Do pressure-demand breathing systems safeguard against inward leakage?

Rescue operations conducted in toxic atmospheres require respiratory protective equipment capable of providing a very high degree of protection. A slight positive pressure in the facepiece throughout the breathing cycle is used in the pressure-demand, compressed-air breathing apparatus to eliminate any inward leakage. In the present work an inward leakage test with sulphur hexafluoride (SF6) was used to measure facepiece penetration in a pressure-demand system at different work loads. During the test, inspiratory-flow pressure variations within the facepiece and heart rate were also measured. Tests were performed on a dummy head and on two subjects. In some tests a poor face seal was introduced by putting an open tubing with an inner diameter of 2 mm and outer diameter of 3 mm under the sealing edge of the mask. It was found that inward leakage was less than 0.0001% under all conditions as long as facepiece pressure was positive. When facepiece pressure fell below zero, which occurred at inhalation peak flows about 300 L/min, an inward leakage was detected. One subject achieved, at extreme work load, an inhalation peak flow around 450 L/min. These results show that pressure-demand systems should be tested with a breathing machine giving peak flows of at least 300 L/min to ascertain the capability of these systems to maintain positive pressure in the facepiece during hard rescue work.