The 11 people taken to the hospital for precautionary measures only were 11 children.
About 50 kids were inside the Boys and Girls Club gymnasium for a radio station pajama party where a Chicago Bear player and Minnesota Viking player were the guests of honor.
Shortly after 7:30 p.m., a few of the kids began feeling sick and the fire department was called. The firefighters immediately took carbon monoxide levels and determined there was a leak coming from the furnace and everyone was immediately evacuated. The kids were put on warming buses while others were taken to the hospital including one boy who called his mom from the party.
"He said, 'Mom, I'm dizzy, I have a headache and feel like I'm going to pass out,'" said Lavandy Goines.
"Carbon monoxide as we all know is deadly and we avoided a very serious problem," said Dist. Chief Patrick Kehoe, Chicago Fire Department.
The fire department was trying to determine if there were carbon monoxide detectors in the building on Monday night.
No one was seriously injured.