Heavy, pungent wildfire smoke from Canada and northern Minnesota is expected to linger over a huge swath of the Midwest and Northeast Friday, after cities like Detroit, Minneapolis and Chicago experienced some of the worst air quality on the planet on Thursday.

(MORE: Track where the smoke is worst and where it’s heading)

In Chicago, residents woke up to a city that “smells like cigarettes outside." 

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Overnight timelapse video captured smoke pouring into the city, transforming a mostly clear skyline into a gray haze by sunrise.

People urged to stay indoors

EPA monitoring also showed some of the nation's highest concentrations of dangerous PM2.5 pollution in northeastern Minnesota, Milwaukee and Toledo.

It’s important to note that tiny PM2.5 particles produced by wildfires can remain suspended in the atmosphere for days, allowing them to drift hundreds of miles from the flames.

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That means communities nowhere near an active wildfire can end up with air quality that's just as unhealthy as places much closer to the source.

Officials in many cities are urging residents to stay indoors. In New York, city workers were handing out masks on the street.

“It’s scary,” Omar Mitchell, 50, told the Associated Press as he looked to the sky. He wore a mask while walking to his restaurant in Detroit. “You don’t know necessarily what the side effects may be. That’s days or months later.”

The haze can also create some surprisingly dramatic effects, reducing visibility enough to hide city skylines.

In the worst cases, smoke can become thick enough to rival dense fog, hindering air travel and making it difficult to see landmarks just a few miles away.

If your area is under an air quality alert, experts recommend limiting outdoor activity, keeping windows closed, and changing your indoor air settings.

See our full checklist of what to do here.