil fuel combustion and industrial activity, as from car engines or concrete plants. Other drivers can include everything from dust storms to volcanoes, cigarettes and air fryers.

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