It is similar to SCUBA (self-contained underwater breathing apparatus), except without the underwater part.
This is a term given to equipment that can provide a worker with 100% of his air supply from a portable source, so as to not require, or endanger himself, by breathing from the environment.
Any time there is a risk of a worker being exposed to fumes, gases, or other harmful agents, SCBA should be used. Obviously, extensive training is needed. As the equipment used requires the knowledge, and practice, to use it in a safe manner.
When used correctly it can provide optimum protection for workers. Firefighters use this type of equipment, along with fire resistant full body clothing, before they enter burning environments. Depending on the working environment, it may very well be required that workers use similar equipment. For example, when working with gas chlorine, if an accident occurs the gas can quickly burn the skin and severly damage the lungs, which can lead to death, if the worker is not adequeatly protected.