Around 75 million people in the US are under air quality alerts related to the wildfires raging across Canada, as officials urge people to limit time spent outdoors and mask up for enhanced protection.
Forecasts show the dangerous air conditions could last for days, but are expected to slowly improve across the East heading into the weekend.
In Quebec, smoke from wildfires across the region -- which sent colossal clouds of heavy smoke through the eastern US -- is now considerably reduced, according to NOAA satellite observations and data from Environment Canada.
Canadian cities, including Ottawa, Montreal and Toronto, are seeing considerable improvement in air quality Thursday.
It took several days for the dense smoke from the Quebec fires to reach US cities including New York, Philadelphia and Washington. Without substantial new smoke entering the US, the dangerous air conditions are expected to improve heading into the weekend. But current weather patterns suggest the smoke will be trapped in impacted areas until it can dissipate, meaning improvements will come slowly.
New York, Charlotte, Detroit, Indianapolis, Delaware and Rhode Island, as well as other areas, are under air quality alerts.
LIVE UPDATES: Millions in US under air quality alerts
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. While New York City saw slight improvements by Thursday, levels are still "very unhealthy" for residents, and the city's air remains the most polluted in the world.
The Federal Aviation Administration issued a ground stop for flights bound for New York's LaGuardia airport Thursday, citing low visibility. The agency is warning that airports from New York to Charlotte could see similar ground stops today.
Philadelphia is under a code red warning, and the city's health department says the elderly, young children and those who are pregnant or have heart or lung conditions could experience serious health effects from the smoke.
"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," James Garrow, spokesperson for the Philadelphia Department of Public Health, told CNN.
The winds are also starting to push the band of smoke farther south into the mid-Atlantic, into cities including Washington, DC, and Baltimore.
Public schools in Yonkers, New York, are closed Thursday. Other 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," Gov. Phil Murphy told CNN's Jake Tapper on Wednesday, urging "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."
Smoke from the wildfires has delivered some of the poorest air quality measures in decades, said 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 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 cancels 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 said.
"I want to be clear, while there may be potential for significantly improved conditions by Friday morning, smoke predictability that far out is low, (it) is difficult to predict the movement of the smoke. ...This is an unpredictable series of events," he added.
