BBC
A blame game has emerged over the dangerous air quality, with US lawmakers saying "American lungs are paying the price for Canadian inaction" and Ontario Premier Doug Ford responding that the US should "send support, send help"
Detroit, Chicago, Washington DC and New York top the list for worst air quality in the world today, as nearly 900 wildfires currently burn across Canada.
Check how they compare to more notorious cities for air pollution.
Winds from the Great Lakes region are driving smoke and ash across the upper Midwest of the US. See how the smoke is moving and when it could improve.
Many places are also expected to experience hot temperatures of more than 90F (32C) and high humidity
Air quality will likely remain a concern through the weekend when Fifa is holding its final matches in Florida and New Jersey.
Washington DC issues safety guidance as smoke hangs over city's iconic monuments
"If you must go outside, wear a mask," says Mayor of Washington DC Muriel Bowser, issuing guidance to residents grappling with the smoky conditions in the city.
Washington DC is in Code Purple - a notice of "very unhealthy air" - Bowser says, and advises "sensitive groups to stay indoors" and everybody else "to limit both time outside and to avoid heavy exertion activities".
She adds that all government activities in the city are cancelled, and outdoor pools and spray parks are closed.
US Republican lawmakers hit out at Canadian PM over wildfires
Several Republican members of the US Congress have lashed out at Canadian Prime Minister Mark Carney, accusing his government of failing to adequately mitigate the risk of wildfires as smoke from blazes in Canada drift into northern US cities.
John James, Jack Bergman, John Moolenaar and Lisa McClain jointly signed a letter to Carney this week, claiming "American lungs are paying the price for Canadian inaction".
"If Canada will not manage its forests to prevent these fires, the United States will look elsewhere, and act on our own, to protect our people. That means our own agencies exploring direct involvement in cross-border fuel reduction and firefighting capacity," the 15 July letter says.
Following US criticism, Carney reportedly said in in Ontario that fighting climate change was the responsibility of all countries, "including the United States".
