Development of a powered helmet respirator suitable for use in coal mines. Final report on CEC Contract 7247/14/041

A powered helmet respirator (PHR), the Racal Airstream AH6, has for several years been made available to selected mineworkers in a number of UK collieries. The AH6 comprises a headshell and visor assembly offering head and eye protection, with a fan-filter system which directs filtered air into the wearer’s breathing zone to provide respiratory protection with minimal breathing resistance being imposed on the wearer. After initial enthusiasm, its usage declined such that by 1982 less than MO per cent of the available AH6’s were in regular use. As a result, the Institute of Occupational Medicine (IOM) undertook to develop an improved PHR which would be more acceptable to mineworkers than the AH6.The development of an IOM prototype PHR suitable for use in coalmines was based upon the appreciation that, to be effective, the PHR had to be simultaneously wearer acceptable and technically efficient; the major difficulty was that these two requirements were likely to be mutually incompatible.as deemed to interfere with work due to its being too bulky, particularly at the rear of the headshell, too heavy and having too low an airflow. A further major shortcoming was that the visor severely interfered with communication, particularly when the wearer was using a tannoy or telephone.Consequently the development was specifically undertaken to ensure that these negative features of the AH6 were minimised as far as possible from the prototype. Given that the long visor of the AH6 caused severe interference with communication, it was decided that a much shorter visor would be required for the prototype. However, it was recognised that such a visor could permit greater leakage of dust into the wearer’s breathing zone than would the long visor of the AH6. The adoption of a short visor effectively excluded the prototype from the scope of the relevant draft European Standards for PHR] and made the maximum of ten per cent total inward leakage specified in the draft standard for type PH1 devices difficult to achieve. It was decided to set the total inward leakage target for the prototype at 20 per cent, i.e., a protection factor of 5. This protection factor is adopted in the relevant European standards for half-facepiece and filtering facepiece respirators which will be available for use in . coalmines on adoption of these standards. [That is, although a lower protection factor for the prototype was adopted than specified in the draft European Standard for PHRs, the protection factor was the same as that which will be specified for other respirators which will be available for use underground].1prEN146 (CEN, 1986)The prototype was also designed to meet the technical and general requirements of the UK coalmining industry. These included the need for the PHR to draw its electrical supply from the normal cap lamp battery, meet electrical intrinsic safety requirements and for the visor and headshell to meet the relevant performance requirements. An additional requirement was that the final PHR should be less expensive to provide, use and maintain than the AH6.A prototype PHR was developed which met all the above performance requirements and which simultaneously minimised those features of the AH6 which had adversely affected wearer acceptability. The prototype was substantially less bulky, lighter and with greater airflow than the AH6 while achieving.a total inward leakage under simulated mining conditions of 16 per cent as against the target maximum of 20 per cent, The design should be less expensive to provide and maintain than the AH6.Following the development of the prototype PHR, a number of devices were produced, and examined in detail in four ergonomic trials. These assessed the effects of the device upon wearers’head mobility, effects upon communication, the physiological cost of wearing the device, and wearer acceptance trials in a simulated coalface and in a coal preparation plant.The mobility trials revealed that the prototype PHH imposed no significantly greater restriction upon movement of the wearer’s head than did the normal pit safety helmet, whereas the AH6 imposeu significantly greater restriction to rearwards movement of the head. The communications trials indicated that the short visor of the prototype imposed about 10 dB less attenuation over the critical speech frequencies between 1 and 4 kHz than did the long visor of the AH6. Given that it is at these frequencies that both age and noise related effects upon hearing are most serious, it is clear that the IOM prototype was a marked improvement over the AH6. The physiological trials indicated that neither the normal pit safety helmet, the AH6 or the IOM prototype caused significant physiological cost as indicated by heart rate.