Canada wildfires spark air quality alerts in Chicago and Milwaukee

The outdoor air quality registered among the world's worst on Tuesday, with a visible haze and burning smell lingering over the region.

Smoke continues to drift south and east across North America in waves.

The Canadian Interagency Forest Fire Centre has said this year is the worst wildfire season on record. 

Cities including Chicago and Milwaukee, as well as parts of Michigan and Ontario, registered air quality index levels in the 200s on a 500-point scale. 

Those are numbers considered "very unhealthy", although lower than the levels seen earlier this month, which breached 400 in New York and other parts of the North East. 

Health authorities said that groups including children, the elderly and people with respiratory problems should limit outdoor activities when air quality is poor. 

Chicago school officials said they would be holding summer programmes indoors on Tuesday. 

Canadian fire authorities say nearly 500 fires are still burning, out of nearly 3,000 recorded so far this season. A total of 7.7m hectares (30,000 square miles) - an area roughly the size of South Carolina - has already been set ablaze.

The fires have released a record 160m tonnes of carbon into the atmosphere, according to the European Union's Copernicus Atmospheric Monitoring Service. 

The smoke is forecast to drift to the south and east over the course of the next few days.