Smoke from wildfires burning in south-central Canada and parts of Minnesota is spreading across the US, prompting air quality alerts in more than 20 states with millions of Americans expected to face unhealthy air conditions this week.

The smoke from the more than 180 active wildfires in northern Ontario briefly made Toronto’s air quality among the worst in the world on Wednesday. By Wednesday night, the smoke had spread across several US states, from Minnesota, where multiple wildfires are also burning, to New York, blanketing the skies in haze and worsening air quality.

The National Aeronautics and Space Administration (Nasa) said that winds carried smoke from the Ontario fires “primarily south-east over much of the southern part of the province, as well as parts of Quebec and the US midwest and north-east, tinting the sky shades of gray and yellow and the sun orange in many areas”.

As of Thursday morning, air quality alerts have been issued for parts of North Carolina, Minnesota, Vermont, Connecticut, New York, Massachusetts, Rhode Island, New Jersey, New Hampshire, Pennsylvania, Illinois, Indiana, Ohio, Delaware, West Virginia, Colorado, Virginia, Maryland, Washington DC, Michigan and Wisconsin.

According to a map of air quality conditions provided by the US Environmental Protection Agency, air quality is at “unhealthy” levels on Thursday morning in parts of Pennsylvania, Delaware, New Jersey, New York, Ohio and Michigan with officials urging residents to “take precautions by reducing outdoor, activity”.

The air quality is rated as “very unhealthy” across parts of Illinois, Wisconsin, Michigan and Ohio, while in certain parts of Michigan, including Detroit, as well as areas of Wisconsin and Minnesota, the air quality is listed as “hazardous”, with officials urging residents there to “stay indoors”.

On Thursday morning, Detroit was listed as the city that currently has the worst air quality in the world on IQAir’s global rankings, followed by Toronto, and then Minneapolis and Chicago.

The National Weather Service in Detroit warned on Thursday that “smoke is expected to be most dense this afternoon and tonight with reduced visibilities”.

In New York, officials extended the city’s heat emergency plan operations, including the opening of hundreds of cooling centers, as “high temperatures and heat indices continue and smoke from significant, still-spreading wildfires in western Ontario, Canada, moves into the region”.

“It’s dangerously hot and smoke from Canadian wildfires has worsened our air quality, which means New Yorkers need to be extra vigilant to stay safe,” New York City’s mayor, Zohran Mamdani, said. He urged residents to stay in air-conditioned spaces, to limit outdoor activity and to stay hydrated.

New York’s governor, Kathy Hochul, announced that more than 100,000 N95-style masks were being made available to counties in need, with additional masks being made available for commuters at Penn Station and Grand Central.

New York officials also warned that this could become New York City’s “most significant smoke event” since 2023, when smoke from Canadian wildfires pushed air quality to hazardous levels, and blanketed the city in an orange haze.

While officials said that this week’s conditions currently “do not indicate a repeat of 2023’s peak levels”, authorities said that they are closely monitoring for any deterioration.

Weather officials in New York City said on Thursday that “wildfire smoke will continue to reduce visibility and air quality through this evening”.

The air quality alert for New York remains in effect until midnight on Thursday, and rain is expected on Saturday, ahead of Sunday’s highly anticipated World Cup Final in New Jersey.

In Boston, forecasters warned on Thursday that “smoky skies are again expected today, which will reduce air quality”.

The Massachusetts senator Ed Markey said that the smoke is “darkening skies and getting in our lungs across the state”.

“With a record El Niño forecast for this year, we are looking at more wildfire smoke days ahead,” he added. “The climate crisis is here and it does not stop at our borders. Stay safe, Massachusetts.”

Elsewhere, forecasters warned that smoky conditions are expected to persist into Friday and potentially into the weekend. In Virginia, forecasters said on Thursday that “additional waves of smoke from Ontario, Canada, wildfires will push south into the area through Friday”, which will create “hazy conditions with diminished air quality”.

In Michigan, smoke is expected to continue through Friday, and perhaps linger across parts of the area into Saturday, and Ohio is also expected to see smoky conditions continue through Friday.