Fire doors are an engineered component which includes the frame, door leaf and any fixtures and fittings. They can be solid or made with a special core, often flax board or a wood composite material. They will be fitted with intumescent seals, installed in the frame or leaf, which seal the door on impact with heat to stop the spread of fire and/or smoke around the edges of the door. Doors will similarly be fitted with smoke seals to prevent the passage of smoke in the early stages of a fire. http://www.firedoorfactory.com/fire-door/steel-fire-door/