Schools across the US east coast cancelled outdoor activities, flights were halted and millions of Americans were urged to stay indoors as smoke from Canadian wildfires drifted south, blanketing cities in a thick orange haze.

The US National Weather Service issued air quality alerts for virtually the entire Atlantic seaboard.

Health officials in 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.

Central Manhattan was shrouded in an orange haze this afternoon (Pic: Gerald Kelly)

The White House said the wildfires are an "alarming example of the ways in which the climate crisis is disturbing our lives".

President Joe Biden is being kept informed of the situation, and called on Americans with fragile health to take proper precautions to protect themselves from air pollution, spokeswoman Karine Jean-Pierre said.

"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 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, including sports practices, field trips and recesses, to protect students.

Comer halts Broadway show due to breathing difficulties

British actress Jodie Comer was forced to halt a matinee performance of Broadway show Prima Facie after experiencing breathing difficulties brought on by poor air quality in New York.

Today's performance was stopped about ten minutes in, with understudy Dani Arlington later set to step in.

US outlet Deadline reported, per an audience member who attended the Prima Facie matinee, that Comer was around three minutes into the performance before coughing and telling a stage manager she could not breathe.

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".

At 12pm (5pm Irish time), Bethlehem, Pennsylvania, was experiencing the worst air-quality in the country, with an AQI reading of 410.

Among major cities, New York had the worst air quality reading in the world this afternoon at 342, about double the reading for chronically polluted cities such as Dubai (168) and Delhi (164), according to IQ Air.

The skyline of lower Manhattan this afternoon

The smoke is crossing the US northern border from Canada, where wildfire season got off to an unusually early and intense start due to persistent warm and dry conditions. Canada is on track for its worst-ever wildfire season.

The skies above New York and many other North American cities grew progressively hazier through today, with an eerie orange tinge filtering through the smoky canopy. The air smelled like burning wood.

Wildfire smoke has been linked with higher rates of heart attacks and strokes, increases in emergency room visits for asthma and other respiratory conditions, and eye irritation, itchy skin and rashes, among other problems.

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.

Tyrone Sylvester, 66, was sitting outside playing chess in Manhattan's Union Square as he has done most days for 30 years.

He had a face mask on for the first time in a long time and said he had never seen air quality as bad in the city as it was today.

"When the sun looks like that," he said, pointing to the bronze-like appearance of the star in the sky, "we know something's wrong".

Traffic goes over the Ed Koch Queensboro Bridge in New York

While wildfires are common in Canada's West, there are blazes across the country, especially in the eastern province of Quebec.

About 3.3 million hectares have already burned - some 13 times the ten-year average - and more than 120,000 people have been at least temporarily forced out of their homes.

Federal 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.

"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."

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.

Canadian Prime Minister Justin Trudeau said: "In coming years we will have to reflect seriously on how we can equip ourselves to deal with this new reality. We will be facing more and more extreme weather events that will cost us a lot more."