The Algonquin-Lake in the Hills Fire Protection District said it has received multiple 911 calls about an unknown odor described as a chemical or burning electrical smell in the air.
The reports, which started late Saturday night or early Sunday morning, span a wide area from central Illinois through Indiana and as far north as the Wisconsin border.
The source of the odor appears to be coming from a distance "quite south" of the Algonquin-Lake in the Hills area, fire officials said. But the source itself is unknown.
The Algonquin-Lake in the Hills Fire Protection District said it is actively monitoring the situation and is coordinating with regional partners and other agencies to gather information.
Officials said to seek medical attention if you experience symptoms such as difficulty breathing, dizziness, nausea, or irritation.
Call 911 only if there is an immediate emergency or life-safety concern, officials added.
Further information was not immediately available.