The Illinois Environmental Protection Agency, in coordination with the National Weather Service Chicago, has declared an Air Pollution Action Day for the greater Chicago metropolitan area, warning that ground-level ozone has reached concentrations classified as "Unhealthy for Sensitive Groups" — a level that poses a direct respiratory health risk to an estimated 9.4 million people across eight Illinois counties.
According to the Illinois EPA, a Chicago Air Pollution Action Day is declared when air quality is forecasted to be at or above the Orange or "Unhealthy for Sensitive Groups" category of the Air Quality Index (AQI) for two or more consecutive days for the Chicago Metropolitan Area — a threshold that explicitly captures sustained, not just brief, episodes of unhealthy air. The alert covered Cook, DuPage, Lake, McHenry, Kane, Kendall, Grundy, and Will counties, with a parallel alert issued for Northwest Indiana.
