Fire doors are used to stop the spread of fire in a building by cutting off one section of the building from another. When the fire doors close, they prevent air, smoke and fire from circulating to other parts of the building. Fire doors are made of several layers of fire proof materials including steel, gypsum, and vermiculite. These combined materials will withstand a fire for a specific period of time. Rating systems that measure the combined components of fire door tell how long a fire door can withstand fire.
Most fire doors are held open by an electromagnet or similar mechanism designed to release the door in the event of a fire. OSHA regulations specify that a fire door may not be tied or propped open at any time. Fire doors may only be held open by a mechanism that will release the doors when a fire alarm is activated.
When a fire alarm is triggered, all fire doors automatically close and because fire doors are self-latching, they create a seal that prevents the spread of fire. This allows most people within the building to proceed to nearby exits when a fire alarm has been activated.