Self contained breathing apparatus fall into two categories. Open Circuit and Closed Circuit. Generally SCBA is used for situations where Rescue Workers such as Firemen need breathable air in a IDLH (Imminent Danger to Life and Health) areas. Unlike PAPR systems, and SCBA unit does not supply air from the immediate surroundings. The unit has a high pressure tank supplying the air to the user. And, Supplied Air Respirators require a long lead hose that may be damaged in a toxic or high heat fire environment.
Also referred to as Industrial Breathing Sets. Generally consisting of a mask, regulator, frame/backpack, hose and high pressure air tank of 2,200 to 4,500 psi, SCBA units are designed to protect the wearer from toxic fumes, spray or contamination.
Open Circuit SCBA Systems Open Circuit Industrial Breathing sets provide air via a high pressure tank, and exhaust exhaled air to the atmosphere once it has been used. It utilizes a regulator, often with two stages. One at the tank, and one at the mask. SCBA systems are generally full face, and provide the wearer with superior visibility. It is important to note that different people, in different physical conditions can dramatically have an impact on usable air time for SCBA units. Generally, larger or less fit individuals will use more air.
Closed Circuit SCBA Systems Closed Circuit SCBA systems involve complex systems that recirculate and recycles exhaled gas. It us generally used for longer duration periods of time, such as mine rescue SCBA and where the confines of the space involved are a consideration.
An excellent article is available on Wikipedia describing SCBA systems in much more detail here