Smoke from Canadian wildfires continue to shroud US cities in a noxious haze, forcing flight delays and cancellations to outdoor activities as environmental groups called for urgent action to tackle climate change.
Residents in the capital Washington awoke to an acrid smell and orange-tinged skies, with the Environment Protection Agency rating parts of the mid-Atlantic region at "Code Maroon," the highest category of the Air Quality Index, signaling hazardous conditions.
This made parts of the United States the most polluted in the world, worse than cities in South Asia and China that normally dominate global rankings, with the situation not expected to improve until the weekend.
"Today's air quality is extremely unhealthy," tweeted the city's Department of Energy and Environment.
"Members of the general public may experience health effects and sensitive groups may experience more serious health issues."
Commuters donned N-95 masks while the National Zoo announced it would close "for the safety of our animals, our staff and our guests."
The Washington Nationals, the capital's Major League Baseball team, announced it was postponing its afternoon game against the Arizona Diamondbacks.
Public schools in the capital canceled all outdoor activities including recess, physical education, athletic practices and competitions.
While the White House postponed an outdoor Pride event until Saturday.
"Today's Pride event on the White House South Lawn will be postponed until Saturday based on the projected air quality in the region," the White House said in a statement.
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The Federal Aviation Administration meanwhile said low visibility had forced it to "manage the flow of traffic safely into New York City, DC, Philadelphia and Charlotte."
Flights bound for New York's La Guardia and to Philadelphia International resumed after a pause.
Environmental groups were quick to draw attention to climate change, which is creating warmer, drier conditions that are increasing the risk and extent of wildfires.
"This is the climate crisis, here and now, causing dangerous air pollution and threatening the health of millions of people," said May Boeve, Chief Executive of 350.org.
Her comments echoed UN chief Antonio Guterres, who tweeted Wednesday: "With global temperatures on the rise, the need to urgently reduce wildfire risk is critical.
"We must make peace with nature. We cannot give up."
'Reminded me of 9/11'
Skies were noticeably clearer in New York compared to the day before, even as the AQI index remained high.
Officials handed out face coverings at train stations, bus depots and parks.
Linda Jiuliano, a 65-year-old secretary, gladly accepted one at Grand Central station in Midtown Manhattan.
"I've never seen anything like it," she told AFP, describing the sepia-tinged smog that engulfed New York on Wednesday as "scary."
"It reminded me a lot of 9/11, seeing the sky all smoky and everything," said Ms Jiuliano, who kept the windows closed and the air conditioner on at her home in Huntington, Long Island.
Meanwhile in Canada, pollution from wildfires is expected to peak today in Toronto, Environment Canada said.
With nearly two million acres affected, according to the Society for the Protection of Forests against Fire (SOPFEU), Quebec is experiencing a historic season.
Twice as many blazes have been recorded this year compared to the average over the past ten years.
The French-speaking province still had more than 150 active fires today, including nearly 90 out of control.
New reinforcements - from the United States, France and Portugal - are expected in the hours and days to come.
More than 12,000 people have been evacuated within the space of a few days.
The situation remains worrying in several regions, explained Stephane Caron, of SOPFEU.
"We are only at the very beginning of this fire season. We are now entering the period when usually there are beginning to be larger fires in Quebec," he said.
The risk of a new outbreak is rated "extreme" by authorities in the western part of Quebec.
These blazes are of high intensity and spread rapidly, and are therefore very complex for firefighters to stop, officials say.
