The group was preparing to board a flight to Denmark when some of them became sick, the Chicago Sun-Times reported Tuesday.
Initially, ambulances took seven people to local hospitals. Two more people were transported later, said Juan Hernandez, a spokesman for Fire Media Affairs.
The group had been on a bus tour before they arrived at the airport.
Hernandez said those people who did not get sick were kept at a hotel "as a precaution," the Chicago Tribune reported.
All those hospitalized were reported in good condition.
The incident is under investigation, the fire department said.