Unusually smoggy skies have upended life for millions of Americans this week as smoke from wildfires in Canada wafts over a large swath of the eastern United States, sparking a slew of Code Red air quality alerts and warnings to stay inside.

For East Coasters accustomed to breathing cleaner air, the wave of pollution has come as a shock, sparking widespread health concerns.

But the smog is temporary. Skies in the region are expected to clear up in the coming days as winds carry the wildfire smoke out over the Atlantic.

That’s not the case in some places around the world. Many people in other countries breathe air of comparably low quality every day.

The burning of fossil fuels and wildfires exacerbated by climate change have created what some experts describe as the biggest danger to global public health. One study last year found air pollution shaves more than two years off the global average life expectancy — more than cigarettes, alcohol or war. And roughly 97 percent of the world’s population lives in places where air pollution exceeds World Health Organization limits.

“Today on the East Coast, people are experiencing air pollution concentrations that hundreds of millions of people around the planet deal with on an average day,” said Michael Greenstone, director of the Energy Policy Institute at the University of Chicago and an author of the study. “We’re getting a taste of what I think is the greatest public health risk on the planet.”

Here are three cities that have contended with significant air pollution — and in some cases cleaned up the air…

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