Smoke from wildfires burning across Canada spread into the northeastern United States on Wednesday, turning skies hazy and yellow in cities including New York and Boston while triggering air quality alerts across several states. Officials warned that smoke could become more widespread through the rest of the week, posing health risks as parts of the region also endure an intense heatwave.

AccuWeather said the smoke outbreak has created conditions reminiscent of the major Canadian wildfire event in 2023, when smoke from Quebec blanketed much of the northeastern US for days.

More than 800 wildfires are currently burning across Canada, including dozens that remain out of control in western Ontario, according to AccuWeather. The Guardian reported that more than 100 active wildfires in northern Ontario turned Toronto’s skies yellow and pushed the city’s air quality to the worst in the world on Wednesday, according to IQAir.

Dramatic footage from near Armstrong, Ontario, showed a train crew surrounded by flames before Canadian National confirmed the crew was safely evacuated.

Toronto was also battling an intense heatwave, with downtown temperatures reaching 37.3°C and runway temperatures at the city’s main international airport climbing to 55°C.

The smoke has spread across the Great Lakes and Northeast, prompting air quality alerts from Wisconsin and Michigan to New York and Massachusetts.

In New York City, the state Department of Environmental Conservation and Department of Health issued an Air Quality Health Advisory covering New York City and Long Island until at least 9 PMWednesday. Officials warned of the possibility of deteriorating air quality, although the timing and extent of ground-level smoke impacts remained uncertain.

The National Weather Service also warned that nearly 100 million people across the US were under “major” or “extreme” heat warning levels as the smoke arrived during an ongoing heatwave. New York City was forecast to reach 99°F, with health officials urging residents to stay indoors in cool, air-conditioned spaces.

Boston recorded an Air Quality Index (AQI) of 147 on Wednesday morning, placing it in the “unhealthy” category, with little improvement expected into Thursday, according to AccuWeather.

AccuWeather forecast that dense smoke would expand further on Thursday, covering much of the Great Lakes and New England, including Detroit, Cleveland, New York and Philadelphia, while hazy conditions spread across much of the northern United States.

Toronto reached an AQI of 220, categorized as “very unhealthy.” Health officials warned that sensitive groups could experience immediate health effects, while healthy individuals might suffer breathing difficulties and throat irritation.

Authorities advised people to limit outdoor activities, especially those in sensitive groups, as smoke exposure can cause breathing problems, throat irritation and itchy eyes. Residents were also encouraged to keep pets indoors to reduce their exposure to poor air quality.

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