Wildfires in Canada, the United States and Mexico often send smoke throughout the continent.

President Donald Trump on Friday threatened to impose tariffs on Canada in retaliation for wildfire smoke, but it was not clear that he has the legal authority to impose such a levy.

Detroit’s air quality was among the worst in the world for major cities, as a lingering high pressure system trapped smoke from dozens of fires in Canada and northern Minnesota, said Steven Freitag, a...