A smoky yellow haze generated by hundreds of Canadian wildfires hovered over a large swathe of the US on Thursday, threatening to make breathing difficult and disrupting air travel for millions of people for a second full day.

The US National Weather Service extended air quality alerts for another day for the East Coast from New England to South Carolina, as well as parts of the Midwest, including Ohio, Indiana and Michigan.

Advised to stay indoors, residents of New York, Washington and other big cities donned masks and scrambled to buy air purifiers to protect themselves from the yellow haze. The haze and low visibility led aviation officials to halt incoming flights to New York.