Toxic smoke from Canadian wildfires could linger over vast swaths of the US for days, officials warned, as millions of Americans remained under air pollution warnings, writes The Guardian.

Across the eastern US residents were again urged to stay inside and limit or avoid outdoor activities on Thursday, as schools in some cities closed, sporting events were canceled and air travel was disrupted. New York City had the worst air quality of any big city in the world on Thursday morning, according to IQAir, while the second worst was Detroit.

Air quality levels were in the 150-200 range on Thursday, marking a slight improvement from Wednesday when record-setting hazardous levels were observed, per Fox Weather.

Smoke from wildfires in Canada has been moving south into the US since May. Hundreds of fires are burning in Canada, from the western provinces to Nova Scotia and Quebec in the east, where there are more than 150 active fires in a particularly fierce start to the summer season.

The weather system driving the smoke south “will probably be hanging around at least for the next few days”, Bryan Ramsey, a US National Weather Service meteorologist, told the Associated Press.

“Conditions are likely to remain unhealthy, at least until the wind direction changes or the fires get put out. Since the fires are raging – they’re really large – they’re probably going to continue for weeks. But it’s really just going be all about the wind shift”, - Ramsey said.