Mystery as Chicago is hit by public health emergency with toxic chemicals filling the air | 23 April 2025 | A concerning air quality mystery has broken out in Chicago, as the city has reportedly been blanketed with toxic chemicals. Air quality maps revealed an enormous cloud of hazardous air sitting over the entire Chicago metropolitan area Wednesday afternoon, with readings reaching 500 on the Air Quality Index (AQI) -- the worst possible score on Google Maps. An AQI of 500 denotes extremely hazardous air quality, where pollutant levels are at or beyond the upper limits of what monitoring systems typically measure which typically only happens during massive wildfires or volcanic eruptions.