Five ambulances were sent to 1120 West 47th Place in Chicago's Back of the Yards neighborhood after a report of several people having a hard time breathing. The Chicago Meat Authority is located at that address. More ambulances were sent to the scene around 20 minutes later.
Chicago Fire Department Spokesman Larry Merritt said a malfunctioning fan caused CO to back up in the building shortly before 9 a.m.
Fire officials tweeted around 9:45 a.m. that the situation was secured. Eight people were transported to the hospital in good condition, five in fair-to-serious condition and four in serious condition, Merritt said. One person refused treatment at the scene.
ABC7 Eyewitness News will update this story as more details are made available.
The Sun-Times Media Wire contributed to this report.