Wildfire smoke crosses into US prompting warnings to stay indoors

Heavy smoke from hundreds of wildfires in Canada has blanketed a swathe of the US from the Great Lakes to parts of the East Coast, prompting health warnings.

Officials are urging residents to stay indoors wherever possible, or wear masks, and avoid the acrid, unhealthy air.

The smoke is coming from fires that are burning primarily in Canada but also in northern Minnesota.

Detroit registered the worst air quality of any city in the world on Thursday, according to monitoring company IQAir, with a pollutant index of 600, twice the level considered "hazardous" by the US Environmental Protection Agency.

Federal data showed dangerous levels of smoke across Minnesota, Michigan, northern Illinois, northern Ohio and into Ontario, with hazardous readings in major cities Minneapolis, Milwaukee and Toronto.

Ten states reported at least some locations with an "unhealthy" reading, stretching from Minnesota to as far south as Maryland.

In the New York City area, a thick haze tinged the morning sky orange and yellow. The Statue of Liberty was barely visible through the obscured Manhattan skyline.

"Today is expected to be the worst day of this event," New York City Mayor Zohran Mamdani said at a gathering on Thursday, local time.

"At 'unhealthy' levels, everyone — not just people with asthma or heart conditions, not just older adults — everyone may feel health effects. So today, every New Yorker should take precautions," he said.

The city was handing out free KN95 face masks at hundreds of libraries, police precincts and firehouses, the mayor said.

Gwen Moseley, 65, was among the first patrons at Rosedale Library in Queens to take advantage of the free masks, saying she was on the road much of the day working as a therapist for children with autism.

"Who wants to be breathing this? It's not healthy," Ms Moseley said.

"When I'm out walking, I can feel the scratchiness in my throat."

Bill Ostrowski, 76, wore a mask as he walked through downtown Chicago, where wildfire smoke shrouded skyscrapers.

"It stinks. It's not a good sign when you wake up in the morning and you can smell the air," he said.

As of Thursday morning, officials tallied 858 fires burning across Canada, including 111 considered out of control.

Most of the fires were in the central provinces of Manitoba, Saskatchewan and Ontario.

"It's basically a river of smoke pouring into the Midwest right now," said Emily Fischer, an atmospheric chemist and professor at Colorado State University.

"This is a direct connection to climate change. This is the climate change that people breathe."

AP/Reuters