Smoke from Canadian wildfires continues to affect air quality across the Midwest, though Monday’s conditions were an improvement for most of the Chicago area.

An Illinois Air Pollution Action Day is declared when weather conditions are such that widespread ozone and or particulate levels are expected to be at or above the Unhealthy for Sensitive Groups (USG) category of the Air Quality Index for all or almost all of Illinois.

Air quality alerts issued Thursday in Illinois because of Canadian wildfire smoke marked the latest bout in an increasingly common summer occurrence. The alert covers 34 Illinois counties designated as having unhealthy air for all residents.