More than 100 million people in the United States are living under air quality alerts caused by Canadian wildfires, the US Environmental Protection Agency has said.
These range from "Code Orange" — unhealthy for sensitive groups — and above, the EPA said in a statement sent to the AFP news agency on Wednesday.
The region "includes much of the Northeast US — extending to Chicago to the west and Atlanta to the south," said the agency, with the Canadian wildfires presumed to be the main cause, though localised emissions and meteorology could also play a factor.
Many parts of the Northeast and mid-Atlantic meanwhile are under "Code Red," an Air Quality Index (AQI) of 151 or above, considered unhealthy for all people.
In Canada's Quebec province, now the epicentre of the devastating wildfires that have ravaged the country, more than 11,000 locals have already been evacuated.
Another 4,000 are expected to flee by the end of Wednesday, said Quebec Premier Francois Legault.
US President Joe Biden said on Twitter that more than 600 US firefighters and other personnel, along with equipment, had been deployed to Canada to help battle the blazes.
Winds have carried smoke hundreds of kilometres from Quebec.
The EPA said it encouraged Americans living in affected areas to check their AQI throughout the day and take steps to reduce their exposure.
"Pay attention to any health symptoms if you have asthma, COPD, heart disease, or are pregnant. Get medical help if you need it," it said.
While most healthy adults and children will recover quickly from smoke exposure and not have lasting effects, people with chronic diseases such as asthma or cardiovascular conditions are at higher risk.
Children, pregnant women and the elderly are also particularly advised to limit outdoor activity.
Fires shroud New York in apocalyptic haze
Canadian wildfires shrouded New York in an apocalyptic smog on Wednesday.
The Big Apple's mayor urged residents to stay indoors as the thick haze of pollution cast an eerie, yellowish glow over Manhattan's famous skyscrapers and delayed flights in and out the city.
"It smells like someone is doing a barbecue," said Nicha Suaittiyanon, a 30-year-old tourist to New York City from Thailand, who complained of "watery and itchy" eyes.
Forty-three-year-old lawyer Hugh Hill said his throat had been "stinging" from the harmful haze, which he likened to the smell of a wood-burning fire.
Like many New Yorkers, he chose to cover his face while out walking his dog in Central Park, normally Manhattan's green lung.
"I don't know if it's psychological or physical, but I know there's some benefit to wearing a mask.