A plenum space is a part of a building that can facilitate air circulation for heating and air conditioning systems, by providing pathways for either heated/conditioned or return airflows, usually at greater than atmospheric pressure. Space between the structural ceiling and the dropped ceiling or under a raised floor is typically considered plenum; however, some drop-ceiling designs create a tight seal that does not allow for airflow and therefore may not be considered a plenum air-handling space.
The cavity/plenum space is typically used to house the communication cables for the building's computer and telephone network; however, it has been proposed that the growing abandonment of cable in plenum spaces may pose a serious hazard in the event of a fire, as once the fire reaches the plenum space the airflow present in the space supplies fresh oxygen to the flame and makes it grow much stronger than it would have otherwise.
Recent testing by ASHRAE has shown that, while flame spread is limited by accumulated cable bundles, other structural concerns may still exist due to increased load on suspended components. As plenum spaces are restricted from use as areas for storage, the principle behind removal of abandoned cable is that regulated removal prevents the use of plenum spaces as a storage area for abandoned cable. In addition, no high-voltage powered equipment is allowed in the plenum space because presence of fresh air can greatly increase danger of rapid flame spreading should the equipment catch on fire.
Diligence is required to make sure that a non-plenum airspace stays that way. A non-plenum airspace can become a plenum airspace by accident if the ductwork is disconnected and not properly repaired and resealed. Ductwork disconnection can occur due to building damage such as earthquakes, aging, or adverse environment causing the metal to corrode and fall apart, or simply negligence on the part of building contractors that leave work unfinished. In all such cases, discovery, and repair of such problems to eliminate unintended plenums is difficult due to the hidden nature, limited space, and difficult access of most installed drop ceilings. For highest fire safety, it is best to assume all drop-ceiling airspaces are plenums, whether or not they are officially designated as one.