NEW YORK (AP) — Heavy, pungent wildfire smoke 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’s unhealthy for anyone, regardless of health conditions. The smoke is coming from fires that are burning primarily in Canada but also in northern Minnesota. A lingering high pressure system has trapped the smoke close to the ground, said Steven Freitag, a National Weather Service meteorologist in Detroit, where air quality was among the worst in the world for major cities.

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

“It’s scary,” Omar Mitchell, 50, said as 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.

The air stinks and the sky 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 months, until it snows in Canada and northern Minnesota, officials have 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 sky 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 orange and yellow and partly obscured Manhattan’s skyline.

City officials opened cooling centers as health officials urged New Yorkers to limit strenuous and prolonged outdoor activities. The city’s schools, parks and other agencies moved activities indoors, rescheduled events and adjusted operations. 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.

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

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 and 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

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

Officials closed the Boundary Waters along the U.S.-Canada border on Tuesday. At the time, 6,000 to 10,000 people were inside, but Superior National Forest staff estimated Wednesday that they’d reached 90% of them, said Karen Harrison, a spokesperson for state and federal agencies involved in the response.

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

“There will be fire on the landscape until fall, and some fire will be burning until snow cover,” 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 (282 kilometers) north of the Minnesota border.

NEW YORK (AP) — It’s an idea whose time, as it were, may have come — again.

The twice-yearly changing of the clocks in the United States could be a thing of the past if legislation currently in Congress that calls for permanent daylight time makes it through. But even as annoying as some find the back-and-forth of the time shift in the spring and the fall, that doesn’t necessarily mean sticking to one would go over well. America has tried it before, most recently in the 1970s, and it didn’t last.

Now it’s a new era, one full of people working at home who didn’t before — and advances in sleep science that tell a more nuanced tale.

Could this time (shift) be the charm?

What’s going on this time around?

The House of Representatives on Tuesday voted overwhelmingly to pass a bill that makes the shift to daylight saving time, when clocks are moved forward one hour, become permanent.

Currently, the shift is forward in spring and back to standard time in fall as a way to give people more daylight time in the summer evenings. But the semi-annual change has few fans – an AP-NORC poll last year found that only 12% of American adults were in favor of it, while almost half opposed it. Proponents of a single time include the American Medical Association and American Academy of Sleep Medicine — outfits to whom daily rhythms are deeply important.

The Senate would have to pass it before it could be signed into law. President Donald Trump has indicated he’s supportive.

So just change it. What’s the big deal?

Not so fast. People may not like making the change, but history shows they also don’t like living with even less morning light in the winter months, when daylight hours are shorter than in summer.

In 1973, Congress passed a law instituting permanent daylight saving time for what was supposed to be a trial period from January 1974 to April 1975. It lasted until October, when it was repealed after public outcry. Among the concerns was worry that schoolchildren would have to get to class in darkness. These days, school starting times have started to shift later.

Kevin Birth, a professor of anthropology at Queens College whose research focuses on cultural concepts of time, was in elementary school in Syracuse, New York, at the time and remembers it vividly. “I had to get up for school and it was like it was midnight,” he said. “It was just pitch black and it remained pitch black into the school day.”

If the U.S. decides to try it again, he said, more has to change than just the clocks. The time zones across the country would need to be adapted as well. The current four zones wouldn’t be adequate – they cover so much ground that sunrise comes at different times in western and eastern parts of each zone.