Washington DC and Philadelphia experienced their worst air quality in years as intense wildfires in Canada continue to impact millions.
The poor conditions have forced event cancellations and grounded flights across the US.
Nearly 100 million people are experiencing very poor air quality in North America.
US President Joe Biden described the fires as a “stark reminder of the impacts of climate change”.
Data from the US Environmental Protection Agency’s Air Quality Index (AQI) shows that cities in North America had the worst air quality in the world on Thursday morning.
Cities including Washington DC, Philadelphia and New York had significantly worse air quality than cities abroad such as Lahore, Dhaka and Hanoi.
The smoke has caused the cancellation of school outings and sporting events, and, in the capitol, the White House’s planned pride celebrations.
The National Zoo was also closed, with its animals, including three giant pandas, taken indoors to shelter.
In nearby Baltimore, residents were wearing masks as they went about their day-to-day activities. One local, Sean Montague, said people “have to put your health first and be cautious”.
At the city’s Inner Harbour, friends Sharifah and Sheila disembarked from a water taxi, eager to hurry indoors.
They said they originally planned to spend the day in Baltimore’s Fells Point, a waterside neighbourhood known for its galleries, shops and outdoor seafood restaurants.
But once on the water, their eyes stung and the smoke was so thick, so they agreed the ride was “miserable” and decided to return home.
Much of the smoke is coming from Quebec, where 150 fires are burning. It is already Quebec’s worst fire season on record.
Some areas of Canada continued to experience very high levels of contamination on Thursday. The city of Janvier in Alberta, for example, had an AQI of 338, far above Washington DC’s 293.
Mr Biden said he spoke to Canadian Prime Minister Justin Trudeau on Thursday and deployed more than 600 firefighters to help battle the blazes in Canada.
On a typical Thursday, Washington DC’s Union Market would usually be packed with customers, dining al fresco in the afternoon sun.
But with smoke thick in the air, dozens of tables and chairs sat empty. A nearby rooftop bar was completely deserted except for a small group of Canadian tourists, who jokingly apologised for the disruption.
One customer, Tori, sat back in a lone Adirondack chair, with a mask tied around her wrist having just travelled from West Virginia.
“As I was driving, I noticed it was more hazy, and I just feel a little bit different too. I had a headache,” she said. “It’s very scary, if you think about it.”
Environment Canada said conditions were worsening in Toronto on Thursday, as more smoke poured in. The agency has 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.
In New York, an orange haze blanketed the city’s skyline and shrouded landmarks including the Statue of Liberty.
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 an area that’s 12 times the 10-year average for this time of year.
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.
Experts say exposure to wildfire smoke can cause a litany of health issues, such as an elevated pulse, chest pain, and inflammation in the eyes, nose and throat.