Orange County Fire Rescue dispatched a hazardous material crew to the University of Central Florida Student Union just before 3:30 p.m. Monday to handle complaints of an odor.
Witnesses said a perimeter was set up around the building with multiple emergency response vehicles.
"People were being told to move back repeatedly by firefighters and staff. The atmosphere was that this was not a drill and something was occurring inside," wrote UCF student Matthew Miller in a news tips to FOX 35.
Crews have determined that the odor was caused by distribution of mace through the building's the air-conditioning filtration system. The building has since reopened.
Eleven people were treated at the scene. University police officers are investigating.