“Air quality and visibility due to wildfire smoke can fluctuate over short distances and can vary considerably from hour to hour,” Environment and Climate Change Canada (ECCC) said in its statement.

The Air Quality Index (AQI), a system used to communicate how much pollution is in the air, is expected to reach the red or unhealthy category in a large swath of Minnesota, and will likely remain so through Saturday.

Wildfire smoke from the Prairies has prompted a special air-quality statement over much of northern Ontario Saturday. Environment Canada says smoke from forest fires is causing reduced visibility and poor air quality that is expected to persist into Sunday for some areas.