Abstract
Industrial half mask respirators [defined as mask covering only the orinasal regions] are normally selected by the safety management for the employees. The choice is normally made on the basis of their theoretical ability to reduce the concentration of airborne contaminants below the Health and Safety Executive [HSE] published Threshold Limit Values [HSE 1977]. However in practice, the degree of protection provided by these respirators is not known and their protective values have long been suspected to be less than the scientific acclaim. In the past , several empirical studies have been carried out to investigate the effectiveness of the respirators but no attempt was made to determine and analyse the factors which cause the reduction of the protective values of respirators. This research project was carried out with an aim to determine the degree of respiratory protection achievable by the commercially available half mask respirators in the United Kingdom. Attempt was made to establish the factors which have direct influence on the protection of respirators. The research necessitated evaluation and development of methods for the accurate measurement of facial contours. Moire fringe photography from a projected coloured grid was the method of ultimate choice. Facial contour measurements were made on a total of 394 individuals mostly from industries which normally use respiratory protective devices. Of this number 184 also had measurements of the inward leakage of the face mask using a Sodium Chloride penetration method. Correlation was made between a number of facial dimensions and leakage values . Other variables which might influence leakage such as training in mask usage, facial hairs , increased respiratory effort [both by voluntary hyperventilation and exercise - induced] , head movements and differences between male and female users were also studied in relation to measured leakage values. During the course of the research, a series of comfirmatory experiments were carried out to provide the supportive evidence for the main line research. All the data obtained were subjected to the relevant statistical treatment. The tolerable level of leakage is 5 % [HSE]. The present study has shown that the leakage of the best mask is greater than this [range 0 % to 26 %]; but other mask can show leakage values of up to 47.9 %. The amount of face seal inward leakage can be attributed to a number of related factors namely the level of training and experience possessed by the subjects in the use of respirators, the presence of beards and the variation in the facial contours and dimensions.