The alert comes as humidity is expected to increase Tuesday, with high temperatures in the mid 80s to low 90s, with heat indices reaching 100 by Wednesday.

Wildfires in Canada have forced tens of thousands of people to leave their homes, and the smoke drifting over the border has sparked air quality concerns across the northern Unite

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.