Poor air quality caused by Canadian wildfires has continued to affect large swathes of North America, with nearly a third of United States residents warned of unhealthy conditions, writes Al Jazeera.

Air quality warnings on Thursday stretched from Wisconsin and northern Illinois across Michigan and into New York and the East Coast, according to the US National Weather Service.

Meanwhile, the US Environmental Protection Agency’s real-time air quality map showed “unhealthy” air from the Midwest border with Canada down to North Carolina and Tennessee. Pockets of “very unhealthy” air were recorded over the cities of Madison, Wisconsin, Cedar Rapids, Iowa and Lansing, Michigan.

All told, more than 100 million US residents in the country of about 330 million were urged to limit prolonged outdoor activities. Those suffering from pulmonary or respiratory diseases were urged to wear masks, while children and the elderly were also advised to minimise or avoid strenuous activities.

Reporting from Chicago, Illinois, Al Jazeera’s John Hendren said that, while conditions have improved slightly, concerns over health risks persist.

“This is a third straight day with a very unhealthy air quality here in Chicago. There’s a 500-point scale for air quality. We have been at over 200 for most of the past 24 hours,” he said. “To give you an idea of how bad that is, at 150 it is the equivalent of smoking seven cigarettes a day if you spend most of the day outside”.

The most recent stretch of hazy days comes as Canada continues to battle its worst early wildfire season on record. In 2023, an area of 8 million hectares (19.8 million acres) had already burned.

As of Wednesday, there were 477 active blazes in Canada, with about half considered out of control, spreading from the Pacific to the Atlantic coasts.

With the hottest months of the year still ahead, experts say there is unlikely to be meaningful reprieve any time soon.