It happened in the Village of South Elgin.
At least 38 people were taken to hospitals.
Fire trucks and ambulances crowded the parking lot as paramedics removed employees from the building.
Some had trouble breathing, others had more serious symptoms. The majority of the 38 people transported to hospitals were from the H & H Sorting Company.
"Some were light-headed, some were getting sick, throwing up. Some were dizzy. Some were laying down on the benches in the hallway," said Mark Yarnell, H & H Sorting Company.
Firefighters quickly determined the employees were suffering from exposure to high levels of carbon monoxide. They found readings nearly 10 times the allowable level throughout the building.
The Elgin O'Hare Commerce Center is an industrial building that houses a number of different companies in about 600,000 square feet.
Investigators believe the source of the carbon monoxide was two forklifts that were operating next door to the H & H Sorting Company.
"Two forklifts in this weather, where everything's enclosed in a building for a prolonged period of time," said South Elgin Fire Chief Joe Cluchey.
Firefighters ventilated the building, which brought the carbon monoxide levels back down. Then to be sure of the source, they started the forklifts and watched the levels immediately rise again.
Paramedics transported people to four different area hospitals including Sherman Hospital in Elgin. Fire department officials say they do not believe any of the injuries were serious, and they expect mostly everyone to be treated and released Friday night.