Plenums are pressures slightly above atmospheric, usually inside air or gas systems as the result of the action of fans or blowers. The plenum is measured in small units of pressure such as ounces per square inch or mm Hg.
Plenum spaces in buildings are spaces for air circulation in air conditioning systems. Structured cabling and fibre optic cable is frequently laid in plenum spaces. To prevent excessive fire risk, there are strict restrictions on the kind of cable installations which are permitted in plenum spaces.