Citation

Bentley RA, Roff MW. J. Occup. Accid. 1984; 5(4): 233-242.

Copyright

(Copyright © 1984, Elsevier Publishing)

DOI

unavailable

PMID

unavailable

Abstract

The protection which any breathing apparatus affords is limited by the effectiveness of the seal between the apparatus and the wearer. All face-masks may leak to some degree, the maximum inward leakage permitted by the British Standard Specification is 0.05 per cent of the wearer's minute volume under conditions of negative demand. This figure is a compromise between the considerations of the toxicity and likely concentrations of gases in irrespirable atmospheres and the time for which such apparatus would normally be used. Extra protection can be achieved if the pressure in the face-mask is maintained at a positive level throughout the breathing cycle, so that any leakage is outward. Compressed air breathing apparatus operating on this principle are commercially available and are widely used in industrial situations. The breathing apparatus used in the mining industry in the U.K. operate on a closed circuit principle and this paper describes a method used to convert such an apparatus to operate at positive pressure. A laboratory prototype apparatus and an experimental face-mask have been constructed. An important feature of this system is the maintenance of the seal between the inner and outer mask, consequently some form of independent inner mask adjustment is necessary. A series of tests at various work rates with the apparatus connected to a breathing simulator has shown that positive pressure can be maintained in the mask outer cavity. A range of leakages at the inner mask sealing edge have been simulated to accommodate the likely levels of fit which may be achieved in practice. Limited wearer trials have shown that the apparatus is comfortable to wear and the normal functioning of the equipment is not impaired.