Millions of people in North America have been advised to wear N95 masks outdoors due to poor air quality levels sparked by intense wildfires in Canada.

New York will begin distributing free masks on Thursday. Canada has said that people should wear a mask if they are unable to remain indoors.

Officials warn that the dangerously smoky conditions are expected to persist into the weekend.

Much of the smoke is coming from Quebec, where 150 fires are burning.

More than 15,000 residents are expected to be forced to evacuate in the province, officials said on Wednesday. It is already Quebec's worst fire season on record.

New York Governor Kathy Hochul announced on Wednesday that New York would distribute one million masks to state residents on Thursday.

"This is a temporary situation. This is not Covid," she said at a news conference. The governor added that New York City buses and trains have high-quality air filtration systems that make them safe forms of travel.

Environment Canada has said that conditions are worsening in Toronto on Thursday, as more smoke pours in.

In a special weather bulletin on Wednesday, the agency recommended that anyone outdoors wear a mask.

"These fine particles generally pose the greatest risk to health. However, respirators do not reduce exposure to the gases in wildfire smoke," the Environment Canada statement said.

Meanwhile, the US Environmental Protection Agency (EPA) classified the air quality in much of the north-east as "unhealthy" especially for people with respiratory concerns.

In total, millions of people around North America are thought to be under a form of air quality warning.

In New York, an orange haze blanketed the city's skyline and shrouded landmarks including the Statue of Liberty.

"We recommend all New Yorkers limit outdoor activity to the greatest extent possible," Mayor Eric Adams warned on Wednesday.

Zoos have brought animals indoors, and in New York, carriage horse rides have been suspended.

On Wednesday, schools in the Washington DC area also cancelled outdoor activities as air quality levels were labelled "code red", while Detroit was listed as the fifth worst major metropolitan area in the world on IQAir's air pollution rankings.

Public health officials have cautioned people not to exercise outside and to minimise their exposure to the smoke as much as possible, as the air poses immediate and long-term health risks.

Canadian officials say the country is shaping up for its worst wildfire season on record.

Experts have pointed to a warmer and drier spring than normal as the reason behind the trend. These conditions are projected to continue throughout the summer.

Fires across Canada have already burned more than 3.8m hectares (9.4m acres) of land - an area 12 times the 10-year average for this time of year.

More than 600 US firefighters have been sent to Canada to assist local officials, the White House announced on Wednesday.

Also on Wednesday, US President Joe Biden and Canadian Prime Minister Justin Trudeau held a phone call to discuss the current situation.

Climate change increases the risk of the hot, dry weather that is likely to fuel wildfires.

The world has already warmed by about 1.2C since the industrial era began, and temperatures will keep rising unless governments around the world make steep cuts to emissions.

How does wildfire smoke affect your health?

Experts say exposure to wildfire smoke can cause a litany of health issues.

Matthew Adams, a professor at the University of Toronto and the director of its Centre of Urban Environments, said immediate effects of inhaling wildfire smoke include shortness of breath, an elevated pulse, chest pain, or inflammation in the eyes, nose and throat.

"On these elevated air pollution days, we'll see an increased number of visits to hospital," Prof Adams told the BBC. "And the people that are visiting the hospital typically have a pre-existing respiratory disease."

But wildfire smoke has also been linked to serious, long-term health issues like cancer or lung disease, Prof Adams said, specifically for people who live in areas that experience frequent forest fires.

This is caused by small particles in the smoke haze, he said, which can enter the bloodstream and other parts of the human body, causing possible DNA mutations and other health issues.

Some studies have also shown that prolonged wildfire smoke exposure can affect pregnant women and their unborn children, Prof Adams added.

For people living in cities far away from the fires but under current air advisories, Prof Adams advised people limit outdoor exercise to avoid breathing in the wildfire smoke.

"Don't get so concerned about it," he said. "Stay inside and reduce your exposure."

But in areas closer to the fires, Prof Adams recommended wearing an N95 mask outside to block inhalation of most of the smoke particles. (BBC)

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