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How do wildland firefighters cope? Physiological and behavioural temperature regulation in men suppressing Australian summer bushfires with hand tools
Journal of Thermal Biology 26 (2001) 381–386
How do wildland firefighters cope? Physiological and behavioural temperature regulation in men suppressing Australian summer bushfires with hand tools G.M. Budd* School of Exercise and Sport Science, Faculty of Health Sciences, The University of Sydney, P.O. Box 170, Lidcombe, NSW 1825, Australia
Abstract Heart rate (HR) and rectal temperature (Tre) of wildland firefighters averaged 152 beats min 1 and 38.21C while they suppressed Australian summer bushfires with hand tools. Contrary to the findings of laboratory studies, HR and Tre were not changed by variations of 36–217 min in work duration; 406–630 W in energy expenditure; 15–341C in wet-bulb globe temperature (WBGT), 7–27% in body fat content; or 31–63 ml min 1 kg 1 body mass in maximum oxygen uptake (VO2max), except for an attenuated effect of VO2max on HR. The stability of HR and Tre is explained by firefighters’ behavioural regulation, guided by negative feedback, of their work rate and heat exposure, and by the unrestricted evaporation of sweat. These findings highlight the limitations of laboratory studies for predicting physiological responses in the workplace. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Workplace standards and guidelines; Control systems; Self pacing; Radiant-heat flux; Heat exchanges; Protective clothing
1. Introduction How do wildland firefighters cope? They undertake prolonged strenuous work, usually in hot summer weather, on wildland fires (‘bushfires’ in Australia) that liberate great quantities of heat. How they manage such a demanding and hazardous job is a challenging question of thermal biology and occupational safety. Agencies responsible for firefighters’ safety and productivity have to make decisions about such matters as their work loads, protective clothing, and fitness requirements. In so doing they are commonly guided by data and predictions derived from laboratory studies. Such studies have shown, for example, that physiological and subjective strainFas reflected in heart rate (HR), rectal temperature (Tre), and the rating of perceived exertion (RPE)Fduring exercise in the laboratory are increased by a higher rate of energy *Tel.: +61-2-9351-9300; fax: +61-2-9351-9204. E-mail address: [email protected] (G.M. Budd).
expenditure (EE), by prolonged work, and by levels of the wet bulb globe temperature index (WBGT) above certain threshold limit values (TLVs), as well as by dehydration, fatness, and low levels of aerobic fitness. But how applicable are these predictions to actual firefighting? Evidence is provided by measurements made on firefighters taking part in an investigation to determine the most intense bushfire that could be suppressed by hand-tool crews (Budd et al., 1996, 1997a). The investigation was part of ‘Project Aquarius’, a wideranging study of bushfire behaviour and control techniques, and was carried out over three successive summers in the dry eucalypt forests of Western Australia and Victoria. On the basis of this evidence, the present paper aims to summarise the interactions between the firefighters’ physiological and behavioural responses, to explain any differences from the predicted responses, and to highlight some practical implications for firefighters and other workers in hot and/or strenuous occupations.
0306-4565/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 6 - 4 5 6 5 ( 0 1 ) 0 0 0 4 8 - 1
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G.M. Budd / Journal of Thermal Biology 26 (2001) 381–386
2. Methods 2.1. Subjects The subjects were a representative group of Australian bushfire fighters. They were men of age (mean and range) 26 (18–45) yr, body mass 71 (52–105) kg, body fat content 14 (7–27)%, and maximum oxygen uptake (VO2max) 47 (31–63) ml O2 min 1 kg 1 body mass. They wore the light cotton or wool coveralls, or trousers and shirt, that are traditionally favoured by Australian bushfire fighters, together with boots and hard hat. 2.2. Experimental fires Four seven-man crews were studied while they attempted to suppress, using their normal firefighting techniques, experimental bushfires that had been lit some 10–50 min earlier on a crosswind line of 50–200 m (Budd et al., 1997b, c). Their fire-control objective was to minimise the area burnt. The hottest fires developed head-fire intensitiesFup to 3280 kW per metre of fire frontFthat were well beyond the limits of control by hand tools, thus exposing the firefighters to a range of intensities they would commonly experience when suppressing summer bushfires and evoking much the same uncertainty and apprehension. No burns or heat disorders were observed. 2.3. Fire suppression technique A crew attempted to stop a fire from spreading by constructing along its flanks, and if possible around the head fire, a firebreakFknown as a ‘fireline’Fabout 1 m wide from which all vegetation had been removed by means of rakehoes (a combination rake and hoe weighing 2.4 kg), brush-hook, and chainsaw. Behind the crew leader and the men with brush-hook and chainsaw the 4 or 5 ‘rakers’ were spaced along the proposed fireline about 10–15 m apart. The rakers advanced by means of the ‘step up’ technique (Luke and McArthur, 1978), in which an individual reaching the end of the work assigned to him calls out ‘step up!’, whereupon every raker steps forward, maintaining his position in the line, until he finds some fresh work to do. This method allows individual firefighters to work at their own preferred pace and thus partially compensates for differences in work capacity.
FRs and NFRs together are collectively known as ‘line rakes’. The line rakes constituted a 4 2 factorial design in which the 4 crews were crossed with the 2 levels of the ‘FNF’ factor (FRs vs. NFRs), and the results were analysed by factorial analyses of variance and covariance. Unless otherwise stated, results cited in this paper refer to FRs and NFRs combined. Over the 26 line rakes the crews worked for 117 (36–217) min at an EE of 516 (406–630) W, corresponding to 45% VO2max (Budd et al., 1997d, h). 2.5. Work environment in fire rakes During fire rakes the firefighters experienced air temperature 29 (19–35)1C, mean radiant temperature 66 (33–96)1C, water-vapour pressure 13 (9–17) Torr, wind speed 1.2 (0.7–1.7) m s 1, and WBGT 26 (17– 34) 1C. 2.6. Measurements Throughout the line rakes observers measured the stresses the firefighters experienced, their work behaviour and fireline productivity, and their physiological and subjective responses. All measurements, described in detail elsewhere (Budd et al., 1996, 1997a), were made by reliable standard techniques using recently-calibrated instruments. In all, 23 km of measured fireline were constructed and 238 man-days of comprehensive measurements were obtainedF179 on the firefighters and 59 on six male scientists, who shared the firefighters’ environment but naturally performed less strenuous work. The results were highly consistent between the four crews, three summers, and two States, and are thus generally applicable to bushfire suppression with hand tools in southern Australia.
3. Physiological temperature regulation During fireline construction firefighters’ physiological responses averaged (mean7s.d.) HR 152714 beats min 1, Tre 38.270.21C, thigh skin temperature 34.571.71C, and sweat rate 11447373 g h 1). They considered the work ‘somewhat hard’ (RPE 13.671.7) and they felt ‘just too warm’. Analysis showed that these responses were mainly due to exertion rather than fire (Budd et al., 1997f).
2.4. Experimental design and work load 3.1. Effects of job demands and personal factors The crews built fireline in the above manner to suppress 15 experimental bushfires (‘Fire rakes’, FRs). On 11 other occasions they built fireline in the same way in the absence of fire (‘Non-fire rakes’, NFRs). Each occasion of building fireline is known as a ‘rake’, and
It was expected, on the basis of laboratory findings and the occupational standards and guidelines derived from them, that firefighters’ HR and TreFthe responses most important to their health and safetyFwould be
G.M. Budd / Journal of Thermal Biology 26 (2001) 381–386
increased in the longer, the more strenuous, or the hotter line rakes, and also in the fatter or less fit firefighters. The firefighters’ responses in formal work tests confirmed these expectations for EE, VO2max and fatness (Brotherhood et al., 1997a, b; Budd et al., 1997h). However, in the line rakes the expectations were not fulfilled (Budd et al., 1997h; Brotherhood et al., 1997b). Simple and multiple regression analyses showed that HR and Tre were not changed by the sixfold variation (36– 217 min) in work duration, nor by the variations of 406– 630 W in EE, 17–351C in air temperature, or 15–341C in WBGT (Fig. 1). There was no evidence of (1) the progressive increase in HR (‘cardiovascular drift’) or decline in sweating (‘sweat suppression’) that commonly accompany prolonged strenuous exercise in the laboratory (Montain and Coyle, 1992; Kerslake, 1972); (2) the greater elevations of HR and Tre expected in more strenuous work; or (3) the increase in Tre predicted by
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current heat-stress guidelines (ACGIH, 1994), despite WBGT being as much as 91C above recommended limits (Fig. 1). Neither were HR or Tre affected by the wide variations in the firefighters’ body fat content (7–27%) or VO2max (31–63 ml O2 min 1 kg 1 body mass), except for an attenuated effect of VO2max on HR. Effects of fire were negligible except for a 356 g h 1 increase in sweat rate; consequently the results are also applicable to other hot and/or strenuous occupations. These surprising findings are explained, as described below, by the firefighters’ appropriate clothing, self pacing, and other behavioural responses.
4. Behavioural temperature regulation The crews’ work practices, evolved by Australian bushfire fighters over decades of practical experience on
Fig. 1. Regressions of rectal temperature on the wet-bulb globe temperature (WBGT) index of heat stress. Each point is the average of all the subjects observed in one rake. The firefighters’ work environment was influenced by fire whereas the fireline weather was not. Vertical reference line shows the threshold limit value (WBGT 25.01C) appropriate to the firefighters’ energy expenditure. None of the regressions were statistically significant (all P > 0:10). (Reproduced, with permission, from Budd et al., 1997h).
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the fireline, unconsciously embodied four key principles for the alleviation of heat stress. They were (1) avoid unnecessary heat, (2) encourage self pacing, (3) help sweat to evaporate, and (4) replace sweat losses by drinking. 4.1. Avoid unnecessary heat Although head-fire intensities were as high as 3280 kW per metre of fire front, with flames extending into the sub-canopy of the forest as much as 30 m above the ground (Budd et al., 1997c), safe work practices and individual behavioural responses together reduced the firefighters’ radiant-heat exposure to an intensity little greater than that of sunlight, which could readily be blocked by light clothing (Budd et al., 1997e, g, h). Safe work practices usually required that the crew work along a flank of the fire in its direction of travel, building fireline roughly parallel to the fire edge and systematically overtaking the head fire with the intention of getting around it when circumstances permitted. These work practices allowed the crew to retire to the safety of already-burnt ground if the fire were to become uncontrollable where they were working, and they also reduced the crew’s heat exposure because fire intensity was less severe on the flanks than at the head fire. The crew leader determined the proximity of the fireline to the fire edge, keeping fairly close to it but dropping back to avoid intense heat or smoke and to bypass obstructions. As a result the crews most commonly worked 3 m (range 1–13 m) from the flames, where radiant heat was tolerable and the fire-induced increase in air temperature averaged less than 31C. Individual behavioural responses provided the fine control of radiant-heat exposure. The firefighters did not wear gloves or face masks, and many of them rolled up their sleeves so as to bare their forearms. The bare skin thus exposed automatically restricted their radiant-heat exposure to comfortable levels (o2 kW m 2), since they had to work far enough from the flames to avoid pain and they involuntarily stepped back from any increase in fire intensity (Budd et al., 1997e). The pain-induced withdrawal reflex is a fundamental safety mechanism, but it is largely disabled if firefighters cover all exposed skin, as is sometimes done overseas (Budd et al., 1997i). Firefighters thus handicapped are at risk of not noticing a dangerous increase in their heat exposure until it is too late to safely withdraw. 4.2. Encourage self pacing Individual firefighters worked at their own preferred pace, which differed twofold among them and was a stable characteristic of their work behaviour (Brotherhood et al., 1997a). Formal work tests showed that small changes in pace, apparently prompted by RPE and the
ventilatory threshold (the upper limit of comfortable breathing), provided a sensitive means for firefighters to adjust their EE and the accompanying physiological and subjective responses (Brotherhood et al., 1997a). Other means of self pacing included changes in the frequency and duration of informal rests and light activities (Budd et al., 1997g). The combined effect was to counterbalance any job demands or personal characteristics that might have disturbed the firefighters’ preferred levels of strain (Budd et al., 1997h, Brotherhood et al., 1997b). To summarise the above two principles, the firefighters’ avoidance of radiant heat controlled the main component of their environmental heat load, while self pacing controlled not only their degree of exertion but also the metabolic heat that constituted (see below) almost three quarters of their total heat load. 4.3. Help sweat to evaporate Calculations of heat exchange showed that to maintain thermal equilibrium the firefighters had to evaporate sweat at an average rate of about 1 litre per hour, and hence required clothing that was sufficiently light, loose, and well ventilated to permit such high rates of evaporation (Budd et al., 1997e). The clothing they wore met these requirements admirably, and resulted in 95% of their secreted sweat being evaporated (Hendrie et al., 1997). By contrast, tests in a climatic chamber (Budd et al., 1997i) confirmed that clothing which did not meet these requirements hindered evaporation, trapped metabolic heat, and caused greater cardiovascular strain, discomfort, and fatigueFpenalties that would outweigh any benefit such clothing could confer during bushfire suppression. The dangers of excessive clothing have been emphasised by injuries and deaths from heat stroke in overdressed bushfire fighters (Reischl, 1980). The calculated heat exchanges (Budd et al., 1997e) showed, contrary to popular expectation, that most of the firefighters’ heat load came not from the fire but from their physical exertion: in FRs the average metabolic heat load (488 W) was more than twice the combined heat load (200 W) from the fire and the weather. The implication for bushfire fighters’ clothing is clearFits main task is not to keep heat out but to let it out. Firefighters’ measured sweat rates agreed with those predicted from measurements of their EE and thermal environment, implying that evaporation was sufficient to completely dissipate their heat loads (Budd et al., 1997f; Hendrie et al., 1997). This implication was confirmed by the constancy of Tre, indicating a state of thermal equilibrium, over the sixfold range of rake duration; and also by the finding that fire increased sweat rate by 356 g h 1 but did not change Tre (Budd et al., 1997h). The sweat rates also confirmed the previous finding that
G.M. Budd / Journal of Thermal Biology 26 (2001) 381–386
the heat load from exertion was more than twice that from fire. 4.4. Replace sweat losses by drinking In this respect the firefighters were less successful: like the subjects of many previous workplace studies, they exhibited the phenomenon of ‘voluntary dehydration’ (Rothstein et. al., 1947; Hendrie et al., 1997). They replaced only 43% of their sweat losses during fireline construction, and consequently became dehydrated by an average of 1.7% (maximum 4.0%) of body mass. Although this degree of dehydration had no discernible effect on HR or Tre (Budd et al., 1997h), RPE was 0.7 unit higher for each 1% body mass of dehydration, in keeping with previous reports of increased RPE and fatigue in dehydrating men (Montain and Coyle, 1992; Brown, 1947). It is clear that bushfire fighting can cause rapid dehydration, and that there is a need for work practices which guarantee an adequate water intake before, during, and after firefighting.
5. Comment The results summarised above illustrate the complex interactions among work practices, work behaviour, and clothing which together reduced the firefighters’ heat load, and which also permitted the free evaporation of sweat that was the essential condition for prolonged work at tolerable levels of physiological and subjective strain. The discrepancies between expectation and observation emphasise the limited applicability of laboratory findings to the workplace. In laboratory studies investigators usually test only one factor at a time (e.g. work load, ambient temperature, aerobic fitness), and they rarely allow the subjects to pace themselves according to their perceived strainFwhereas in most workplaces numerous factors that may increase or reduce strain are simultaneously present in varying degree, can interact with each other (Nunneley, 1989), and can provoke behavioural responses to counteract their effects (Gun and Budd, 1995). In terms of control theory (Hardy and Hammel, 1963), laboratory studies exemplify an ‘open loop’ system in which a stimulus is independent of the response to it, whereas the workplace exemplifies a ‘closed loop’ system in which the stimulus is influenced, via negative feedback, by the response and is adjusted so as to maintain the response at a particular level. Thus firefighters’ EE declined slightly with fire and with warmer weather, although not with rake duration. Self pacing has also been invoked to explain the paradoxical findings of two seasonal studies of industrial workers in hot climates (Parikh et al., 1976; Gertner et al., 1984).
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Because of self pacing and other behavioural responses, stresses encountered in the workplace seem more likely to result in reduced productivity than in excessive strain. This was apparent in the finding that firefighters’ productivity and efficiency (productivity per unit EE) declined slightly with rake duration and with fire, although not with warmer weather. Excessive strain would seem most likely to occur in those occupations (e.g. military training, competitive sport) where self pacing is too limited to be effective.
6. Conclusions The results presented in this paper demonstrate the effectiveness of the firefighters’ behavioural responses. They allowed each man to regulate his work rate and his exposure to radiant heat, and thus to maintain his physiological and subjective responses at safe and sustainable levels over a wide range of job demands and personal factors. The results also confirm the excellence of the firefighters’ light cotton or wool clothing, which shielded them from radiant heat yet did not impede the free evaporation of sweat at the high rates required. Finally, they highlight the limitations of laboratory studies for predicting physiological responses in the workplace.
Acknowledgements I am grateful to Josephine Bastian and Phil Cheney for helpful criticism.
References ACGIH, 1994. Threshold limit values for chemical substances and physical agents, and biological exposure indices, for 1994–1995. Heat stress. American Conference of Governmental Industrial Hygienists, Cincinnati, pp. 84–90. Brotherhood, J.R., Budd, G.M., Hendrie, A.L., Jeffery, S.E., Beasley, F.A., Costin, B.P., Wu Zhien, Baker, M.M., Cheney, N.P., Dawson, M.P., 1997a. Project Aquarius. 3. Effects of work rate on the productivity, energy expenditure, and physiological responses of men building fireline with a rakehoe in dry eucalypt forest. Int. J. Wildland Fire 7(2), 87–98. Brotherhood, J.R., Budd, G. M., Hendrie, A.L., Jeffery, S.E., Beasley, F.A., Costin, B.P., Wu Zhien, Baker, M.M., Cheney, N.P., Dawson, M.P., 1997b. Project Aquarius. 11. Effects of fitness, fatness, body size, and age on the energy expenditure, strain, and productivity of men suppressing wildland fires. Int. J. Wildland Fire 7(2), 181–199.
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Brown, A.H., 1947. Water shortage in the desert. In: Adolph, E.F., et al. (Ed.), Physiology of Man in the Desert. Interscience, New York, pp. 136–159. Budd, G.M., Brotherhood, J.R., Hendrie, A.L., Cheney, N.P., Dawson, M.P., 1996. Safe and Productive Bushfire Fighting with Hand Tools. Australian Government Publishing Service, GPO Box 84, Canberra ACT 2601, 46 pp. Budd, G.M., Brotherhood, J.R., Hendrie, A.L., Cheney, N.P., Dawson, M.P., 1997a. Special Issue: Project Aquarius. Stress, strain, and productivity in wildland firefighters. Int. J. Wildland Fire 7 (2), 69–218. Budd, G.M., Brotherhood, J.R., Hendrie, A.L., Jeffery, S.E., Beasley, F.A., Costin, B.P., Wu Zhien, Baker, M.M., Cheney, N.P., Dawson, M.P., 1997b. Project Aquarius. 1. Stress, strain, and productivity in men suppressing Australian summer bushfires with hand tools: background, objectives, and methods. Int. J. Wildland Fire 7(2), 69–76. Budd, G.M., Brotherhood, J.R., Hendrie, A.L., Jeffery, S.E., Beasley, F.A., Costin, B.P., Wu Zhien, Baker, M.M., Cheney, N.P., Dawson, M.P., 1997c. Project Aquarius. 4. Experimental bushfires, suppression procedures, and measurements. Int. J. Wildland Fire 7(2), 99–104. Budd, G.M., Brotherhood, J.R., Hendrie, A.L., Jeffery, S.E., Beasley, F.A., Costin, B.P., Wu Zhien, Baker, M.M., Cheney, N.P., Dawson, M.P., 1997d. Project Aquarius. 5. Activity distribution, energy expenditure, and productivity of men suppressing free-running wildland fires with hand tools. Int. J. Wildland Fire 7(2), 105–118. Budd, G.M., Brotherhood, J.R., Hendrie, A.L., Jeffery, S.E., Beasley, F.A., Costin, B.P., Wu Zhien, Baker, M.M., Cheney, N.P., Dawson, M.P., 1997e. Project Aquarius. 6. Heat load from exertion, weather, and fire in men suppressing wildland fires. Int. J. Wildland Fire 7(2), 119–131. Budd, G.M., Brotherhood, J.R., Hendrie, A.L., Jeffery, S.E., Beasley, F.A., Costin, B.P., Wu Zhien, Baker, M.M., Cheney, N.P., Dawson, M.P., 1997f. Project Aquarius. 7. Physiological and subjective responses of men suppressing wildland fires. Int. J. Wildland Fire 7(2), 133–144. Budd, G.M., Brotherhood, J.R., Hendrie, A.L., Jeffery, S.E., Beasley, F.A., Costin, B.P., Wu Zhien, Baker, M.M., Cheney, N.P., Dawson, M.P., 1997g. Project Aquarius. 9. Relative influence of job demands and personal factors on the energy expenditure, strain, and productivity of men suppressing wildland fires. Int. J. Wildland Fire 7(2), 159–166. Budd, G.M., Brotherhood, J.R., Hendrie, A.L., Jeffery, S.E., Beasley, F.A., Costin, B.P., Wu Zhien, Baker, M.M., Cheney, N.P., Dawson, M.P., 1997h. Project Aquarius.
10. Effects of work, weather, and fire on the energy expenditure, strain, and productivity of men suppressing wildland fires. Int. J. Wildland Fire 7(2), 167–180. Budd, G.M., Brotherhood, J.R., Hendrie, A.L., Jeffery, S.E., Beasley, F.A., Costin, B.P., Wu Zhein, Baker, M.M., Hoschke, B. N., Holcombe, B.V., Cheney, N.P., Dawson, M.P., 1997i. Project Aquarius. 13. The thermal burden of high insulation and encapsulation in wildland firefighters’ clothing. Int. J. Wildland Fire 7(2), 207–218. Gertner, A., Israeli, R., Cassuto, Y., 1984. Effects of work and motivation on the heart rates of chronic heat-exposed workers during their regular work shifts. Ergonomics 27 (2), 135–146. Gun, R.T., Budd, G.M., 1995. Effects of thermal, personal and behavioural factors on the physiological strain, thermal comfort and productivity of Australian shearers in hot weather. Ergonomics 38 (5), 1368–1384. Hardy, J.D., Hammel, H.T., 1963. Control system in physiological temperature regulation. In: Hardy, J.D. (Ed.), TemperatureFits Measurement and Control in Science and Industry. Part 3. Biology and Medicine. Reinhold, New York, pp. 613–625. Hendrie, A.L., Brotherhood, J.R., Budd, G.M., Jeffery, S.E., Beasley, F.A., Costin, B.P., Wu Zhien, Baker, M.M., Cheney, N.P., Dawson, M.P., 1997. Project Aquarius. 8. Sweating, drinking, and dehydration in men suppressing wildland fires. Int. J. Wildland Fire 7(2), 145–158. Kerslake, D. McK., 1972. The Stress of Hot Environments. University Press, Cambridge, 316 pp. Luke, R.H., McArthur, A.G., 1978. Bushfires in Australia. Australian Government Publishing Service, Canberra, 359 pp. Montain, S.J., Coyle, E.F., 1992. Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. J. Appl. Physiol. 73, 1340–1350. Nunneley, S.A., 1989. Heat stress in protective clothingFinteractions among physical and physiological factors. Scand. J. Work, Environ. Health 15 (Suppl 1), 52–57. Parikh, D.J, Pandaya, C.B., Ramanathan, N.L., 1976. Applicability of the WBGT index of heat stress to work situations in India. Indian J. Med. Res. 64, 327–335. Reischl, U., 1980. Personal heat stress monitoring. In: DukesDobos, F.N., Henschel, A. (Eds.), Proceedings of a NIOSH Workshop on Recommended Heat Stress Standards. DHHS (NIOSH) Publication No. 81–108. National Institute for Occupational Safety and Health, Cincinnati, pp. 110–128. Rothstein, A., Adolph, E.F., Wills, J.H., 1947. Voluntary dehydration. In: Adolph, E.F., et al. (Ed.), Physiology of Man in the Desert. Interscience, New York, pp. 254–270.
How do wildland firefighters cope? Physiological and behavioural temperature regulation in men suppressing Australian summer bushfires with hand tools G.M. Budd* School of Exercise and Sport Science, Faculty of Health Sciences, The University of Sydney, P.O. Box 170, Lidcombe, NSW 1825, Australia
Abstract Heart rate (HR) and rectal temperature (Tre) of wildland firefighters averaged 152 beats min 1 and 38.21C while they suppressed Australian summer bushfires with hand tools. Contrary to the findings of laboratory studies, HR and Tre were not changed by variations of 36–217 min in work duration; 406–630 W in energy expenditure; 15–341C in wet-bulb globe temperature (WBGT), 7–27% in body fat content; or 31–63 ml min 1 kg 1 body mass in maximum oxygen uptake (VO2max), except for an attenuated effect of VO2max on HR. The stability of HR and Tre is explained by firefighters’ behavioural regulation, guided by negative feedback, of their work rate and heat exposure, and by the unrestricted evaporation of sweat. These findings highlight the limitations of laboratory studies for predicting physiological responses in the workplace. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Workplace standards and guidelines; Control systems; Self pacing; Radiant-heat flux; Heat exchanges; Protective clothing
1. Introduction How do wildland firefighters cope? They undertake prolonged strenuous work, usually in hot summer weather, on wildland fires (‘bushfires’ in Australia) that liberate great quantities of heat. How they manage such a demanding and hazardous job is a challenging question of thermal biology and occupational safety. Agencies responsible for firefighters’ safety and productivity have to make decisions about such matters as their work loads, protective clothing, and fitness requirements. In so doing they are commonly guided by data and predictions derived from laboratory studies. Such studies have shown, for example, that physiological and subjective strainFas reflected in heart rate (HR), rectal temperature (Tre), and the rating of perceived exertion (RPE)Fduring exercise in the laboratory are increased by a higher rate of energy *Tel.: +61-2-9351-9300; fax: +61-2-9351-9204. E-mail address: [email protected] (G.M. Budd).
expenditure (EE), by prolonged work, and by levels of the wet bulb globe temperature index (WBGT) above certain threshold limit values (TLVs), as well as by dehydration, fatness, and low levels of aerobic fitness. But how applicable are these predictions to actual firefighting? Evidence is provided by measurements made on firefighters taking part in an investigation to determine the most intense bushfire that could be suppressed by hand-tool crews (Budd et al., 1996, 1997a). The investigation was part of ‘Project Aquarius’, a wideranging study of bushfire behaviour and control techniques, and was carried out over three successive summers in the dry eucalypt forests of Western Australia and Victoria. On the basis of this evidence, the present paper aims to summarise the interactions between the firefighters’ physiological and behavioural responses, to explain any differences from the predicted responses, and to highlight some practical implications for firefighters and other workers in hot and/or strenuous occupations.
0306-4565/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 6 - 4 5 6 5 ( 0 1 ) 0 0 0 4 8 - 1
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G.M. Budd / Journal of Thermal Biology 26 (2001) 381–386
2. Methods 2.1. Subjects The subjects were a representative group of Australian bushfire fighters. They were men of age (mean and range) 26 (18–45) yr, body mass 71 (52–105) kg, body fat content 14 (7–27)%, and maximum oxygen uptake (VO2max) 47 (31–63) ml O2 min 1 kg 1 body mass. They wore the light cotton or wool coveralls, or trousers and shirt, that are traditionally favoured by Australian bushfire fighters, together with boots and hard hat. 2.2. Experimental fires Four seven-man crews were studied while they attempted to suppress, using their normal firefighting techniques, experimental bushfires that had been lit some 10–50 min earlier on a crosswind line of 50–200 m (Budd et al., 1997b, c). Their fire-control objective was to minimise the area burnt. The hottest fires developed head-fire intensitiesFup to 3280 kW per metre of fire frontFthat were well beyond the limits of control by hand tools, thus exposing the firefighters to a range of intensities they would commonly experience when suppressing summer bushfires and evoking much the same uncertainty and apprehension. No burns or heat disorders were observed. 2.3. Fire suppression technique A crew attempted to stop a fire from spreading by constructing along its flanks, and if possible around the head fire, a firebreakFknown as a ‘fireline’Fabout 1 m wide from which all vegetation had been removed by means of rakehoes (a combination rake and hoe weighing 2.4 kg), brush-hook, and chainsaw. Behind the crew leader and the men with brush-hook and chainsaw the 4 or 5 ‘rakers’ were spaced along the proposed fireline about 10–15 m apart. The rakers advanced by means of the ‘step up’ technique (Luke and McArthur, 1978), in which an individual reaching the end of the work assigned to him calls out ‘step up!’, whereupon every raker steps forward, maintaining his position in the line, until he finds some fresh work to do. This method allows individual firefighters to work at their own preferred pace and thus partially compensates for differences in work capacity.
FRs and NFRs together are collectively known as ‘line rakes’. The line rakes constituted a 4 2 factorial design in which the 4 crews were crossed with the 2 levels of the ‘FNF’ factor (FRs vs. NFRs), and the results were analysed by factorial analyses of variance and covariance. Unless otherwise stated, results cited in this paper refer to FRs and NFRs combined. Over the 26 line rakes the crews worked for 117 (36–217) min at an EE of 516 (406–630) W, corresponding to 45% VO2max (Budd et al., 1997d, h). 2.5. Work environment in fire rakes During fire rakes the firefighters experienced air temperature 29 (19–35)1C, mean radiant temperature 66 (33–96)1C, water-vapour pressure 13 (9–17) Torr, wind speed 1.2 (0.7–1.7) m s 1, and WBGT 26 (17– 34) 1C. 2.6. Measurements Throughout the line rakes observers measured the stresses the firefighters experienced, their work behaviour and fireline productivity, and their physiological and subjective responses. All measurements, described in detail elsewhere (Budd et al., 1996, 1997a), were made by reliable standard techniques using recently-calibrated instruments. In all, 23 km of measured fireline were constructed and 238 man-days of comprehensive measurements were obtainedF179 on the firefighters and 59 on six male scientists, who shared the firefighters’ environment but naturally performed less strenuous work. The results were highly consistent between the four crews, three summers, and two States, and are thus generally applicable to bushfire suppression with hand tools in southern Australia.
3. Physiological temperature regulation During fireline construction firefighters’ physiological responses averaged (mean7s.d.) HR 152714 beats min 1, Tre 38.270.21C, thigh skin temperature 34.571.71C, and sweat rate 11447373 g h 1). They considered the work ‘somewhat hard’ (RPE 13.671.7) and they felt ‘just too warm’. Analysis showed that these responses were mainly due to exertion rather than fire (Budd et al., 1997f).
2.4. Experimental design and work load 3.1. Effects of job demands and personal factors The crews built fireline in the above manner to suppress 15 experimental bushfires (‘Fire rakes’, FRs). On 11 other occasions they built fireline in the same way in the absence of fire (‘Non-fire rakes’, NFRs). Each occasion of building fireline is known as a ‘rake’, and
It was expected, on the basis of laboratory findings and the occupational standards and guidelines derived from them, that firefighters’ HR and TreFthe responses most important to their health and safetyFwould be
G.M. Budd / Journal of Thermal Biology 26 (2001) 381–386
increased in the longer, the more strenuous, or the hotter line rakes, and also in the fatter or less fit firefighters. The firefighters’ responses in formal work tests confirmed these expectations for EE, VO2max and fatness (Brotherhood et al., 1997a, b; Budd et al., 1997h). However, in the line rakes the expectations were not fulfilled (Budd et al., 1997h; Brotherhood et al., 1997b). Simple and multiple regression analyses showed that HR and Tre were not changed by the sixfold variation (36– 217 min) in work duration, nor by the variations of 406– 630 W in EE, 17–351C in air temperature, or 15–341C in WBGT (Fig. 1). There was no evidence of (1) the progressive increase in HR (‘cardiovascular drift’) or decline in sweating (‘sweat suppression’) that commonly accompany prolonged strenuous exercise in the laboratory (Montain and Coyle, 1992; Kerslake, 1972); (2) the greater elevations of HR and Tre expected in more strenuous work; or (3) the increase in Tre predicted by
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current heat-stress guidelines (ACGIH, 1994), despite WBGT being as much as 91C above recommended limits (Fig. 1). Neither were HR or Tre affected by the wide variations in the firefighters’ body fat content (7–27%) or VO2max (31–63 ml O2 min 1 kg 1 body mass), except for an attenuated effect of VO2max on HR. Effects of fire were negligible except for a 356 g h 1 increase in sweat rate; consequently the results are also applicable to other hot and/or strenuous occupations. These surprising findings are explained, as described below, by the firefighters’ appropriate clothing, self pacing, and other behavioural responses.
4. Behavioural temperature regulation The crews’ work practices, evolved by Australian bushfire fighters over decades of practical experience on
Fig. 1. Regressions of rectal temperature on the wet-bulb globe temperature (WBGT) index of heat stress. Each point is the average of all the subjects observed in one rake. The firefighters’ work environment was influenced by fire whereas the fireline weather was not. Vertical reference line shows the threshold limit value (WBGT 25.01C) appropriate to the firefighters’ energy expenditure. None of the regressions were statistically significant (all P > 0:10). (Reproduced, with permission, from Budd et al., 1997h).
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the fireline, unconsciously embodied four key principles for the alleviation of heat stress. They were (1) avoid unnecessary heat, (2) encourage self pacing, (3) help sweat to evaporate, and (4) replace sweat losses by drinking. 4.1. Avoid unnecessary heat Although head-fire intensities were as high as 3280 kW per metre of fire front, with flames extending into the sub-canopy of the forest as much as 30 m above the ground (Budd et al., 1997c), safe work practices and individual behavioural responses together reduced the firefighters’ radiant-heat exposure to an intensity little greater than that of sunlight, which could readily be blocked by light clothing (Budd et al., 1997e, g, h). Safe work practices usually required that the crew work along a flank of the fire in its direction of travel, building fireline roughly parallel to the fire edge and systematically overtaking the head fire with the intention of getting around it when circumstances permitted. These work practices allowed the crew to retire to the safety of already-burnt ground if the fire were to become uncontrollable where they were working, and they also reduced the crew’s heat exposure because fire intensity was less severe on the flanks than at the head fire. The crew leader determined the proximity of the fireline to the fire edge, keeping fairly close to it but dropping back to avoid intense heat or smoke and to bypass obstructions. As a result the crews most commonly worked 3 m (range 1–13 m) from the flames, where radiant heat was tolerable and the fire-induced increase in air temperature averaged less than 31C. Individual behavioural responses provided the fine control of radiant-heat exposure. The firefighters did not wear gloves or face masks, and many of them rolled up their sleeves so as to bare their forearms. The bare skin thus exposed automatically restricted their radiant-heat exposure to comfortable levels (o2 kW m 2), since they had to work far enough from the flames to avoid pain and they involuntarily stepped back from any increase in fire intensity (Budd et al., 1997e). The pain-induced withdrawal reflex is a fundamental safety mechanism, but it is largely disabled if firefighters cover all exposed skin, as is sometimes done overseas (Budd et al., 1997i). Firefighters thus handicapped are at risk of not noticing a dangerous increase in their heat exposure until it is too late to safely withdraw. 4.2. Encourage self pacing Individual firefighters worked at their own preferred pace, which differed twofold among them and was a stable characteristic of their work behaviour (Brotherhood et al., 1997a). Formal work tests showed that small changes in pace, apparently prompted by RPE and the
ventilatory threshold (the upper limit of comfortable breathing), provided a sensitive means for firefighters to adjust their EE and the accompanying physiological and subjective responses (Brotherhood et al., 1997a). Other means of self pacing included changes in the frequency and duration of informal rests and light activities (Budd et al., 1997g). The combined effect was to counterbalance any job demands or personal characteristics that might have disturbed the firefighters’ preferred levels of strain (Budd et al., 1997h, Brotherhood et al., 1997b). To summarise the above two principles, the firefighters’ avoidance of radiant heat controlled the main component of their environmental heat load, while self pacing controlled not only their degree of exertion but also the metabolic heat that constituted (see below) almost three quarters of their total heat load. 4.3. Help sweat to evaporate Calculations of heat exchange showed that to maintain thermal equilibrium the firefighters had to evaporate sweat at an average rate of about 1 litre per hour, and hence required clothing that was sufficiently light, loose, and well ventilated to permit such high rates of evaporation (Budd et al., 1997e). The clothing they wore met these requirements admirably, and resulted in 95% of their secreted sweat being evaporated (Hendrie et al., 1997). By contrast, tests in a climatic chamber (Budd et al., 1997i) confirmed that clothing which did not meet these requirements hindered evaporation, trapped metabolic heat, and caused greater cardiovascular strain, discomfort, and fatigueFpenalties that would outweigh any benefit such clothing could confer during bushfire suppression. The dangers of excessive clothing have been emphasised by injuries and deaths from heat stroke in overdressed bushfire fighters (Reischl, 1980). The calculated heat exchanges (Budd et al., 1997e) showed, contrary to popular expectation, that most of the firefighters’ heat load came not from the fire but from their physical exertion: in FRs the average metabolic heat load (488 W) was more than twice the combined heat load (200 W) from the fire and the weather. The implication for bushfire fighters’ clothing is clearFits main task is not to keep heat out but to let it out. Firefighters’ measured sweat rates agreed with those predicted from measurements of their EE and thermal environment, implying that evaporation was sufficient to completely dissipate their heat loads (Budd et al., 1997f; Hendrie et al., 1997). This implication was confirmed by the constancy of Tre, indicating a state of thermal equilibrium, over the sixfold range of rake duration; and also by the finding that fire increased sweat rate by 356 g h 1 but did not change Tre (Budd et al., 1997h). The sweat rates also confirmed the previous finding that
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the heat load from exertion was more than twice that from fire. 4.4. Replace sweat losses by drinking In this respect the firefighters were less successful: like the subjects of many previous workplace studies, they exhibited the phenomenon of ‘voluntary dehydration’ (Rothstein et. al., 1947; Hendrie et al., 1997). They replaced only 43% of their sweat losses during fireline construction, and consequently became dehydrated by an average of 1.7% (maximum 4.0%) of body mass. Although this degree of dehydration had no discernible effect on HR or Tre (Budd et al., 1997h), RPE was 0.7 unit higher for each 1% body mass of dehydration, in keeping with previous reports of increased RPE and fatigue in dehydrating men (Montain and Coyle, 1992; Brown, 1947). It is clear that bushfire fighting can cause rapid dehydration, and that there is a need for work practices which guarantee an adequate water intake before, during, and after firefighting.
5. Comment The results summarised above illustrate the complex interactions among work practices, work behaviour, and clothing which together reduced the firefighters’ heat load, and which also permitted the free evaporation of sweat that was the essential condition for prolonged work at tolerable levels of physiological and subjective strain. The discrepancies between expectation and observation emphasise the limited applicability of laboratory findings to the workplace. In laboratory studies investigators usually test only one factor at a time (e.g. work load, ambient temperature, aerobic fitness), and they rarely allow the subjects to pace themselves according to their perceived strainFwhereas in most workplaces numerous factors that may increase or reduce strain are simultaneously present in varying degree, can interact with each other (Nunneley, 1989), and can provoke behavioural responses to counteract their effects (Gun and Budd, 1995). In terms of control theory (Hardy and Hammel, 1963), laboratory studies exemplify an ‘open loop’ system in which a stimulus is independent of the response to it, whereas the workplace exemplifies a ‘closed loop’ system in which the stimulus is influenced, via negative feedback, by the response and is adjusted so as to maintain the response at a particular level. Thus firefighters’ EE declined slightly with fire and with warmer weather, although not with rake duration. Self pacing has also been invoked to explain the paradoxical findings of two seasonal studies of industrial workers in hot climates (Parikh et al., 1976; Gertner et al., 1984).
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Because of self pacing and other behavioural responses, stresses encountered in the workplace seem more likely to result in reduced productivity than in excessive strain. This was apparent in the finding that firefighters’ productivity and efficiency (productivity per unit EE) declined slightly with rake duration and with fire, although not with warmer weather. Excessive strain would seem most likely to occur in those occupations (e.g. military training, competitive sport) where self pacing is too limited to be effective.
6. Conclusions The results presented in this paper demonstrate the effectiveness of the firefighters’ behavioural responses. They allowed each man to regulate his work rate and his exposure to radiant heat, and thus to maintain his physiological and subjective responses at safe and sustainable levels over a wide range of job demands and personal factors. The results also confirm the excellence of the firefighters’ light cotton or wool clothing, which shielded them from radiant heat yet did not impede the free evaporation of sweat at the high rates required. Finally, they highlight the limitations of laboratory studies for predicting physiological responses in the workplace.
Acknowledgements I am grateful to Josephine Bastian and Phil Cheney for helpful criticism.
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