The shroud of polluted air emanating from Canadian wildfires pushed further down the Atlantic Seaboard, blanketing Washington DC, in an unhealthy haze and prompting many residents of the nation's capital to stay indoors.
Traffic was light and trains less crowded than usual as many companies in the city told employees to work from home. Some non-essential municipal services were suspended, including parks and recreation, road construction and waste collection.
The Washington Nationals baseball team called off its home game, while the National Zoo shut down for the day.
It was the worst case of wildfire smoke blanketing the US Northeast in more than 20 years, according to private forecasting service AccuWeather.
The US National Weather Service extended air quality alerts from New England to South Carolina, as well as parts of the Midwest, including Ohio, Indiana and Michigan. The smoke reached as far as Norway by Thursday, according to scientists tracking the level of particulates in Scandinavia.
Millions of Americans have been advised to stay indoors if possible to avoid respiratory issues and other health problems that could arise due to the high levels of fine particulates in the atmosphere.
Government data showed air quality readings above the "hazardous" level in Washington on Thursday morning.
Many people wore masks outside as a thick layer of smoke veiled the capital. The dense smog, which reeked of ash, erased the top of the Washington Monument from view.
"This problem is likely to continue or worsen through Friday," Washington Mayor Muriel Bowser said in a tweet. "We urge residents and visitors to follow precautions."
BRIGHTER SKIES OVER NEW YORK
In New York, which spent much of Wednesday covered in a sickly haze, air remained some of the worst in the world on Thursday, with pollution similar to that found in perennially smoggy cities such as Dhaka and Delhi, according to IQAir, a Swiss technology company.
Though a smell of burning wood lingered, the skies were brighter on Thursday morning than on Wednesday, but as evening approached, the haze appeared to be thickening again.
After two days in which schools called off outdoor activities, including sports practices and recess, New York's public school system announced that its more than 1 million students would shift to remote learning on Friday.
The smoke prompted aviation officials to halt incoming flights to major airports in New York and Philadelphia from the Northeast, Mid-Atlantic region and Ohio for a second day. All flights bound for the airport in Newark, New Jersey, a major New York-area airport, were delayed.
Smoky conditions are likely to persist until Sunday, when a new storm system shifts the direction of prevailing winds, National Weather Service meteorologist Peter Mullinax said.
But researchers have said wildfires will occur more often and at higher intensity due to climate change.
"Public knowledge is a huge issue," said Keith Bein of the Air Quality Research Center at the University of California, Davis. "People need to realise that this is not a one-off event. These fires are going to be the new normal."
While the conditions have disrupted daily life for millions of Americans, the impact on the US economy is likely to be limited and short-lived, according to Ryan Sweet, chief US economist at Oxford Economics.
In Canada, where the smoke originated, the federal government gave Toronto a "high risk" rating based on the air quality on Thursday afternoon, while other cities such as Ottawa and Montreal were at "low risk" as smoky conditions eased.
The country is enduring its worst-ever start to wildfire season, with thousands forced from their homes.
The US has dispatched more than 600 firefighters to Canada to help the country battle the blazes. President Joe Biden, who has called the wildfires another reminder of the dangers of climate change, said US officials were monitoring air quality and aviation delays.