Millions across the US and Canada will remain at risk of inhaling potentially harmful air from wildfires in Quebec on Thursday as more officials urge people to limit time spent outdoors and mask up for enhanced protection.

Colossal clouds of heavy smoke from more than 430 active wildfires raging across Canada have descended on parts of the US Northeast and Mid-Atlantic, enveloping neighborhoods, parks and school grounds in an orange haze filled with possible pollutants. And forecasts suggest it may take several days for the air to clear.

With about 75 million people under air quality alerts in the US, some school districts in New York, Maryland, Virginia, and Washington, DC, have canceled outdoor activities while New Jersey's governor encouraged local school districts to do the same.

"It's either bad or really bad, depending where you are," New Jersey Gov. Phil Murphy told CNN's Jake Tapper on Wednesday, encouraging "young kids, seniors, anybody with heart of lung issues, to stay inside. If you have to go out, consider wearing a good, snug-fitting N95 mask."

Winds are forecast to continue pushing thick smoke farther south into the Mid-Atlantic, meaning the same ominous smoke that delivered some of the world's worst air quality to New York City this week could smother Delaware, Maryland, northern Virginia and Washington, DC, by Thursday morning.

Late Wednesday night, the air quality index in New York City topped 320, meaning it was "hazardous" or level 6 of 6, the worst designation from AirNow.gov, an air quality data site maintained in partnership by several government agencies. The air quality of the city slightly begins to improve early Thursday morning, albeit still at a "very unhealthy" level 5 of 6.

Philadelphia and Harrisburg, Pennsylvania, were the most affected metro areas early Thursday morning, with air level quality at "hazardous." Other major cities in the US that face "unhealthy" or "unhealthy for sensitive groups" levels include Baltimore, Newark, Pittsburgh, Detroit, Cleveland, Cincinnati, Indianapolis and Washington, DC.

The smoke from the wildfires has delivered some of the poorest air quality measures in decades, according to Mark Zondlo, an atmospheric chemist specializing in air quality monitoring and professor of civil and environmental engineering at Princeton University.

"What's making it really unique -- aside from the fires being huge by themselves -- is the air is staying really close to the ground. So, instead of being wafted up and dispersing throughout the atmosphere or being in the layer 10,000 feet above us, it's basically hugging the ground, and therefore it's not dispersing," Zondlo told CNN.

"The weather pattern is such that it's funneling that smoke plume, keeping it tight close to the ground, and it's coming for a bullseye right for us."

Air quality in Canada has been on the decline as the ferocious blazes trigger evacuation orders, including for about 7,000 people in Quebecois town of Chibougamau.

US President Joe Biden and Canadian Prime Minister Justin Trudeau discussed the wildfires' impacts on air quality on Wednesday, according to a statement from Trudeau's office.

"Both leaders acknowledged the need to work together to address the devastating impacts of climate change," the statement read.

Biden has directed federal firefighting resources to aid in stopping the fires, the White House said, adding that more than 600 firefighters and support personnel have already been deployed.

While those conditions persist, experts and officials alike have been urging people to stay indoors as much as possible and wear N95 or KN95 face masks when outside to ensure they're properly protected.

NYC to cancel all outdoor city events

Wildfires that lead to such poor air quality have become more common and severe as the planet warms from the impacts of human-induced climate change, experts have said.

"We typically see these impacts with wildfires in the Western US and in the Mountain West," said Dr. Peter DeCarlo, an associate professor in the Department of Environmental Health and Engineering at Johns Hopkins University.

"The East Coast is generally a little bit more insulated from this type of thing. Our forests tend to be wetter and don't burn as much, but looking forward with climate change, while this is kind of a unique experience that we're seeing right now, it may become a lot less unique and a little bit more common in the future."

As New York's air remains compromised, the state is providing one million N95 masks to those who need them, the governor announced Wednesday evening.

About 400,000 of those masks will be distributed at New York state parks and public transit stations, among other locations, Gov. Kathy Hochul said. An additional 600,000 masks will be available at Homeland Security stockpiles for local governments to pick up, she added.

"Simply stay indoors. Outdoors is dangerous in just about every part of our state," the governor said, calling the poor air quality "unprecedented."

To that end, outdoor events held by New York City have been canceled, and city beaches will also remain closed, Mayor Eric Adams said Wednesday.

While conditions may improve overnight into Thursday morning, air quality during the afternoon and evening hours will plummet again, Adams indicated.

"I want to be clear, while there may be potential for significantly improved conditions by Friday morning, smoke predictability that far out is low, is difficult to predict the movement of the smoke. ...This is an unpredictable series of events," he added.

Air quality alerts extended

Elsewhere, officials in Pennsylvania and Delaware issued a "code red" to warn residents of the potentially harmful air quality.

A code red was issued in Philadelphia Wednesday, warning that the elderly, young children and those who are pregnant or have heart or lung conditions could experience serious health effects from the smoke, according to James Garrow, spokesperson for the Philadelphia Department of Public Health.

"For those who are not considered to be in a sensitive group, we are asking those folks to avoid strenuous activities outdoors like jogging or exercising," Garrow told CNN.

For Delaware, an air quality alert and a Code Red Action Day are in effect through Thursday. State officials have advised residents to limit time spent outdoors and stay in a space with filtered air.

Additionally, Rhode Island's air quality alert has been extended through Thursday, citing heavy smoke and unhealthy particles on the air quality index.