The US National Weather Service issued air quality alerts for virtually the entire Atlantic seaboard.
Wildfires are common in Canada's western provinces but this year flames have mushroomed rapidly in the country's east, making it the worst-ever start to the season.
About 3.8 million hectares have already burned, 15 times the 10-year average, Canada's Minister of Emergency Preparedness Bill Blair said.
"Across the country as of today, there are 414 wildfires burning, 239 of which are determined to be out of control," he told a briefing.
The giant eastern province of Quebec is among the worst affected.
"We've... seen continued impacts to critical infrastructure in Quebec such as roads and rural closures, telecommunication interruptions and high voltage power lines being threatened by the growing fires," Blair said.
Quebec Premier Francois Legault earlier said the province was able to fight 40 fires at the same time.
"But we have 150 fires so we have to make sure that we focus where the problems are the more urgent," he told reporters.
South of the border, health officials in US states from Vermont to South Carolina and as far west as Ohio and Kansas warned residents that spending time outdoors could cause health problems due to the amount of fine particulates in the atmosphere.
New York's world-famous skyline, usually visible for kilometres, appeared to vanish underneath the otherworldly veil of smoke, which some residents said made them feel unwell.
"I can't breathe. It makes breathing difficult," Mohammed Abass said as he walked down Broadway in Manhattan.
"I've been scheduled for a road test for driving, for my driving licence today, and it was cancelled."
The reduced visibility caused by the haze forced the US Federal Aviation Administration to slow or halt some flights into New York City at LaGuardia International Airport and Newark Liberty International Airport.
Schools up and down the east coast called off outdoor activities to protect students.
In some areas, the Air Quality Index (AQI), which measures major pollutants including particulate matter produced by fires, was well above 400, according to Airnow, which sets 100 as "unhealthy" and 300 as "hazardous".
The skies above New York and many other North American cities grew progressively hazier through Wednesday, with an eerie yellowish tinge filtering through the smoky canopy.
The air smelled like burning wood.
A Home Depot in Manhattan sold out of air purifiers and masks as residents scrambled to protect themselves.
New York Road Runners cancelled events intended to mark Global Running Day.
"This is not the day to train for a marathon or to do an outside event with your children," New York Mayor Eric Adams advised.
"If you are older or have heart or breathing problems or an older adult, you should remain inside."
City pedestrians donned face masks in numbers that recalled the worst days of the coronavirus pandemic.
