Plenum is a fire-safety rating awarded to cables that have undergone rigid tests as outlined by the NFPA (National Fire Protection Association). Plenum rated cables are fire-resistant, produce low smoke when burned, and do not have metal conduits. Plenum cables are named after the areas in buildings in which they were designed to be routed, the plenum space, which can be defined as air circulation space above dropped ceilings or below raised floors. Before plenum rated cables, in the event of a fire, ordinary cables running through plenum space excited the spread of fire. The plenum cable rating was created to ensure cables routed through plenums do less to spread the fire.
Riser is a fire-safety rating designed for cables that are to be run vertically, from floor to floor, in their routing. Riser rating tests are not as stringent as plenum rating tests but do require riser cables to self-extinguish and prevent flames from traveling upward when aflame.
LSZH (Low Smoke Zero Halogen) is the designation for a certain kind of plastic developed as a safety precaution. In fiber optics, LSZH cable is jacketed with this plastic. Because many non-LSZH cable jackets contain various halogens and heavy metals, the smoke they produce upon burning is not only toxic and potentially fatal to humans and animals but can mix together in high temperatures to create corrosives such as hydrochloric acid. LSZH plastics are dense in metal hydrates, a chemical combination high in water molecules, greatly reducing the amount of smoke they produce when burned and dramatically reducing the toxicity and corrosive reactions resultant of its burning.