Wildfires in Canada are continuing to blanket its southern neighbors in smog, triggering new air quality alerts in the northern high plains, Midwest and Great Lakes, according to the National Weather Service. While the agency predicts the haze should wane by Monday, sensitive groups could still be at risk in the upcoming week.

Smoke from blazes in Alberta and British Columbia moved south across Minnesota following a cold front Friday, according to the state’s pollution control agency, prompting an air quality alert that was extended until Sunday afternoon.

As of Saturday, nearly 900 fires have burned in a record-breaking year for the Great White North, according to the Canadian Interagency Forest Fire Centre.

Canada’s eastern provinces such as Quebec, Ontario and Nova Scotia have been hit particularly hard this year by large and at time uncontrollable infernos. But British Columbia, along Canada’s west coast, had the second-highest number of active fires followed by Alberta and Ontario.

What is AQI, or air quality index?

The air quality index, or AQI, measures the air quality and level of health concern it presents as measured by the Environmental Protection Agency.

Air quality alerts are indicators the air is unsafe to breathe for certain people. Alerts are triggered by a number of factors, including the detection of fine-particle pollution — known as “PM 2.5” — which can irritate the lungs.

Pollution is detected by a system of monitors on the ground “constantly taking measurements of the amount of chemicals and particles in the air,” said Susan Anenberg, professor and department chair of environmental and occupational health at George Washington University.

What is the forecast? US air quality map