Authorities in New York, Toronto and Ottawa on Tuesday warned residents about the health risks from air polluted by smoke from unprecedented early summer wildfires in eastern Canada.
An unusually early and intense start to wildfire season has set Canada on track for its worst-ever year as warm and dry conditions are forecast to persist for months.
There are blazes in nearly all of Canada's 10 provinces and territories, with Quebec the worst impacted due to multiple fires caused by lightning.
The New York State Department of Environmental Conservation issued a health advisory for counties including New York, Bronx, and Queens.
The state recommended residents consider limiting strenuous outdoor physical activity to reduce the risk of adverse health effects.
The Canadian capital of Ottawa, which neighbours Quebec, was covered in haze on Tuesday morning, with air quality in category 10+, the worst level on Environment Canada's Air Quality Health Index, indicating "very high risk".
"Smoke plumes from local forest fires as well as forest fires in Quebec have resulted in deteriorated air quality," Environment Canada said in an air quality alert for Ottawa.
The air over Toronto was also polluted and conditions could persist through most of this week, the government run weather agency said.
Wildfire smoke can harm health even at low concentrations, and people with lung or heart diseases as well as older adults, children, and pregnant women were at higher health risk from wildfire smoke, Environment Canada said.
Canada's Foreign Minister Melanie Joly on Tuesday thanked United States, Mexico, South Africa and France for sending in firefighters to help.
Wildfires are common in Canada's western provinces, but this year flames have been mushrooming rapidly in eastern Canada, forcing home evacuations and the federal government to send in the military.
About 3.3 million hectares have already burned - some 13 times the 10-year average - and more than 120,000 people have been at least temporarily forced out of their homes.
"I want to highlight that people need to continue to listen to local authorities on how to stay safe, including in places like Ottawa that are affected by smoke from wildfires," Prime Minister Justin Trudeau told reporters in Ottawa.