Seasonal heat acclimatization in wildland firefighters

Journal of Thermal Biology 45 (2014) 134–140

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Seasonal heat acclimatization in wildland firefighters Brianna Lui, John S. Cuddy, Walter S. Hailes, Brent C. Ruby n Montana Center for Work Physiology and Exercise Metabolism, The University of Montana, 32 Campus Drive, Missoula, MT 59812-1825, United States

art ic l e i nf o

a b s t r a c t

Article history: Received 26 February 2014 Received in revised form 27 August 2014 Accepted 27 August 2014 Available online 1 September 2014

The purpose of this study was to determine changes in physiological markers of heat acclimatization across a 4-month wildland fire season. Wildland firefighters (WLFF) (n ¼12) and non-WLFF (n ¼14) were . assessed pre- and post-season for body mass, percent body fat, and peak VO2 . Both groups completed a . 60-min heat stress trial (walking at 50% of peak VO2 ) in a climate controlled chamber (43.3 °C, 33% RH) pre and post-fire season (May through September). During the trials, core (Tc) and skin (Tsk) temperatures, heart rate (HR), physiological strain index (PSI), and rating of perceived exertion (RPE) were measured. There were no differences pre or post-season between the WLFF and non-WLFF groups . in body mass, percent body fat, or peak VO2 . During the 73 days where the WLFF were involved in direct wildland fire suppression, daily high temperature for the WLFF was higher compared to the non-WLFF, 30.6 75.4 °C and 26.9 7 6.1 °C, respectively, p o0.05. Tc was lower at post-season compared to preseason (po 0.05) for the WLFF at 30, 45, and 60 min (pre 30, 45, and 60: 37.97 0.3, 38.3 70.3 and 38.5 70.3 °C, respectively; post 30, 45, and 60: 37.8 70.3, 38.1 7 0.3 and 38.2 70.4 °C, respectively). For WLFF, PSI was lower (po 0.05) at 15, 30, 45, and 60 min at post-season compared to pre-season (4.27 0.7, 5.67 0.9, 6.5 7 0.9, and 7.1 71.1 for 15, 30, 45, and 60 min pre-season, respectively; 3.67 0.8, 4.97 1.0, 5.77 1.2, 6.37 1.3 for 15, 30, 45, and 60 min post-season, respectively). For WLFF, RPE was lower during the post-season trial at 30, 45, and 60 min (pre 30, 45, and 60: 11.77 1.4, 12.3 71.2, and 13.5 71.4, respectively; post 30, 45, and 60: 10.7 71.2, 11.37 1.3, and 11.971.5, respectively), po0.05. There were no differences between pre and post-season for the non-WLFF for Tc and PSI, but RPE was lower at 15 min during the pre-season trial. WLFFs demonstrated significant decreases in Tc, PSI, and RPE during controlled heat stress after the season. Since an age and fitness-matched control group experienced no indication of heat acclimatization, it is suggested that the long-term occupational heat exposure accrued by the WLFFs was adequate to incur heat acclimatization. & 2014 Published by Elsevier Ltd.

Keywords: Heat stress Physiological strain index Heat-related illness Heat injury

1. Introduction Wildland fire suppression is a seasonal occupation that involves working long hours in unpredictable, hazardous conditions and often under high ambient heat (Cuddy et al., 2008, 2011; Ruby et al., 2003, 2002). The specific duties of a wildland firefighter (WLFF) include hiking, digging fire lines, clearing brush, standing lookout, and chain sawing on steep terrain (Cuddy et al., 2011; Ruby et al., 2003, 2002). The daily energy demands of wildfire suppression in the United States range between 12–26 MJ d  1

Abbreviations: WLFF, wildland firefighter; HRI, heat-related illness; Tc, core . temperature; Tsk, skin temperature; ANOVA, analysis of variance; VO2, volume of oxygen; %ΔPV, percent change in plasma volume; HR, heart rate; PSI, physiological strain index; RPE, rating of perceived exertion n Corresponding author. Tel.: þ 1 406 243 2117; fax: þ1 406 243 6252. E-mail addresses: [email protected] (B. Lui), [email protected] (J.S. Cuddy), [email protected] (W.S. Hailes), [email protected] (B.C. Ruby). http://dx.doi.org/10.1016/j.jtherbio.2014.08.009 0306-4565/& 2014 Published by Elsevier Ltd.

(2800–6200 kCal d  1), with work rates estimated to be 6.5– 10 kcal min  1 (20–30 ml kg min  1) (Ruby et al., 2002; Sharkey and Davis, 2008). Work shifts last approximately 12–16-h for 14days before a 2-day rest (Cuddy et al., 2008, 2011; Ruby et al., 2003). The long hours, coupled with periods of vigorous muscular work, result in a loss of muscle glycogen, which is accelerated by heat stress (Cheuvront et al., 2010) and can ultimately impair physical performance if proper feeding strategies are not utilized (Cheuvront et al., 2010; Cuddy et al., 2011). This, along with the addition of protective clothing and high ambient heat, challenges whole body homeostasis causing physiological and psychological stress (Montain et al., 1994; Ruby et al., 2003) that can potentially lead to heat stress and heat-related illness (HRI) (Cuddy and Ruby, 2011). Heat-related illness is a physiological consequence of heat stress that occurs when the body's metabolic heat gain exceeds the evaporative cooling capacity rendering its inability to adequately thermoregulate (Bouchama and Knochel, 2002; Cheung and McLellan, 1998; Cheuvront et al., 2010; Sawka et al., 2001;

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Wendt et al., 2007). In one case study, heat exhaustion occurred in an experienced 24-year old WLFF who was working in the middle of the summer, during the hottest part of the day. Even though he had been drinking an abundance of fluids, he ultimately succumbed to heat exhaustion (core temperature (Tc) 440 °C). In this particular case, the WLFF collapsed and was evacuated from the fire line for medical attention and recovered from the injury (Cuddy and Ruby, 2011). Another instance occurred in July 2011 when an experienced 23-year old WLFF was on an initial attack assignment at a fire in Palo Pinto County, TX. He worked much of the day in extremely hot ambient temperatures (441 °C) and as 4 p.m. approached, he reported that he was hot and had a headache. Within minutes, he collapsed at the scene and arrived at the hospital approximately 1-h after the incident. The WLFF died due to heatstroke despite no apparent irregularities in hydration or electrolyte status (Wood et al., 2011). The ability to reduce the risk for HRI is a critical concern as it can be life threatening (Bouchama and Knochel, 2002; Cuddy and Ruby, 2011; Wood et al., 2011). High aerobic fitness aids in protecting the body from heat stress due to improved tolerance to the heat (Shvartz et al., 1977; Wenger, 1988). However, poor heat acclimatization is a well-known risk factor in many occupational and recreational HRI cases (Bonauto et al., 2007). In individuals with lower heat tolerance, Tc will rise at a much quicker rate compared to heat resilient individuals (Epstein, 1990). Natural heat acclimatization is not well documented but is assumed to contribute to the adaptations necessary for safely working in outdoor occupations. Early season acclimatization and the changes that occur due to natural heat acclimatization during outdoor, seasonal wild fire suppression have not been evaluated. Understanding the seasonal adaptations that WLFFs accrue will provide insight into increased preparedness and reduce the risk for HRI (Bouchama and Knochel, 2002; Montain et al., 1994; Nielsen et al., 1993; Sawka et al., 2001; Wendt et al., 2007). The purpose of this study was to determine changes in physiological markers of heat acclimatization across a 4-month wildland fire season. We hypothesized that physiological markers of heat stress would respond due to the seasonal occupational heat exposure from wildland fire suppression.

2. Methods 2.1. Participants Twenty-six healthy males volunteered for this study. This included WLFF (n ¼12) and an age and fitness-matched control group of non-WLFF (n ¼14) from the local community. The WLFF were a local Type II Hand Crew, but traveled to fight fire in Colorado in June, New Mexico in July, and Montana during July and August. The control group was a group of non-WLFFs who were recreationally active (exercised at a rating of perceived exertion (RPE) between 13 and 17, and between 30–120 min d  1). The major difference between the WLFF and the control group was the control group did not have daily, extended exposure to the heat like the WLFF group (10 þ h d  1). Prior to data collection, all participants completed a University Institutional Review Board approved written consent form. 2.2. Preliminary testing Preliminary testing was done pre and post-fire season (May and September, respectively) before the experimental trials. Participants fasted for at least 3-h prior to performing a maximal . aerobic capacity (peak VO2) test and body composition assess. ment. Peak VO2 testing was done on a motorized treadmill

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(Fullvision, Inc., Newton, KS) and started with a 3 min warm up. After the warm up, the participants performed an incremental protocol, which started at 1.8 m s  1 and 0% incline. After 1 min, the incline increased to 4% and continued to increase 2% each minute until a maximum of 10% incline was reached. Speed remained constant at 1.8 m s  1 until minute 6, at which time it was increased to 2.0 m s  1 and continued to increase 0.18 m s  1 each minute until the participant reached volitional exhaustion. Expired gases were analyzed every 15-s throughout the test using a calibrated metabolic cart (Parvomedics, Inc., Sandy, UT). Body composition was determined by hydrodensitometry with estimated residual volume. Body weight was recorded on a dry weight scale (Befour Inc., Cedarburg, WI) and height was measured. Body density was assessed using a calibrated underwater weighing tank with computerized scales (Exertech, Dresbach, MN). Body density was converted to percent body fat using estimated residual volume (Goldman and Becklake, 1959) and the Siri equation (Siri, 1993). 2.3. Experimental design The experimental heat trials were conducted pre and post-fire season (May and September, respectively). In the 10-h period before the heat trials, participants fasted, but were permitted to consume water ad-libitum. Upon arrival to the laboratory, participants voided their bladder and a nude body weight was obtained on a calibrated scale (Ohaus, Pine Brook, NJ). All urine used in the estimation of sweat rate was collected for measurement after this initial void. The participants then inserted a rectal thermometer (Mallinckrodt Medical, St. Louis, MO), which continuously measured Tc throughout the trial. A heart rate (HR) monitor (Polar Electro, Kempele, FL) was affixed around the chest and a wired skin temperature (Tsk) sensor (Mallinckrodt Medical, St. Louis, MO) was adhered to the left pectoralis muscle. Blood samples (5 mL) were collected before and after the 60-min exercise sessions from an antecubital vein into a heparinized vacutainer vial (Becton, Dickinson and Co., Franklin Lakes, NJ). The participants walked at . 50% of their peak VO2 in a climate controlled chamber (Tescor, Warminster, PA) at a constant temperature . of 43.3 °C and 33% relative humidity for 60 min. The mean VO2 during the experimental heat trials averaged 27.3 73.4 ml kg min  1, an intensity indicative of typical wildland fire suppression tasks (Sharkey and Davis, 2008). In September (post-season), the participants repeated the heat trial at the same absolute work rate (identical speed and grade) as the pre-season trial. Throughout each heat trial, participants were given a measured amount of water (0.7 g kg  1) every 15-min. RPE was measured at the start of the trial and every 15 min. After exercise, participants voided and urine was collected for measurement. A post-exercise nude body weight and blood sample was collected as previously described. 2.4. Physiological measurements 2.4.1. Core and skin temperature Tsk and Tc were continually monitored during each laboratory trial with a digital data logger (Physitemp Instruments Inc., Clifton, NJ). A rectal thermistor was inserted 15 cm past the anal sphincter and a skin temperature thermistor was placed on the left pectoralis muscle 2.5 cm medial and 2.5 cm superior from the nipple. Data was collected every ½ second throughout the exercise session at 0.5-s intervals using DASYLab Software (Measurement Computing Co., Norton, MA). 2.4.2. Heart rate HR was collected using a Polar RS800CX heart rate monitor (Polar Electro Inc., Lake Success, NY). Data was recorded at 5-s

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intervals and the time point HR values at each minute were used for analysis.

3. Results 3.1. Anthropometrics

2.4.3. Sweat loss Whole body sweat loss was determined by the difference in nude body mass before and after exercise, urine output and fluid intake (corrected for respiratory water loss (Mitchell et al., 1972) and water vapor pressure (Fox et al., 1993)). Sweat loss was converted to sweat rate relative to body surface area (Mosteller, 1987).

There were no differences pre or post-season between the WLFF and non-WLFF groups in body mass, percent body fat, or . . peak VO2, Table 1. There was a main effect for time for peak VO2 1 from pre to post-season, 54.6 76.7 and 57.0 78.2 ml kg min for pre and post-season, respectively, p o0.05. 3.2. Sweat rate and plasma volume shift

SweatRate(g m−2 min−1) = (BWpre + Liquid Ing)– (BWpost + Urine weight + Resp Water Loss) 2.4.4. Physiological strain index PSI was determined with HR and Tc using an established equation (Buller et al., 2008; Moran et al., 1998) where “t” is core temperature/HR at the time of reading, and “0” is core temperature/HR at time zero, prior to the start of exercise. −1

PSI = 5 (Tcore(t) −Tcore(0) )(39.5−Tcore(0) )

+ 5(HR t −HR 0 )(180–HR 0 )−1

2.4.5. Blood Immediately following the blood draw, 2 capillary tubes were filled with 100 ml of blood each and placed in a hematocrit (Hct) centrifuge for 3 min (A13, Jouan, Winchester, VA). The proportion of blood cells to serum was measured in each tube and the average of the two was used to determine Hct. Remaining whole blood was used to determine hemoglobin (Hb) content using the colorimetric cyanmethemoglobin procedure per manufacturer instructions (Drabkin's reagent, Sigma, St. Louis, MO). Changes during the heat stress trials in acute plasma volume (%ΔPV) were evaluated using pre and post-exercise values of Hb and Hct as previously described by Dill and Costill (1974). 2.4.6. Exercise and work history Exercise and work history records were obtained in September for both the control group and the WLFF. For the control group, the minutes of exercise they participated in each week was collected via questionnaire and interview. For WLFF, shift logs and work schedules were reviewed and recorded (dates and locations of fire work) from the entire summer. 2.5. Weather data WLFF were on site for direct fire suppression activities for 73 out of 120 days during the experimental time period, May through September. The other 47 days they were exposed to the same environment as the non-WLFF, as both groups lived in the same local community. To evaluate differences in environmental exposure, weather data from the 73 days where WLFF and non-WLFF were exposed to different environments was evaluated. 2.6. Statistical analyses All dependent variables were analyzed using mixed design 2-way ANOVA with repeated measures for time. Weather data was analyzed using an independent 2-tailed t-Test. Statistical significance was set at p o0.05 for all analyses. Descriptive data is reported as mean 7SD and graphic values are reported as mean 7SEM. Data was analyzed using SPSS (Version 13.0 for Windows: SPSS Inc., Chicago, IL).

Sweat rate was higher at the post-season trial compared to the pre-season trial, 6.5 71.1 vs. 6.17 1.3 g m  2 min  1, respectively, main effect for time, p o0.05. There were no differences in the change in plasma volume between groups or from pre to postseason, Table 2. 3.3. Blood There was a trial-by-time interaction for hematocrit, see Table 2. WLFF demonstrated changes from pre- to post-season, while the non-WLFF exhibited no differences from pre- to postseason. 3.4. Weather data During the 73 days where the WLFF were involved in direct wildland fire suppression, daily high temperature for the WLFF was higher compared to the non-WLFF, 30.67 5.4 °C and 26.97 6.1 °C, respectively, po 0.05. 3.5. Core temperature There was a trial-by-time interaction for Tc, with post-season values being lower at 30, 45, and 60 min for the WLFF group, po 0.05, Fig. 1. Post-season core temperature at 60 min was lower for the WLFF compared to the non-WLFF group, p o0.05. There were no differences for the non-WLFF group from pre-season to post-season. Time points 15, 30, 45, and 60 min were each higher compared to all prior time points for all 4 trials, po 0.05, Fig. 1. 3.6. Skin temperature The trial-by-time interaction was not significant for Tsk. Tsk was higher pre-season compared to post-season at 15 min (36.6 70.5 °C vs. 36.4 70.6 °C, respectively), 30 min (36.9 7 0.5 °C vs. 36.7 70.7 °C, respectively), 45 min (37.17 0.6 °C vs. 36.87 0.7 °C, respectively), and 60 min (37.2 70.8 °C vs. 36.97 0.8 °C, respectively), main effect for time, p o0.05. Table 1 Mean 7SD. The wildland firefighter (WLFF) and non-wildland firefighter (nonWLFF) physical characteristics pre- and post-firefighting season (May and September, respectively). Characteristic

WLFF Pre-season

Non-WLFF Post-season Pre-season Post-season

Age (yr) 27 74 Height (cm) 1797 8 Weight (kg) 79.0 7 10.5 78.8 7 9.4 Body fat (%) 15.2 76.1 14.7 76.4 Peak VO2 (ml kg  1 min  1) 53.2 7 5.2 55.0 7 7.6* n

Indicates main effect for time, p o 0.05.

257 4 1807 7 77.7 7 8.8 77.6 7 8.7 14.17 3.9 14.9 74.3 55.9 7 7.8 58.7 7 8.6*

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Table 2 Mean 7 SD. The wildland firefighter (WLFF) and non-wildland firefighter (non-WLFF) analysis of blood pre- and post- heat trial as well as sweat rate pre- and post- season (May and September, respectively). Characteristic

WLFF

Non-WLFF

Pre-season Pre-trial Body weight (kg) Hct (%) Change in plasma volume (%) Sweat loss (kg) Sweat rate (g m  2 min  1) n

†

Post-season Post-trial

79.3 7 10.8 78.7 710.8 47.1 72.1 47.7 7 1.8  1.1 712.8 1.5 70.3 12.7 7 1.8

Pre-trial

Pre-season Post-trial

78.6 7 9.6 78.1 79.5 49.37 2.3†n 48.47 2.2n  3.3 710.2 1.6 7 0.2 13.0 7 1.1

Pre-trial

Post-season Post-trial

78.4 7 8.9 77.9 7 8.8 47.5 7 2.4 48.57 2.9†  3.0 7 5.3 1.5 7 0.4 12.5 7 2.9

Pre-trial

Post-trial

77.5 7 8.7 77.0 7 8.6 47.5 7 2.6 48.5 72.5†  3.17 9.4 1.5 7 0.5 13.0 7 4.1

po 0.05 vs. pre-season. p o0.05 vs. pre-trial.

3.7. Heart rate

3.8. Physiological strain index

The trial-by-time interaction was not significant for HR. HR increased from the previous time point at 15, 30, 45, and 60 min both pre-season (817 15, 140716, 148 7 16, 153 717, 156717 bpm for 0, 15, 30, 45, and 60 min, respectively) and post-season (80 712, 141 712, 150 714, 153 714, and 1587170 bpm for 0, 15, 30, 45, and 60 min, respectively), main effect for time, po 0.05.

There was a trial-by-time interaction for PSI, with post-season values being lower at 15, 30, 45, and 60 min for the WLFF group, po 0.05, Fig. 2. There were no differences for the non-WLFF group from pre-season to post-season. Time points 15, 30, 45, and 60 min were each higher compared to all prior time points for all 4 trials, p o0.05, Fig. 2.

Fig. 1. Changes in Tc during exercise in the heat for WLFF (A) and Non-WLFF (B). n p o 0.05 pre-season vs. post-season; †p o0.05 vs. non-WLFF; ‡p o 0.05 15, 30, 45, and 60 min were each higher compared to time 0 within each trial both pre- and post-season.

Fig. 2. Changes in PSI during exercise in the heat for WLFF (A) and non-WLFF (B). n p o0.05 pre-season vs. post-season; ‡p o0.05 15, 30, 45, and 60 min were each higher compared to time 0 within each trial both pre- and post-season.

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Fig. 3. Changes in RPE during exercise in the heat for WLFF (A) and non- WLFF (B). n p o 0.05 pre-season vs. post-season; †p o0.05 vs. non-WLFF; ‡p o 0.05 15, 30, 45, and 60 min were each higher compared to time 0 within each trial both pre- and post-season.

3.9. Rating of perceived exertion There was a trial-by-time interaction for RPE, with post-season values lower at 30, 45, and 60 min for the WLFF group and for the non-WLFF group at 15 min, p o0.05, Fig. 3. Pre-season PSI at 45 min was lower for the WLFF compared to the non-WLFF group, and post-season PSI at 15, 30, 45, and 60 min was lower for the WLFF group compared to the non-WLFF group, p o0.05. For the WLFF, each time point was higher than the previous one for preand post-season except for 30 and 45 min pre- and post-season, p o0.05. For the non-WLFF, 15, 30, 45, and 60 min were each higher compared to all prior time points during the pre- and postseason trials except for 45 and 60 min post-season, p o0.05.

4. Discussion The current study demonstrates that the WLFF experienced heat acclimatization across the fire season, as confirmed by significant decreases in Tc, PSI, and RPE at post-season compared to pre-season during a controlled heat stress evaluation. When compared to a non-heat acclimatized individual, these physiological adaptations could decrease the risk for HRI (Bonauto et al., 2007; Fox et al., 1964). In contrast, an age and fitness-matched control group of non-WLFF did not incur any adaptions large enough to decrease Tc, PSI, or RPE during the same time period (May through September). The participants in the study had a high

. level of aerobic fitness, evidenced by a peak VO2 of 53.2 75.2 and 55.97 7.8 ml kg min  1 for the WLFF and non-WLFF, respectively. Collectively, study participants experienced a small but statisti. cally significant increase (2.4 7 3.9 ml kg min  1) in peak VO2 across the study time period (main effect for time), but there were no pre or post-differences between groups. The WLFF worked on wildfires 61% of the time during the summer, for a total of 73 out of 120 days between May and September, with daily high temperatures 3.7 °C higher for the WLFF group. Considering that the typical 14 h shift has about 8 h of warm environmental exposure, the WLFF were exposed to approximately 600-h of occupational heat exposure. The non-WLFFs accumulated 200-h of outdoor exercise (running, cycling, hiking, etc.), though not all exercise time was during the heat of the day. The considerable amount of occupational heat exposure time (600 h) is most responsible for the adaptations incurred by the WLFF across the fire season. Combined, the subtle reductions of Tc and PSI led to a decreased perception in discomfort while exercising in the heat. The WLFFs demonstrated a significant decrease in RPE post-season compared to pre-season for over half of the post-season heat trial, and had a lower RPE compared to the non-WLFF group postseason at all exercise time points (Fig. 3). For the non-WLFF, RPE was not different pre and post-season except for a higher postseason value at 15 min. Both groups shared similar rates of increase in RPE during the 60 min heat trials, though both RPE and Tc will inevitably increase during an exercise protocol in the heat as previously demonstrated in other studies (Simmons et al., 2008). The ability to perceive the reduction in heat strain underscores the fact that WLFFs should be mindful of their perceived exertion while working and take breaks as needed to reduce the possibility of dangerously high Tc, and ultimately, HRI in environments they are not fully acclimatized to. Since RPE was significantly reduced from pre to post-season, WLFFs may inadvertently develop a false sense of thermoregulatory adaptation. Since laboratory acclimation studies show decreases in Tc ranging from 0.2–0.8 °C (Aoyagi et al., 1998; Bass et al., 1955; Cheung and McLellan, 1998; Nadel et al., 1974; Shvartz et al., 1977), the observed decrease in Tc of 0.2 °C over the course of 4 months represents a minimal change. Despite the significant decreases in Tc and PSI observed, WLFF may not be acquiring as much protection as they perceive in regards to a reduction in HRI risk. This sets the stage for potential HRI if WLFFs do not adhere to appropriate work:rest cycles. As previously stated, “a WLFF should periodically withdraw from the heat and find cooler refuge if feeling excessively hot from exposure to high ambient/fire temperatures” (Cuddy and Ruby, 2011). The effects of acclimation in previous laboratory studies demonstrates physiological changes beginning to occur within a few days, with full adaptation occurring within 7–14 days (Wendt et al., 2007). These changes include decreases in HR, Tsk, and Tc and an increase in sweat rate at the same absolute work rate (Cheung and McLellan, 1998; Nielsen et al., 1993; Wenger, 1988). Thus, thermoregulation and exercise performance in the heat is improved while reducing PSI and decreasing the risk of HRI (Bonauto et al., 2007; Bouchama and Knochel, 2002; Buller et al., 2008; Buono et al., 2009; Hargreaves, 2008; Sawka and Coyle, 1999). It has also been suggested that acclimation in laboratory conditions and natural acclimatization have the same effect (Frisancho, 1993; Wenger, 1988), yet, the time course and workload requirements to achieve the same physiological adaptations outside the controlled laboratory environment have not been described. In the current study, Tc, PSI, and RPE decreased post-season for the WLFF despite any differences in seasonal %ΔPV or sweat rate (Table 1). It has been suggested that plasma volume increases early on in acclimatization, improving cardiovascular responses and lowering Tc due

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to an improvement in heat transfer and dissipation. The increases in plasma volume, however, drop back to pre-acclimatization levels after 14 days of heat exposure, which may explain why % ΔPV did not change across the fire season (Bass et al., 1955). Sweat rate demonstrated a significant main effect for time and increased by 5 7 9% during the heat stress trial from pre- to postseason for all participants. However, there was no difference between groups, despite the significant decrease in Tc, PSI, and RPE post-season for WLFF. An explanation for the decreased Tc, PSI, and RPE could be the WLFF commenced sweating earlier in the post-season trial compared the pre-season trial, as Tsk for WLFF plateaued by 30 min to 36.5 70.8 °C, whereas the non-WLFF did not plateau in Tsk until 45 min to 37.070.6 °C. The difference in time to Tsk plateau alludes to the concept that a trained and heat acclimatized individual is able to dissipate heat better than a trained, non-heat acclimatized individual (Nadel et al., 1974). It has been suggested that high aerobically fit individuals (4 55 ml kg  1 min  1) acclimate to the heat quicker than unfit individuals (Cheung and McLellan, 1998). This has been attributed to an increased capacity for heat dissipation, evaporative heat loss and a lower resting HR and Tc at the onset of exercise (Shvartz et al., 1977; Wenger, 1988). Training education and material provided to seasonal employees should emphasize the importance of starting the fire season with a high aerobic capacity. Higher aerobic capacity can provide thermal protection and reduce thermal strain during exercise in the heat (Cuddy et al., 2013). It should be emphasized that higher aerobic fitness will more likely provide early-season, early-shift protection and reduce the risk of exertional heat strain prior to the development of occupation specific acclimatization. The collective benefits of increased early season fitness and heat acclimatization would decrease the rate of rise in Tc and HR, which could better maintain PSI values below 7.0 during work rates commonly associated with the tasks of wildfire suppression. Individuals who have “high heat strain” may be susceptible to HRI and can be identified with elevated values of PSI ( 47.0), as previously explained by Moran et al. (Moran et al., 1998). As a result of seasonal work, the WLFFs in the current study demonstrated lower Tc at the post-season and thus, “low heat strain” PSI values (o7.0). These physiological changes were observed during the heat stress trials, as it brought 6 out of the 12 WLFFs from “high heat strain” pre-season, down to 3 out of the 12 post-season. The remaining 9 WLFFs who had “low” PSI post-season, also demonstrated a reduction in HR and Tsk compared to the 3 WLFFs who did not demonstrate a reduction in PSI over the season. In contrast, 10 out of the 14 non-WLFFs experienced “high heat strain” both pre and post- season. Based on the observed changes in Tc and PSI, the majority of the WLFFs demonstrated greater physiological tolerance to heat stress post-season. In summary, at the end of the wildland fire season, WLFFs demonstrated significant decreases in Tc, PSI, and RPE during controlled heat stress, with the most noticeable differences between groups appearing with RPE. A possible change in the sweat response pattern (earlier onset of sweating) of the WLFFs may serve as one potential mechanism responsible for the observed reductions. Since an age and fitness-matched control group experienced no indication of heat acclimatization, it is suggested that the long-term occupational heat exposure (600 h) accrued by the WLFFs was adequate to incur heat acclimatization. Since the changes in the markers of heat acclimatization were subtle, training material should emphasize the importance of high pre/ early season aerobic fitness. Moreover, WLFFs should remain consistently aware of their interaction between the environment and work rate in order to mitigate the risk for HRI.

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Acknowledgments We wish to express sincere appreciation to the wildland firefighter volunteers as well as their supervisors who allowed them to participate in this study.

References Aoyagi, Y., McLellan, T.M., Shephard, R.J., 1998. Effects of endurance training and heat acclimation on psychological strain in exercising men wearing protective clothing. Ergonomics 41, 328–357. Bass, D.E., Kleeman, C.R., Quinn, M., Henschel, A., Hegnauer, A.H., 1955. Mechanisms of acclimatization to heat in man. Medicine 34, 323–380. Bonauto, D., Anderson, R., Rauser, E., Burke, B., 2007. Occupational heat illness in washington state, 1995–2005. Am. J. Ind. Med. 50, 940–950. Bouchama, A., Knochel, J.P., 2002. Heat stroke. N. Engl. J. Med. 346, 1978–1988. Buller, M.J., Latzka, W.A., Yokota, M., Tharion, W.J., Moran, D.S., 2008. A real-time heat strain risk classifier using heart rate and skin temperature. Physiol. Meas. 29, N79–N85. Buono, M.J., Martha, S.L., J.H., H., 2009. Peripheral sweat gland function is improved with humid heat acclimation. J. Therm. Biol. 34, 127–130. Cheung, S.S., McLellan, T.M., 1998. Heat acclimation, aerobic fitness, and hydration effects on tolerance during uncompensable heat stress. J. Appl. Physiol. 84, 1731–1739. Cheuvront, S.N., Kenefick, R.W., Montain, S.J., Sawka, M.N., 2010. Mechanisms of aerobic performance impairment with heat stress and dehydration. J. Appl. Physiol. 109, 1989–1995. Cuddy, J.S., Buller, M., Hailes, W.S., Ruby, B.C., 2013. Skin temperature and heart rate can be used to estimate physiological strain during exercise in the heat in a cohort of fit and unfit males. Mil. Med. 178, e841–e847. Cuddy, J.S., Ham, J.A., Harger, S.G., Slivka, D.R., Ruby, B.C., 2008. Effects of an electrolyte additive on hydration and drinking behavior during wildfire suppression. Wilderness Environ. Med. 19, 172–180. Cuddy, J.S., Ruby, B.C., 2011. High work output combined with high ambient temperatures caused heat exhaustion in a wildland firefighter despite high fluid intake. Wilderness Environ. Med. 22, 122–125. Cuddy, J.S., Slivka, D.R., Tucker, T.J., Hailes, W.S., Ruby, B.C., 2011. Glycogen levels in wildland firefighters during wildfire suppression. Wilderness Environ. Med. 22, 23–27. Dill, D.B., Costill, D.L., 1974. Calculation of percentage changes in volumes of blood, plasma, and red cells in dehydration. J. Appl. Physiol. 37, 247–248. Epstein, Y., 1990. Heat intolerance: predisposing factor or residual injury? Med. Sci. Sports Exerc. 22, 29–35. Fox, E., Bowers, R., Foss, M., 1993. The Physiological Basis for Exercise and Sport, 5th ed. Brown and Benchmark Publishers, Madison, WI. Fox, R.H., Goldsmith, R., Hampton, I.F., Lewis, H.E., 1964. The nature of the increase in sweating capacity produced by heat acclimatization. J. Physiol. 171, 368–376. Frisancho, A.R., 1993. The Study of Human Adaptation Human Adaptation and Accomodation: Enlarged and Revised Edition of Human Adaptation. The University of Michigan Press, Ann Arbor, MI, pp. 4–5. Goldman, H.I., Becklake, M.R., 1959. Respiratory function tests; normal values at median altitudes and the prediction of normal results. Am. Rev. Tuberc. 79, 457–467. Hargreaves, M., 2008. Physiological limits to exercise performance in the heat. J. Sci. Med. Sport 11, 66–71. Mitchell, J.W., Nadel, E.R., Stolwijk, J.A., 1972. Respiratory weight losses during exercise. J. Appl. Physiol. 32, 474–476. Montain, S.J., Sawka, M.N., Cadarette, B.S., Quigley, M.D., McKay, J.M., 1994. Physiological tolerance to uncompensable heat stress: Effects of exercise intensity, protective clothing, and climate. J. Appl. Physiol. 77, 216–222. Moran, D.S., Shitzer, A., Pandolf, K.B., 1998. A physiological strain index to evaluate heat stress. Am. J. Physiol. 275, R129–R134. Mosteller, R.D., 1987. Simplified calculation of body-surface area. N. Engl. J. Med. 317, 1098. Nadel, E.R., Pandolf, K.B., Roberts, M.F., Stolwijk, J.A., 1974. Mechanisms of thermal acclimation to exercise and heat. J. Appl. Physiol. 37, 515–520. Nielsen, B., et al., 1993. Human circulatory and thermoregulatory adaptations with heat acclimation and exercise in a hot, dry environment. J. Physiol. 460, 467–485. Ruby, B.C., Schoeller, D.A., Sharkey, B.J., Burks, C., Tysk, S., 2003. Water turnover and changes in body composition during arduous wildfire suppression. Med. Sci. Sports Exerc. 35, 1760–1765. Ruby, B.C., et al., 2002. Total energy expenditure during arduous wildfire suppression. Med. Sci. Sports Exerc. 34, 1048–1054. Sawka, M.N., Coyle, E.F., 1999. Influence of body water and blood volume on thermoregulation and exercise performance in the heat. Exerc. Sport Sci. Rev. 27, 167–218. Sawka, M.N., et al., 2001. Physiologic tolerance to uncompensable heat: Intermittent exercise, field vs laboratory. Med. Sci. Sports Exerc. 33, 422–430. Sharkey, B.J., Davis, P.O., 2008. Hard Work. Human Kinetics, Champaign, IL.

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B. Lui et al. / Journal of Thermal Biology 45 (2014) 134–140

Shvartz, E., et al., 1977. Heat acclimation, physical fitness, and responses to exercise in temperate and hot environments. J. Appl. Physiol.: Respir. Environ. Exerc. Physiol. 43, 678–683. Simmons, S.E., Mundel, T., Jones, D.A., 2008. The effects of passive heating and head-cooling on perception of exercise in the heat. Eur. J. Appl. Physiol. 104, 281–288. Siri, W.E., 1993. Body composition from fluid spaces and density: analysis of methods. Nutrition 9, 480491.

Wendt, D., van Loon, L.J., Lichtenbelt, W.D., 2007. Thermoregulation during exercise in the heat: strategies for maintaining health and performance. Sports Med. 37, 669–682. Wenger, C.B., 1988. Human Heat Acclimatization Human Performance Physiology and Environmental Medicine at Terrestrial Extremes. Benchmark Press, Inc., Indianapolis, IN, pp. 153–197. Wood, V., et al., 2011. In: B.o.L. M.-T.F. Service (Ed.), Serious Accident Investigation Factual Report. CR 337 Fatality. Mineral Wells, TX.