Self-contained breathing apparatus



A self-contained breathing apparatus (SCBA) is a respirator worn to provide an autonomous supply of breathable gas in an atmosphere that is immediately dangerous to life or health from a gas cylinder. They are typically used in firefighting and industry. The term self-contained means that the SCBA is not dependent on a remote supply of breathing gas (e.g., through a long hose). They are sometimes called industrial breathing sets. Some types are also referred to as a compressed air breathing apparatus (CABA) or simply breathing apparatus (BA). Unofficial names include air pack, air tank, oxygen cylinder or simply pack, terms used mostly in firefighting. If designed for use under water, it is also known as a scuba set (self-contained underwater breathing apparatus).

An open circuit SCBA typically has three main components: a high-pressure gas storage cylinder, (e.g., 2216 to 5500 psi, about 150 to 374 atmospheres), a pressure regulator, and a respiratory interface, which may be a mouthpiece, half mask or full-face mask, assembled and mounted on a framed carrying harness.

A self-contained breathing apparatus may fall into one of three categories: open-circuit, closed-circuit, or continuous-flow.

Closed-circuit
The closed-circuit type, also known as a rebreather, operates by filtering, supplementing, and recirculating exhaled gas. It is used when a longer-duration supply of breathing gas is needed, such as in mine rescue and in long tunnels, and going through passages too narrow for a big open-circuit air cylinder. Before open-circuit SCBA's were developed, most industrial breathing sets were rebreathers, such as the Siebe Gorman Proto, Siebe Gorman Savox, or Siebe Gorman Salvus. An example of modern rebreather SCBAs would be the SEFA.

As of 1987 (30 CFR Part 11)
Duration of closed-circuit SCBAs is somewhere between 1-4 hours. A closed-circuit SCBA system is negative-pressure, increasing the risk of leaks.

There are two types of closed-circuit SCBA according to NIOSH: To reduce pressure buildup from use, a pressure-relief valve with saliva trap is included. Closed-circuit SCBAs are also noticeably smaller than open-circuit ones.
 * Uses compressed oxygen.
 * Uses an oxygen-generating solid. This involves a chemical reaction between potassium superoxide with exhaled water and carbon dioxide. A chlorate candle has to be struck to start the device.

Self-contained self-rescue devices are also closed-circuit SCBAs, working on the same principles, being designed for emergency use in mines, and lasting about one hour.

As of 1987 (30 CFR Part 11)
An open-circuit SCBA does not recirculate air; it instead allows respired air to be exhausted outside. While 30 CFR 11 does not restrict the gas that can be used (although compressed air is usually chosen), use of compressed oxygen is not allowed due to the system's exposure to outside air. Duration is usually limited to 30-60 minutes.

There are two types of open-circuit SCBA according to NIOSH:
 * Demand: 2000 psi to the regulator from the main valve, plus a bypass valve in case of failure, with a two-stage regulator reducing pressure to 50-100 psi.
 * Pressure-Demand: Similar to demand, but with a spring in the diaphragm, which holds the admission valve open, for continuous air flow to the facepiece.

NIOSH emphasizes that facepieces between both SCBA types cannot be interchanged, but certain SCBAs can be switched to both 'demand' and 'pressure-demand' operation. However, both modes require different training.

Continuous-flow
Escape SCBAs, also known as ESCBAs, come with hoods, are meant for escapes only, and are operated in continuous flow mode.

Escape only SCBAs, designed for escape from IDLH situations, regardless of type, are usually limited to 3-10 minutes.

Facepiece
SCBAs usually come with full-facepieces, but can also come with half-mask or mouthpiece in demand or pressure-demand mode, though use of mouthpieces are limited to escapes only, as of 1987.

Hoods and helmets are limited to continuous flow mode only, and are also used in air-line respirators in addition to escape-only SCBAs.

Usage
There are two major application areas for SCBA: firefighting, and industrial use in confined spaces.

Regulation and standards
In the United States and Canada, SCBAs used in firefighting must meet guidelines established by the National Fire Protection Association, NFPA Standard 1981. If an SCBA is labeled as "1981 NFPA compliant", it is designed for firefighting. The current version of the standard was published in 2018. These standards are revised every five years. Similarly, the National Institute for Occupational Safety and Health (NIOSH) has a certification program for SCBA that are intended to be used in chemical, biological, radiological, and nuclear (CBRN) environments.

Any SCBA supplied for use in Europe must comply with the requirements of the Personal Protective Equipment Directive (89/686/EEC). In practice this usually means that the SCBA must comply with the requirements of the European Standard EN 137:2006. This includes detailed requirements for the performance of the SCBA, the marking required, and the information to be provided to the user. Two classes of SCBA are recognised, Type 1 for industrial use and Type 2 for firefighting. Any SCBA conforming to this standard will have been verified to reliably operate and protect the user from -30 °C to +60 °C under a wide range of severe simulated operational conditions.

Human factors
SCBA is intended to be personal protective equipment, but its use is not without cost. The weight of the unit and work of breathing affect the work capacity and agility of the wearer, and the full-face mask, while protecting the face and eyes from heat, smoke, and toxic gases, also reduces peripheral vision and awareness of the surroundings. The weight and harness straps may limit tidal volume, ventilation rate, and oxygen consumption, and heart rate may increase in comparison with the same exercise levels without the equipment. Shoulder harness straps of heavy SCBA can reduce free motion of the thorax which affects breathing.