A breathing apparatus or breathing set is equipment which allows a person to breathe in a hostile environment where breathing would otherwise be impossible, difficult, harmful, or hazardous, or assists a person to breathe. A respirator, medical ventilator, or resuscitator may also be considered to be breathing apparatus. Equipment that supplies or recycles breathing gas other than ambient air in a space used by several people is usually referred to as being part of a life-support system, and a life-support system for one person may include breathing apparatus, when the breathing gas is specifically supplied to the user rather than to the enclosure in which the user is the occupant.
Breathing apparatus may be classified by type in several ways:
- by breathing gas source: self-contained gas supply, remotely supplied gas, or purified ambient air,
- by environment: underwater/hyperbaric, terrestrial/normobaric, or high altitude/hypobaric,
- by breathing circuit type: open, semi-closed, or closed circuit,
- by gas supply type: constant flow, supply on demand, or supplemental,
- by ventilatory driving force: the breathing effort of the user, or mechanical work from an external source,
- by operational pressure regime: at ambient pressure or in isolation from ambient pressure,
- by gas mixture: air, oxygen enriched air, pure oxygen or mixed gases,
- by purpose: underwater diving, mountaineering, aeronautical, industrial, emergency and escape, and medical.
The user respiratory interface is the delivery system by which the breathing apparatus guides the breathing gas flow to and from the user. Some form of facepiece, hood or helmet is usual, but for some medical interventions an invasive method may be necessary.
Any given unit is a member of several types. The well-known recreational scuba set is a self-contained, open circuit, demand supplied, high pressure stored air, ambient pressure, underwater diving type, delivered through a bite-grip secured mouthpiece.