A thick, hazardous haze is disrupting daily life for millions of people across the US and Canada, blotting out skylines and turning skies orange.
About 3.8 million hectares have already been burned, with more than 400 fires across Canada leaving 20,000 people displaced.
The US has sent more than 600 firefighters and equipment to Canada and other countries are also helping.
The smoky blanket billowing from wildfires in Quebec and Nova Scotia, sending plumes of fine particulate matter as far away as North Carolina, is expected to persist on Thursday and possibly into the weekend.
That means at least another day, or more, of a dystopian-style detour that's chased players from sports fields, 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 weather system that's driving the great Canadian-American smoke out - a low-pressure system over Maine and Nova Scotia - "will probably be hanging around at least for the next few days”, US National Weather Service meteorologist Bryan Ramsey said.
“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 to be all about the wind shift.”
Across the eastern US, officials warned residents to stay inside and limit or avoid outdoor activities, extending “Code Red” air quality alerts in some places for a third day as forecasts showed winds continuing to push smoke-filled air south.
In Washington, Mayor Muriel Bowser ordered schools to cancel outdoor recess, sports and field trips on Thursday.
In suburban Philadelphia, officials set up an emergency shelter so people living outside could take refuge from the haze.
New York Governor Kathy Hochul said the state was making a million N95 masks - the kind prevalent at the height of the COVID-19 pandemic - available at state facilities, including 400,000 in New York City.
She also urged residents to stay put.
Canadian Prime Minister Justin Trudeau spoke to President Joe Biden by phone on Wednesday.
Trudeau's office said he thanked Biden for his support and 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-ever wildfire season.
It started early on drier-than-usual ground and accelerated quickly.
Smoke from the blazes has been lapping into the US since last month but intensified with recent fires in Quebec, where about 100 were considered out of control on Wednesday.
The smoke was so thick in Canada's capital, Ottawa, that office towers just across the Ottawa River were barely visible.
Eastern Quebec received some rain on Wednesday, but Montreal-based Environment Canada meteorologist Simon Legault said no significant rain was expected for days in the remote areas of central Quebec where the wildfires are more intense.
In the US, federal officials paused some flights bound for New York's LaGuardia Airport and slowed planes to Newark and Philadelphia because smoke was limiting visibility.
Major League Baseball's Yankees and Phillies had their games postponed.
On Broadway, Hamilton and Camelot cancelled Wednesday performances and Killing Eve star Jodie Comer left a matinee after 10 minutes because of difficulty breathing.
