The risk of heat exhaustion at a deep underground metalliferous mine in relation to body-mass index and predicted Fb 2 max

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1 Occup. Med. Vol. 50, No. 4, pp , 2000 Copyright 2000 Lippincott Williams & Wilkins for SOM Printed in Great Britain. All rights reserved /00 INTRODUCTION The risk of heat exhaustion at a deep underground metalliferous mine in relation to body-mass index and predicted Fb 2 max A. M. Donoghue* and G. P. Bates 1 *The Minerals Industry Safety and Health Centre, University of Queensland, Brisbane, Australia; and \The School of Public Health, Curtin University, Perth, Australia The risk of heat exhaustion at a deep underground metalliferous mine was assessed in relation to the body-mass index (BMI) and predicted maximal oxygen uptake (Vo 2 max) of miners, using case-control methodology. Sixty-five cases of acute heat exhaustion and controls were studied. Heat exhaustion cases had a significantly higher BMI than controls (P=0.006). The odds ratios increased with BMI. For a BMI of , compared to a BMI of less than the odds ratio was 3.63 (95% confidence interval, ). Vo 2 max was not significantly lower in cases than controls. The odds ratios for heat exhaustion increased with decreasing Vo 2 max, but not significantly. The sample size provided 80% power of detecting an odds ratio of 2.5 or greater. Deep underground miners should be advised to maintain a BMI of Selection of miners on the basis of BMI should not be used as an alternative to satisfactory engineering controls such as ventilation and refrigeration. Key words: Aerobic fitness; heat exhaustion; heat strain; heat stress; maximal oxygen uptake; mining; obesity; thermal; underground. Occup. Med. Vol. 50, , 2000 Heat stress increases with mine depth because of the geothermal gradient and auto-compression. Ventilation in deep mines is often poor, particularly in development headings where frequent repair and extension of the ventilation line are required. Refrigeration is expensive and rarely generous enough to make thermal conditions comfortable. Many underground mines do not use refrigeration at all. Obesity and poor aerobic fitness may increase the risk of heat exhaustion or heat stroke during work in these conditions. In a recent case series from a deep underground metalliferous minej the median body-mass index (BMI) of miners suffering heat exhaustion was 29.6, which is at the upper end of the World Health Organization's grade 1 overweight range of Correspondence to: Dr A. Michael Donoghue, Senior Research Fellow, The Minerals Industry Safety and Health Centre, Frank White Annexe, University of Queensland, Brisbane QLD 4072, Australia. Tel: ; fax: a.m.donoghue@minmet.uq.edu.au Received December 1999; accepted in final form 8 March ' 2 Median maximal oxygen uptake (Fo 2 max) predicted by cycle ergometry was 41.1 ml/kg/min, just within the normal range for 'non-athletes' aged 30-39, of ml/kg/min. 1 ' 3 In environmental chamber studies, body mass, BMI, percentage body fat and reduced surface area:volume ratio have been consistently and significantly related to increased heat strain measured by core temperature and heart rate during paced work. 4 " 7 Military personnel who have suffered heat exhaustion or heat stroke during physical training have been found to have significantly higher body mass or BMI and significantly lower surface area: volume ratio than control subjects. 5 ' 8 ' 9 For aerobic fitness measured by Fo 2 max the relationship is less clear. In chamber studies of paced work under compensable thermal conditions (i.e. a thermal steady state is achievable), Fo 2 max has either had a weak negative (protective) effect on heat strain measured by core temperature or no significant effect at all. 5 ' 7 ' 10 Fo 2 max has however had a significant negative (protective) effect on heat strain measured by heart rate. 7 ' 10 For paced work during uncompensable thermal conditions (i.e. a thermal steady state is not achievable for anyone),

2 260 Occup. Med. Vol. 50, 2000 there is some disagreement. Fo 2 max has been reported to significantly prolong the tolerance time and increase the tolerable core temperature. 11 Another study, however, did not find that a higher Ko 2 max increased the core temperature at exhaustion. 12 Studies of aerobic fitness in military personnel who have suffered heat exhaustion or heat stroke during physical training are also in disagreement. One found significantly elevated odds ratios for increasing lengths of time taken to complete a 1.5-mile training run, 8 while another reported no significant difference in Ko 2 max between cases and controls. 5 The latter was, however, a much smaller study. The effect of obesity and aerobic fitness on the risk of heat exhaustion in underground miners has not been investigated to the best of our knowledge. In order to determine this we undertook the following case - control study. METHODS The study was undertaken at a deep underground metalliferous mining complex in tropical arid Australia. Incident cases and controls were studied during the 1- year period 1 October 1997 to 31 September All were employee miners in the two mines operating between 1200 and 1800 m depth. The cases formed part of a larger series published recently, which were derived from all five mines at this location. 1 The case definition required: that the subject be an employee of one of the two underground mines operating between 1200 and 1800 m depth; there to be a history of significant heat exposure and the presence of one or more symptoms of heat exhaustion commencing after starting work (headache, dizziness, fatigue, nausea, vomiting, transient loss of consciousness); the reasonable clinical exclusion of alternative diagnoses. Contractor cases were excluded so that a more valid comparison could be made with employee controls. The only exception to the symptom criteria was one case presenting during the study period with heat cramps alone. Seventy-three cases of heat exhaustion in employees from the two mines of interest were recorded during the study period by staff of the mine's medical centre. All work-related illness or injury is required to be reported to the mine's medical centre, which is open 24 hours/day, seven days/week. Usually, heat exhaustion cases are reported by the emergency telephone line and are retrieved from underground by four-wheel drive ambulance and taken to the medical centre. Sixty-seven (91.8%) were seen acutely by AMD, another medical officer, or in one case by an occupational health nurse and considered eligible for the study. Sixty-five (89.0%) returned after recovery from heat exhaustion for measurement of height, weight and Fb 2 max. Height and weight were measured and BMI calculated as weight/height 2 (kg/m 2 ). Sub-maximal exercise testing by cycle ergometry was undertaken according to the Astrand and Rodahl protocol, giving predicted Fo 2 max (ml/kg/min). For cases, these measurements were made following recovery from heat illness, on a rostered day off, or on a first day back before going underground. For controls the measurements were undertaken as part of routine periodic health assessment and surveillance before going underground for a shift. One hundred and nineteen controls were studied. All employee miners sent for periodic health assessment and health surveillance during the study period were selected as controls, provided that they had not suffered heat exhaustion during the study period. The cases and controls represented 49.5% of the employee workforce in these two mines. The study was approved by Curtin University's Human Research Ethics Committee. Statistical analysis The mean, 95% confidence interval for the mean, median, standard deviation and range were calculated for all parameters (Microsoft Excel). The significance of differences in age, height, weight, BMI and Fo 2 max between the cases and controls, were determined using the Mann-Whitney [/test (SPSS). Odds ratios and their 95% confidence intervals were calculated for the risk of heat exhaustion occurring according to stratified age, height, weight, BMI and Fo 2 max (Epi-Info). RESULTS Table 1 lists the case and control descriptive data for age, height, weight, BMI and Fo 2 max. P values are given for the significance of differences in these variables between cases and controls. Table 2 lists the numbers, percentages and odds ratios for cases and controls stratified by age, height, weight, BMI and Fo 2 max. There were no significant differences in age between cases and controls and the odds ratio for heat exhaustion did not increase with age. There were also no significant differences in height between cases and controls. Weight and BMI were significantly greater in cases than controls and the odds ratios for heat exhaustion increased significantly with these variables. The odds ratios for BMI were greater than those for weight alone. Fo 2 max was not significantly lower in cases than controls. The odds ratios for heat exhaustion increased with decreasing Fo 2 max, but not significantly. The sample size provided 80% power of detecting an odds ratio of 2.5 or greater. DISCUSSION By comparison with underground coal miners from Spain 13 ' 14 and Czechoslovakia, 15 the miners in our study

3 A. M. Donoghue and G. P. Bates: Heat exhaustion at an underground mine 261 Table 1. Age, height, weight, BMI and Vo 2 max for heat exhaustion cases and controls Variable n Mean 95% Cl (mean) Median SD Range P value Age Height Weight BMI VOz max P value determined by Mann-Whitney test Table 2. Numbers, percentages and odds ratios for heat exhaustion cases and controls, stratified by age, height, weight, BMI and Vo z max Variable Age (years) < Height (m) <1.72 Weight (kg) < BMI < Vo 2 max CO CO CM % OR, odds ratio. 95% Cl, the 95% confidence interval for the odds ratio. CO CO CM CM in CM % OR % Cl have a higher mean BMI (cases 29.7, controls 28.2, Spain , Czechoslovakia 27.4) and higher mean Fo 2 max (cases 42.4, controls 44.4, Spain , Czechoslovakia 35.4). An elevated BMI is clearly a significant risk factor for heat exhaustion in these miners and the risk increases with increasing BMI. This is entirely consistent with the findings of the experimental chamber studies and military studies of heat illness outlined in the introduction. 4 " 9 The physiological principles underlying this increase in risk may include: 5 ' 6 ' 8 ' 9 an increased heat production rate for a given physical work rate; a decreased surface area to volume ratio, reducing heat loss; an insulation effect of fat. With regard to risk control it is clearly desirable for miners to avoid obesity or lose weight if they are already obese. A recent large-scale prospective cohort study reported minimum all-cause mortality in relation to BMI occurs at a BMI of in men and in women. It is likely therefore that by maintaining a BMI below 27.0 miners would reduce both the risk of heat

4 262 Occup. Med. Vol. 50, 2000 exhaustion at work and the risk of all-cause mortality. Of course miners require a well-developed musculature in order to undertake heavy physical work and this should be borne in mind when advising miners about the need for weight loss. It is likely however that adequate musculature is afforded without needing to exceed a BMI of 27, given that 48.7% of controls were in this category and were presumably capable of undertaking their duties satisfactorily. This conclusion is supported by a study of lifting capacity in relation to BMI among 988 United States Army men 17. The maximum mean lifting capacity of just over 140 lbs (64 kg) occurred at a BMI of 28. There was a decline to approximately 130 lbs (59 kg) at a BMI of 24, 120 lbs (54 kg) at a BMI of and 110 lbs (50 kg) at a BMI of 19. Ko 2 max appears not to be a significant risk factor for heat exhaustion in these miners. The limited sample size precludes exclusion of a relationship, but if one does exist the odds ratio is most likely less than 2.5. The lack of a relationship is consistent with the experimental chamber studies of heat strain during compensable heat stress outlined in the introduction. 5 ' 7 ' 10 It seems likely that these employee miners self-pace effectively, reducing their work rate during periods of increased heat stress. Those who are less fit probably work at a lower rate than the others, thereby preserving the aerobic capacity required for thermoregulation. In 1970 Wyndham et al. found that the rectal temperature after 4 h of paced work (stepping) in humid heat was dependent on the percentage of Fo 2 max required by the individual for the stepping exercise. 18 This relationship was observed before and after acclimatization in recruits to the South African gold mines. They cautioned against men with low Ko 2 max working in hot conditions. Of course this was for paced work and the lack of a significant relationship between Ko 2 max and heat exhaustion in our study suggests that self-pacing was occurring in the miners we studied. Wyndham previously demonstrated that there was no relationship between Fo 2 max and productivity, and concluded individuals set their own pace depending on motivation. 19 This could be influenced strongly by different supervisors although the severity of supervision or the consequences in terms of heat illness were not discussed. 20 More recent studies have confirmed the ability of people to self-pace consistently in compensable heat. ' 22 However, when uncompensable conditions are approached and a goal is set, people tend to set their own pace too high and develop heat strain even in conditions where a slower rate would have achieved the goal. 23 This is an important warning for mining where miners are paid on the basis of their productivity. It is likely when uncompensable conditions are approached that miners will continue to push themselves too hard and develop heat illness. This is another possible reason why aerobic fitness appears not to be a significant determinant of heat exhaustion in these miners. Both the fit and unfit may be pushing themselves too hard in arduous conditions, albeit at different work rates, slower in the case of the unfit. Heat exhaustion in deep underground miners has been shown to be relatively common, and associated with unpleasant symptoms and significant haematological and biochemical disturbances. 1 Clearly it is important to minimize the risk of heat exhaustion in underground mines according to the hierarchy of hygiene controls. Engineering controls such as ventilation and refrigeration should be employed as much as practicable. Miners should not be selected on the basis of BMI as an alternative to satisfactory engineering controls. Rather, advice and support to maintain a healthy BMI should be offered by mining companies. In order to minimize the risk of heat exhaustion in deep underground miners the BMI should be less than 27. Miners should not be advised, however, to pursue a BMI of less than 24 as allcause mortality increases and maximum lifting capacity declines. The use of Fc^max to select miners with a lower risk of heat exhaustion is not supported by the findings of this study. Because of the limited sample size it remains possible that there is a relationship between Fo 2 max and heat exhaustion but the odds ratio would be most likely less than 2.5. REFERENCES 1. Donoghue AM, Sinclair MJ, Bates GP. Heat exhaustion in a deep underground metalliferous mine. Occup Environ Med 2000; 57: World Health Organization. Physical Status: the Use and Interpretation of Anthropometry. WHO Technical Report Series; Vol 854. WHO: Geneva; Wilmore JH, Costill D. Physiology of Sport and Exercise. 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