When a fire starts in a closed room, heat is generated causing smoke and gases to rapidly expand. This creates a higher or "positive" pressure in the fire room than in the surrounding area. In addition, the hot gases rise due to their lower density and greater bouyancy.

Full scale room fire experiments have shown that these forces generally result in a sort of equilibrium once the fire is fully developed where negative pressure in the room relative to the outside exists in the lower part of the room and positive pressure exists in the higher levels. The area where these two zones meet is known as the "Neutral Pressure Plane". This plane is generally horizontal and the pressure in the room becomes increasingly more positive the higher it is measured above this plain and more negative with increasing distance below the neutral pressure plane.

In the case of fire door tests, there has been a substantial change over the course of the last several years in recognition of the need to make tests more closely simulate the type of pressure profile that is felt to be a good representation of the way many fires actually behave. It has been recognized that the level of pressure difference is less critical than the fact that it exists and tends to drive a flow of hot gases and flame out of the fire area. Thus most US fire test standards for fire doors and windows now include a specific requirement for location of the "neutral pressure plane" relative to the assembly height. This level is fixed at 40" above the sill for door assemblies and 2/3 down from the top of window assemblies. These locations are consistent with observation during room fire experiments and also correspond to the levels already specified in most international fire test standards.