Old Orchard Residences in northwest suburban Skokie was evacuated due to a carbon monoxide leak.
The cause was a malfunctioning furnace system.
Carbon monoxide alarms went off at around 8 a.m. which probably helped save from a major disaster.
Six people were sent to area hospitals with symptoms of nausea and lightheadedness. No serious injuries were reported.
The cause of the leak is still under investigation.