Where an orinasal mask is used as an internal component of a fullface mask, helmet or hood, its function is generally to reduce dead space, allowing effective use of demand supplied gas or a breath driven rebreather loop.
They are not necessary if the breathing gas supply is abundantly in excess of physiological requirements in a free-flow or powered rebreather loop.
In the most basic cases a disposable mask may be made of porous paper or non-woven fabric held in place by two elasticated webbing straps around the back of the head.
For more critical applications and where re-use is intended, soft flexible polymers or elastomers are generally used for the seal.
The straps may be synthetic rubber or elasticated woven webbing, and the frame may be moulded plastic, to which accessories such as filters and valves can be fitted.