Wildfire smoke degrades air quality in swaths of Ontario, with Toronto ranking among the worst in the world on Monday.

The Chicago area recorded ozone levels of 156 late Monday afternoon. Levels above 100 are considered unhealthy. Residents with respiratory issues, children and the elderly are advised to stay indoors and limit exposure to air outside until Tuesday evening.

The National Weather Service’s Chicago office attributed the alert to unhealthy ozone levels and lingering effects of smoke from wildfires in Canada.