The Self-Contained Breathing Apparatus (SCBAs) worn by the cast are Scott 4.5 Air-Pak 75s. This is one of the most popular brands and models of SCBA with fire departments across America for its durability, reliability, and ease of use. However, the masks worn by the cast members are unrealistic. They are nonfunctional and were made specifically for the series with the anti-fog interior facepiece and exterior voice amplifier removed, the lens elongated, and the regulator mounted at an odd angle to clearly show the actors' faces. These masks would fog badly if they were actually flowing air. Real SCBA masks are put on separately from the regulator, which is connected after the mask is on, and disconnected before the mask is removed. The air demand and exhalation valves make a loud noise when operating, and muffle the voice of the wearer.
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 often seen on Chief Boden, Mouch, Otis and some others) are not technically an issue, but some departments may have internal regulations that restrict them.