Permissible wearing times for rescue personnel using a new self-contained breathing apparatus. Final report on CEC Contract 7258 – 04/146/08

In the course of their work, British Coal’s Mines Rescue men wear self-contained breathing apparatus. They depend upon this to provide them with a safe, respirable gas supply in potentially irrespirable atmospheres. When they wear the apparatus in hot environments, the time for which they are permitted to work is laid down in standard tables. These tables, known as the ‘Lind Tables’ after the research physiologist responsible for their development, specify the maximum safe period in terms of the wet and dry bulb temperatures of the working climate. In recent years, British Coal’s Central Mines Rescue Service have developed a new compressed oxygen breathing apparatus, the SEFA, standing for Selected Elevated Flow Apparatus. It has been produced with the express purpose of improving the wearability/tolerability of self-contained breathing apparatus by careful attention to specific details such as breathing resistance and inspiratory temperature, both known to affect the physiological responses to work in the heat. It is intended that this new device will replace both forms of breathing apparatus currently in use within British Coal.The objective of this project was to carry out climate chamber studies of the physiological responses to wearing the SEFA apparatus in the heat, in order to determine new permissible working time charts. There has for some time been a feeling within the Rescue Service that the Lind charts, which catered for the most severe rescue conditions, were unduly restrictive in most situations. For example, as well as their rescue and other heavy duties,rescue men are frequently required to carry out inspection and environmental sampling visits where the workload may be comparatively light. As a consequence, it was initially decided that heavy and light workload charts would also be produced to be used at the discretion of Rescue Service supervisory staff.with a wet bulb temperature of 28�, to a high of 49� dry bulb, 41 � wet bulb. Airflow was minimal throughout, representing the ‘worst-case’ situation which is frequently encountered when rescue men enter blocked or closed off districts. These 12 climates, two work rates and two flow rates therefore yielded 48 different climate/work/flow combinations. Each of these was scheduled to be carried out by 12 men. The two workloads simulated Rescue Service activities underground. The light workload represented men carrying out a survey, walking round a district taking temperature and gas samples. The heavy work task simulated men walking into a district carrying resuscitators etc., building a stopping and returning to the fresh air base with an injured colleague. Throughout his time in the climate chamber, each subject was monitored for heart rate and core temperature using a computerised on-line physiological data monitoring and storage system developed by ourselves in a previous project. Heart rate was monitored in the conventional manner with three ecg electrodes fixed to the chest. The use of electrodes with microporous adhesive carriers has been found in previous studies to ensure good adhesion in hot, sweaty conditions and it proved possible to maintain adequate monitoring throughout. Core temperature was measured using the insulated auditory meatus technique.A thermistor probe projects through a plastic ear moulding into the ear canal. The insulation provided by this moulding is supplemented by insulating the outer ear to prevent inaccuracies arising due to hot air warming the probe directly. This technique has previously been shown by ourselves and others to provide a reliable measure of core temperature in hot conditions. With many subjects stabilising their temperature below 38 �C at dry bulb temperatures approaching 50 �C there can be little doubt that leakage of warm air into the ear canal was not a problem.A series of withdrawal criteria for early termination of sessions was agreed with British Coal Medical Service. The main one of these was reaching an ear canal temperature of 38.5 �C. This was considerably more conservative than the limits employed by Lind. Firstly, Lind utilised rectal temperature which has been shown to respond more slowly to changes in heat load. Secondly, he adopted a limiting value of 38.8�C and was only able to record temperatures every five minutes due to the limited technology then available. As we have found in our studies, particularly at the higher temperatures, core temperatures can rise very rapidly once the body’s regulatory mechanisms are exhausted and this may well account for the reports that a number of his subjects had to be helped from the chamber.An additional physiological criterion of a heart rate of 180 beats per minute was also employed. Finally, if either the experimenters or the medical staff present were unhappy with any aspect of the subject’s behaviour or appearance, or if the subject himself felt unable to continue, the session was terminated. As a result of these safeguards, no subject was taken ill during any session.A total of 27 full-time and 66 part-time rescue brigadesmen participated in the studies. They were aged between 25 and 45 (mean 34 yrs), and had a mean fitness score of 86.6 � 6.89 (range 74.5 – 100.5). The 93 subjects completed a total of 428 wearings. For analysis, the data for all wearings were summarised either to the time at which core temperature, heart rate or personal discomfort led to withdrawal of the subject from the chamber (withdrawal time), or to the limiting time (60 or 120 minutes, depending on selected flowrate) at which the wearing was terminated if withdrawal-had not occurred.Plots of the ‘limiting times1 against climatic temperature were analysed statistically to produce curves which satisfactorily fitted the data. Descriptions of the distribution of values around these curves were then used to plot 95 percent confidence intervals. The lower (97.5th percentile) curve of the confidence interval was then used to derive ‘working times’ for a series of wet and dry bulb temperatures in the style of the original Proto charts for both heavy and light work options.At this point, it was decided by Rescue Service management not to pursue the light workload option and no further work was carried out with these data.A comparison of the 97.5th percentile working times with the maximum safe periods permitted for the Proto apparatus indicated that the times permitted for the Proto were a few minutes longer than the 97.5th percentile times at higher temperatures. This could be explained by the lower limiting body temperature criterion adopted, combined with the use of aural rather than rectal temperature.As a result of this analysis, it was decided by British Coal Mines Rescue and Medical Services, in consultation with the Mines and Quarries Inspectorate and the Institute, to adopt working times which combined the two sets of data. Support for this decision was provided by the lack of any evidence of heat related problems with the operational use of the Proto apparatus since the introduction of the charts more than 30 years ago.A chart of permissible wearing times for the SEFA apparatus was produced in accordance with this decision. “”