NEW YORK (AP) — Officials warned residents to stay inside and limit outdoor activities Thursday as a thick, hazardous haze of wildfire smoke loomed over daily life for millions of people across the U.S. and Canada for a third day and was expected to persist as long as the weekend.

In a Canadian fire season that is just getting started but could well become the worst on record, more than 400 blazes — over a third of them in Quebec — burned Thursday. The smoke billowing from the fires sent plumes of fine particulate matter as far away as North Carolina and northern Europe.

The weather system driving the great Canadian-American smokeout — a low-pressure system over Maine and Nova Scotia — extended “Code Red” air quality alerts in some places as forecasts showed winds continuing to push smoke-filled air south.

U.S. National Weather Service meteorologist Bryan Ramsey said that means the smoke will probably be hanging around for a few days.

“Conditions are likely to remain unhealthy, at least until the wind direction changes or the fires get put out,” Ramsey said. “Since the fires are raging — they’re really large — they’re probably going to continue for weeks. But it’s really just going be all about the wind shift.”

The smoke-clogged air has created eerily silhouetted skylines as it chased players from ballfields, actors from Broadway stages, delayed thousands of flights and sparked a resurgence in mask wearing and remote work — all while raising concerns about the health effects of prolonged exposure to such bad air.

The smoke has moved over Greenland and Iceland since June 1, and was expected to reach Norway on Thursday, the Norwegian Climate and Environmental Research Institute said, but wasn’t expected to be a health concern.

In Washington D.C., the roads were unusually clear as many stayed home and about half the people on the streets wore their pandemic-era masks. The local government canceled all outdoor activities at public schools and local senior centers, closed all public parks, suspended work by city road construction and paving crews, and delayed trash collection.

The poor air quality also forced the closing of the National Zoo, a popular tourist attraction in the nation’s capital. Zoo officials said they were taking the action “for the safety of our animals, our staff and our guests.”

In suburban Philadelphia, officials set up an emergency shelter so people living outside can take refuge from the haze.

New York Gov. Kathy Hochul said at a briefing Thursday in Albany that “this continues to be a public health crisis,” with the worst air quality since at least the 1960s.

“This is not over. We might get a little respite, but I don’t want people to let down their guard and to become complacent about this because we have to be prepared for the winds to shift. This is the unknown,” Hochul said.

New York City is seeing a “higher than usual” number of asthma-related visits to the emergency room, city Health Department spokesperson Pedro Frisneda said Thursday, estimating they were in the “low hundreds.”

The city’s public schools announced classes would be remote Friday, a decision mostly affecting high schoolers as pupils in most other grades already had a scheduled day off.

More the blazes burning across Canada have left 20,000 people displaced. The U.S. has sent more than 600 firefighters and equipment to Canada. Other countries are also helping.

Canadian Prime Minister Justin Trudeau spoke to President Joe Biden by phone Wednesday. Trudeau’s office said he thanked Biden for his support and that both leaders “acknowledged the need to work together to address the devastating impacts of climate change.”

Canadian officials say this is shaping up to be the country’s worst wildfire season ever. It started early on drier-than-usual ground and accelerated quickly. Smoke from the blazes has been flowing into the U.S. since last month but intensified with recent fires in Quebec, where about 100 were considered out of control Wednesday.

In a statement Thursday, President Joe Biden called the residual smoke in the eastern U.S. “another stark reminder of the impacts of climate change” and said Cabinet officials were monitoring the smoke’s impact on travel and air quality.

Environment and Climate Change Canada issued a special air quality statement warning of high levels of pollution and deteriorated air quality as a result of smoke from the fires in Toronto, the country’s most populous city. The city also adjusted programming in response to the air quality, with some city-run outdoor recreation programs being canceled or moved indoors while outdoor activities at city-run day cares were suspended.

In the U.S., federal officials delayed some flights bound Thursday morning for New York’s LaGuardia Airport; Newark, New Jersey; and Philadelphia because smoke was limiting visibility. The Federal Aviation Administration said in a statement that it will likely need to take steps to manage the flow of air traffic into Washington, D.C., and Charlotte, North Carolina, as well.

Major League Baseball’s Yankees, Phillies and Nationals had their games postponed. Hochul said air quality could affect the Belmont Stakes scheduled for Saturday in the New York City area.

On Broadway, “Hamilton” and “Camelot” canceled Wednesday performances and “Prima Facie” star Jodie Comer left a matinee after 10 minutes because of difficulty breathing. The show restarted with an understudy, show publicists said.

It was not to be at Central Park’s outdoor stage, either. Shakespeare in the Park canceled its Thursday and Friday performances of “Hamlet,” saying ’tis not nobler in the mind to suffer the slings and arrows of wretched air.

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Gillies reported from Toronto. Associated Press journalists Michael Hill in Albany, New York; and Ashraf Khalil and Seung Min Kim in Washington, D.C., contributed to this report.

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