Some scenes which take place in the Firehouse 51 kitchen are filmed in the actual firehouse kitchen, others are filmed on a studio set. If you see a door to the left of the cooking area, that is the studio set; if you see a drinking fountain, that's the actual firehouse.
Anthony Ferraris (Tony, Squad 3) is still an active firefighter. He helps plan how each incident occurs in the show for accuracy.
Filmed in Chicago. Firehouse 51 is actually Station 18, which is located at 1360 S. Blue Island Ave in Chicago. "Molly's Bar," owned by characters Herrmann, Dawson, and Otis, is filmed in the real Chicago bar "Lottie's" in Bucktown.
Scenes at the fire academy are filmed at the actual Chicago Fire Department's Quinn Fire Academy, located at 558 W. De Koven Street, Chicago. The academy is built on the former site of the O'Leary Farm, which is believed to be the origin of the Great Chicago Fire in 1871.
Most departments, Chicago FD included, do not allow firefighters on duty to grow out facial hair on the cheeks or on the lower lip or chin as Lt. Severide often does. It interferes with achieving a tight seal on the SCBA (air) mask as the seal gasket on the rests against those parts of the face. Firefighters with too much facial hair will also fail the mask fit test for the same reason. Mustaches, as seen on Chief Boden, Mouch, Otis and some others, are not technically an issue.