Plenum means "air bag". A Plenum area, is the air return for an air conditioning system. In most buildings, the area above a drop ceiling or under a raised floor is used as the air return (source of air) for the air conditioning. Those drop ceiling and raised floors are also where wire is often installed. If wire and cable is installed in a drop ceiling or raised floor, it is out of sight. If that cable were burning, it would give off toxic fumes. These would be fed to the rest of the building by the air conditioner as a result people could be injured even though they are a long way from the fire.
Need for Plenum
Plenum cables are tested in a special way (UL 910 Steiner Tunnel Test) to make sure the cable will not allow the fire to continue burning and spread. this doesn't mean the wires will work: all wires will eventually melt under heat. It is just that they won't be the fuel for the fire.
Plenum Rated Materials
There are many materials which can meet plenum specifications. The most common is TeflonTM by DuPont, which comes in many forms: FEP, TFE, PTFE, all of which are used in cables. There are other materials, such as SolefTM and HalarTM which can meet plenum specifications. There are also PVC versions, such as FlamarrestTM from Belden which can meet plenum specifications in certain constructions. There is also a lot of work on combined materials such as a data cable where one or two pairs out of four are one material and the others are another material. These combinations have successfully passed Plenum tests and significantly reduce dependence on high-cost plenum-rated materials.
Cost of Plenum
Materials required to pass plenum testing makes plenum-rated cables much more expensive than non-plenum-rated designs. However, with the advent of alternate compounds and mixed compounds, the cost of plenum cables has dropped dramatically. At one time, plenum cables were 3 to 5 times the cost of non-plenum. With newer materials and techniques, they are 2 to 3 times the cost.