NEW YORK (AP) – Heavy, pungent smoke from Canadian wildfires darkened skies in the U.S. on Thursday from the Great Lakes to parts of the East Coast, reducing visibility and prompting warnings that breathing the air outside could be dangerous.

Officials in many cities urged residents to stay inside or wear masks outside as air quality reached unhealthy to hazardous levels, meaning it is unhealthy for anyone, regardless of health conditions.

“It’s scary,” Omar Mitchell, 50, said as he looked he looked to the sky. He wore a mask while walking to his restaurant in Detroit. “You don’t know necessarily what the side effects may be. That’s days or months later.”

Microscopic particles can lodge deep in the lungs and enter the bloodstream, leading to heart and lung problems and contributing to other long-term health issues.

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 National Weather Service meteorologist in Pontiac, Michigan.

“Sure enough, it arrived in force here and it’s really pretty extreme levels,” said Mr. Freitag, who noted that visibility in some areas was reduced to a half mile.

The Air Stinks and Glows Yellow in Some Places

All of Michigan and much of Minnesota were under a hazardous air quality alert. In the Chicago area, air quality ranged from very unhealthy to hazardous.

National Weather Service meteorologist Jake Petr said even if winds from the northwest clear skies as expected later this week, the smoky air could keep returning until the fires are out. That could take weeks or longer.

“Anytime we have something that could bring air from that region until the fires are over, it could conceivably dip back into the area,” Petr said.

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,” said Ostrowski.

In St. Paul, Minnesota, the air was “glowing yellow,” said Brent Williams, head of the soil, water and climate department at the University of Minnesota. The area “could be looking at weeks to months of continued smoke and flare-ups off and on as the winds blow in different directions,” he said.

A study published this year found that long-term exposure to tiny particles from wildfire smoke contributed to an average of 24,100 deaths a year in the lower 48 states. Long-term exposure can make existing health problems worse and lead to a range of chronic and deadly issues, including respiratory illness, cardiovascular and neurological diseases, and premature death.

New York City Cancels Activities and Hands Out Masks

In the New York City area, a thick haze tinged the morning sky in orange and yellow and partly obscured Manhattan’s prominent skyline.

City officials opened cooling centers as health officials urged New Yorkers to limit strenuous and prolonged outdoor activities and to stick to air-conditioned spaces as much as possible. State officials distributed tens of thousands of face masks at transit hubs and other major locations.

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

“I’m trying to stay as much in the house as I can,” Ms. Moseley said as she waited to meet a young client at the library. “Who wants to be breathing this? It’s not healthy. When I’m out walking, I can feel the scratchiness in my throat.”

The city’s schools, parks and other agencies moved activities indoors, rescheduled events and adjusted operations.

Smoke eased a bit but was expected to thicken again by late afternoon or evening, possibly lasting overnight, weather service meteorologist Maureen Hastings said. She said it might move south for a while on Friday, but return at night.

The state Department of Environmental Conservation warned that there was a potential for temporary spikes of “very unhealthy” air quality from Buffalo in the state’s western corner to Rochester by Lake Ontario, Syracuse in the central region, down to the greater New York City area.

Philadelphia officials urged people to avoid strenuous activity and stay inside or wear N95 or KN95 masks outside.

“Today is not the day to start your marathon training plan,” said Dr. Palak Raval-Nelson, the city’s public health commissioner.

Minnesota Fires Are Spreading

Meanwhile, forest rangers on Thursday combed a remote wilderness area in Minnesota for anyone who might still be there days after wildfires led to its closure.

Officials closed the Boundary Waters along the U.S.-Canadian border on Tuesday. At the time, 6,000 to 10,000 people were inside, but Superior National Forest staff estimated Wednesday that they had reached 90 percent of them, said Karen Harrison, a spokesperson for state and federal emergency management agencies.

She said Thursday that smoke is making it difficult for helicopters to fly, and fires in the wilderness are spreading despite firefighting efforts.

“There will be fire on the landscape until fall, and some fire will be burning until snow cover,” Ms. Harrison said.

The Royal Canadian Air Force successfully evacuated 11 Minnesota teenagers and four staff members Wednesday from wildfires in an Ontario provincial park about 175 miles north of the Minnesota border.