Officials in many cities urged residents to stay inside or wear masks outside as air quality reached unhealthy to hazardous levels, meaning it's unhealthy for anyone, regardless of health conditions.
The smoke is coming from fires that are burning primarily in Canada but also in northern Minnesota. A lingering high pressure system has trapped the smoke close to the ground, said Steven Freitag, a National Weather Service meteorologist in Detroit, where air quality was among the worst in the world for major cities."
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This is the shocking moment people are trapped inside a train engulfed by flames.
It's just one example of how, across the world, it has been a summer of fire.
More than 800 wildfires are burning in Canada alone, many of them out of control.
In terrifying scenes from Ontario, flames surrounded a train with people inside.
An orange haze engulfed other major Canadian cities.
And in northern US states like Michigan and Minnesota, air quality has been plummeting.
Even in New York, it's bordering unhazardous.
In California, crews battle to stop these flames reaching thousands of homes.
In Europe, France's worst wildfire in more than a century is now under control.
The Fontainebleau blaze destroyed more than 5,000 acres of forest.
The country has registered 11,000 fires this year alone.
Scotland's Cairngorms are more accustomed to snow than fire.
This year, one in the Highlands burned for 24 hours.
Now under control, risk of more remain.
13 people are now dead in Spain.
It too has experienced some of its worst wildfires on record.
In the northeast, firefighters operate in truly apocalyptic scenes.
