It was late Tuesday afternoon when a group of senior executives from the Related Companies, one of New York’s largest property developers, ascended the Edge observation deck atop 30 Hudson Yards to show off what is typically one of the city’s grandest views. They were startled by what they found.

“You can’t see anything right now,” said Jeff Blau, Related’s chief executive. “I’ve never seen it so bad.”

By the next day, hazy air that had drifted from Canadian wildfires hundreds of miles away would turn hazardous, prompting health warnings for millions of people across eastern Canada and the US north-east, upsetting everything from air travel and schools to Broadway shows.

New York City health officials reported an increase in visits to hospital emergency rooms for asthma as anxious residents found themselves reaching for Covid-19 masks they thought they no longer needed. They also consulted weather maps and fiddled with smartphone apps that showed the city’s air pollution had outpaced Delhi as the world’s worst. Even without data on airborne particulates, the otherworldly orange and pale yellow skies with an apocalyptic cast made clear that all was not right.

“I went outdoors and basically said, what the hell is this?” Eric Adams, New York City’s mayor, said on Wednesday.

Helen Mannion, a commuter from Long Island, was also stunned. “Yesterday turned so quickly. It was scary,” she said as she waited outside New York Penn Station for a bus on Thursday. The donning of masks, the official warnings and the sudden realisation that one was in the grips of a larger threat were all too familiar. “Everyone’s got PTSD from Covid,” Mannion said, then asked: “Why is it always in New York that all these catastrophes happen?”

People in the world’s media capital habitually struggle to look beyond their own shores — as northern Californians, long accustomed to forest fire pollution, amply complained this week. If they did so, they might discover the plight of others was even worse. In Quebec, in particular, emergency services were overwhelmed by more than 150 fires on Thursday, most of which were deemed out of control. Many were in remote areas, or typically served by volunteer fire departments. Entire towns were being evacuated.

“Some fires are under control, some not,” François Bonnardel, Quebec’s public security minister, told reporters on Thursday — a day after US President Joe Biden promised to send additional firefighters to help. “We’re looking at these fires every hour, we’re hoping to tell Quebecers they will be able to go back home, but in the short term, it won’t be possible.”

From Canada’s boreal forests to the furthest reaches of polluted air in the south-eastern US, a common question emerged this week: was this a freak event for the eastern seaboard or the beginning of a chronic summer affliction?

Adams was pessimistic. “While this may be the first time we’ve experienced something like this of this magnitude, let’s be clear, it’s not the last,” the mayor warned, blaming climate change. Many scientists shared that view. Still, as with most matters involving the interplay of weather, climate and human activity, precise predictions were harder to come by.

The smoke has been generated by a particularly intense Canadian wildfire season. Earlier this spring, fires in Alberta, the western province that is Canada’s main oil-producing region, forced tens of thousands of people to leave their homes. More recently, blazes have taken hold in the forests in the eastern provinces of Quebec and Nova Scotia. The 4.3mn hectares that have burnt was well above the annual average of the past decade, according to the Canadian Interagency Forest Fire Centre.

“This is an unusually bad and unusually early fire season, and that is true across all of the provinces in Canada,” said Carly Phillips, a researcher at the Union of Concerned Scientists, an environmental advocacy group. “It has been a very warm and very dry spring. If vegetation has more opportunities to lose moisture, it becomes ignitable much more quickly.”

That might not necessarily affect New Yorkers and Philadelphians. But the smoky air has been shuttled to the north-eastern US by a combination of northerly and northwesterly winds and an unusually stubborn area of low atmospheric pressure over New England, according to the US National Weather Service. 

It forecast that the weather pattern would continue for the next few days, bringing more bouts of concentrated and dense smoke over the mid-Atlantic region into the weekend before a new weather system arrives early next week. 

While the north-east may experience a brief reprieve, Zack Taylor, a meteorologist with the NWS, warned that the smoke could return this summer. 

“The direct source of this is obviously the fires in Canada. Until those fires are contained, or more controlled, smoke is going to continue to be wafted into the atmosphere. Depending on wind directions and weather patterns — some of it will probably at times drop into the United States.” 

Timely rains would help, noted Phillips, of the Union of Concerned Scientists. But she also took the longer view. “The bigger question is about these patterns that are occurring not just over a few weeks, but over decades into the future. Fire activity is likely to increase in future — it can vary year to year, but the trend will increase.” 

In the meantime, those on the ground — far removed from the fires and the statistical models — were left to cope as their means allowed. Some in New York City heeded the mayor’s pleas to stay at home, or had the luxury of working from modern high rises, like Hudson Yards, with advanced air filtration.

Then there were those labouring at a downtown construction site on Hudson Street, where a new office building is rising for the Disney company. Construction workers said they were given masks and briefed on safety precautions. By midday on Thursday, many had ditched them and work appeared to carry on as usual.

Gustavo Ajche, a founding member of Los Deliveristas Unidos, a New York City delivery workers’ association, said he experienced headaches and eye irritation after his shift on Tuesday. Still, he hopped on his cycle on Wednesday — this time wearing an N95 mask. He handled more takeout orders than usual, presumably from so many sheltering indoors.

“It felt like we were under attack, like if it was some other pandemic,” Ajche said of the smoke. The city, he added, “looked like a movie of what the future might look like”.