Section 904 addresses smoke control systems designed to manage smoke movement and maintain tenable conditions during fires. Smoke control systems include mechanical pressurization creating positive pressure differences, smoke exhaust systems removing smoke, and natural ventilation through open windows or vents. Hospitals and nursing homes require smoke control systems including stairwell pressurization maintaining at least 0.05 inches of water pressure difference. Stairwell pressurization keeps smoke out of exit stairs while allowing normal passage. Return air must be exhausted from the building rather than recirculated when smoke is detected. Atrium buildings with multiple stories open to a central void require smoke control to prevent smoke stratification and maintain upper floor safety. The smoke layer must be maintained above occupied head height (7 feet minimum) to keep smoke away from occupants. Engineering analysis is required to verify system performance using fire dynamics simulation. Smoke control systems must be inspected, tested, and commissioned before occupancy. Maintenance personnel must be trained in system operation and testing. Backup power must be sufficient for complete system operation during fire.