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Heat Loss Is Impaired in Older Men on the Day after Prolonged Work in the Heat

NOTLEY, SEAN R.; MEADE, ROBERT D.; D’SOUZA, ANDREW W.; FRIESEN, BRIAN J.; KENNY, GLEN P.

Medicine & Science in Sports & Exercise: September 2018 - Volume 50 - Issue 9 - p 1859–1867
doi: 10.1249/MSS.0000000000001643
APPLIED SCIENCES
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Purpose Prolonged work in the heat may exacerbate the rise in core temperature on the next work day, especially in older workers who display impairments in whole-body heat loss that increase body heat storage and core temperature relative to young adults during heat stress. We therefore evaluated whether whole-body heat loss in older adults was impaired on the day after prolonged work in the heat.

Methods Whole-body heat exchange and heat storage were assessed in nine older (53–64 yr) men during three 30-min bouts of semirecumbent cycling at fixed rates of metabolic heat production (150 [Ex1], 200 [Ex2], 250 W·m−2 [Ex3]), each separated by 15-min recovery, in hot-dry conditions (40°C, 20% relative humidity), immediately before (day 1), and on the day after (day 2) a prolonged, work simulation (~7.5 h) involving moderate-intensity intermittent exercise in hot-dry conditions (38°C, 34% relative humidity). Total heat loss (evaporative ± dry heat exchange) and metabolic heat production were measured using direct and indirect calorimetry, respectively. Body heat storage was quantified as the temporal summation of heat production and loss.

Results Total heat loss (mean ± SD) during Ex1 did not differ between days 1 and 2 (151 ± 15 and 147 ± 14 W·m−2, respectively; P = 0.27), but was attenuated on day 2 during Ex2 (181 ± 15 W·m−2) and Ex3 (218 ± 16 W·m−2) relative to day 1 (192 ± 14 and 230 ± 19 W·m−2, respectively; both P < 0.01). Consequently, body heat storage throughout the protocol on day 2 (276 ± 114 kJ) was 31% greater than on day 1 (191 ± 87 kJ; P < 0.01).

Conclusions Prolonged work in the heat causes next-day impairments in whole-body heat loss, which exacerbate heat storage and may elevate the risk of heat injury on the following day in older workers.

Human and Environmental Physiology Research Unit, School of Human Kinetics, University of Ottawa, Ottawa, CANADA

Address for correspondence: Dr. Glen P. Kenny, Ph.D., University of Ottawa, School of Human Kinetics, 125 University, Room 367, Montpetit Hall, Ottawa, Ontario, Canada K1N 6N5; E-mail: gkenny@uottawa.ca.

Submitted for publication February 2018.

Accepted for publication April 2018.

Performing back-to-back days of prolonged (10–12 h), arduous work in the heat is common for workers in many industries (e.g., mining, electric utilities, firefighting, military). These work conditions can cause dangerous increases in core temperature over the course of a shift (1–4), which may progress to heat-related illnesses and even death (5). It is well established that a worker’s susceptibility to heat illness during physically demanding work in the heat is greater when heat stress occurs on the preceding day (6–8), and more recent field reports indicate that the rise in core temperature among firefighters (age, 31 ± 8 yr) (9) and electric utilities workers (38 ± 12 yr) (10) is greater on the second of consecutive days of work in the heat. Taken together, those findings indicate that the physiological strain (i.e., rises in core temperature, cardiovascular strain and fluid depletion) associated with prolonged work in the heat may cause next-day effects, which impair the body’s physiological capacity to dissipate heat, and therefore, exacerbate the rise in core temperature and the risk of heat-related injury. However, although we recently showed that, when evaluated under controlled-laboratory conditions, prolonged work in the heat does not significantly modify whole-body heat loss on the next day in young men (26 ± 4 yr) (11), it remains uncertain if this conclusion holds for the rising number of older workers (50–65 yr) (12), who display marked impairments in whole-body heat loss relative to young adults that worsen with increases in the combined exercise and environmental (net) heat load (13,14). Indeed, these age-related differences in thermoregulatory function may explain the discrepancy between our results in young adults (11) and those obtained in previous field studies involving some older workers (9,10). Individual data from the study involving electric utilities workers lend support to this possibility (10), where the relative increase in core temperature on the second relative to the first work day in the oldest worker (54 yr) was similar to the youngest worker (24 yr), even though the older worker spent approximately 30% more of the second relative to the first work day at rest. It is possible therefore that prolonged, arduous work in the heat may cause reductions in whole-body heat loss, which exacerbate the rise in core temperature and the risk of heat illness on the next day in older workers, particularly during work conditions eliciting higher net heat loads.

Previous evaluations of thermoregulatory function in older adults over consecutive days are surprisingly sparse (15–18) and have not replicated the duration (10–12 h), and thus, physiological strain associated with a prolonged work day in the heat. To our knowledge, the only exception was a recent evaluation of heat strain in middle-age volunteer and career firefighters (41 ± 17 yr) during three 2-h work circuits performed over four consecutive days in a hot environment with sleep restriction (19). However, only some of the workers studied exceeded the minimum age (~40 yr) at which thermoregulatory function is known to be impaired as a function of aging (20), and those investigators were unable to assess potential between-day differences in either local or whole-body heat loss. Consequently, it remains unclear whether prolonged occupational heat strain reduces whole-body heat loss on the next work day in older workers (50–65 yr) or whether the magnitude of those reductions is heat load dependent.

Because this information may form important considerations for existing safe-work guidelines (e.g., American Conference of Governmental and Industrial Hygienists (ACGIH) Threshold Limit Values [TLV®]) (21), which do not consider age-related or potential next-day effects on heat strain, we felt it was essential to evaluate whether a prolonged work day in the heat modulates whole-body heat loss on the next day in older workers as an extension of our previous work with young men (11). To achieve this objective, we used direct calorimetry to assess time-dependent changes in whole-body heat exchange and storage in older men (50–65 yr) during heat stress tests (2.5 h each) performed on the same day before (day 1), and on the day after (day 2) a prolonged work simulation (7.5 h), which involved moderate intensity, intermittent work in hot, dry conditions to replicate the physiological demands of arduous occupations (e.g., mining, electric utilities, firefighting, military). Each heat stress test consisted of three work bouts performed at increasing fixed rates of metabolic heat production in similarly hot, dry conditions to determine the net heat load at which prolonged heat strain may influence heat loss on the next day. We hypothesized that whole-body total heat loss would be reduced on day 2 relative to day 1 and that the magnitude of those differences would be greater with increases in net heat load.

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METHODS

Ethical Approval

The experimental protocol was approved by the University of Ottawa Health Sciences and Science Research Ethics Board and is in agreement with the Declaration of Helsinki. Written and informed consent was obtained from all volunteers before their participation in the study.

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Participants

Nine healthy older men participated in this study (mean ± SD: age, 59 ± 4 yr; mass, 76.8 ± 5.4 kg; height, 1.75 ± 0.04 m; body surface area, 1.92 ± 0.06 m2; body fat, 20.7% ± 5.6%; peak aerobic power, 3.41 ± 0.70 L·min−1). Participants were habitually active, performing an average of 3 to 4 days of structured, aerobic exercise per week for a duration of 30 to 60 min per session. To ensure these participants were reflective of older workers required to perform arduous work in the heat, we recruited individuals who possessed similar physical characteristics, activity levels and aerobic fitness to a group of older, career firefighters recruited for a previous study (22). Testing was conducted during the months August to December where the daily minimum and maximum air temperature averaged 4.8°C ± 10.2°C and 13.9°C ± 11.9°C, respectively. Participants were nonsmokers, were not taking any medication, and did not report a history of cardiovascular, respiratory, or metabolic disease.

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Procedures

Overview

These procedures are a replication of those employed previously to evaluate the next-day effects of a prolonged work day in the heat on thermoregulatory function in young men (11), with modifications and sources for additional detail provided. Participants completed one preliminary session and one experimental trial separated by >48 h. The experimental trial spanned a two-day period (~29 h) comprising of a heat stress test (7:00 AM to 10:00 AM, day 1), work simulation (10:00 AM to 6:00 PM), recovery and sleep period (6:00 PM, day 1 to 7:00 AM, day 2) and a second, identical heat stress test (7:00 AM to 10:00 AM, day 2).

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Preliminary session

Body height, mass, surface area, and density, as well as peak aerobic power were determined during the preliminary visit. Body surface area was derived (23) from measures of standing height (model 2391; Detecto, Webb City, MO) and body mass (IND560; Mettler Toledo Inc., Mississauga, ON, Canada). Body density was measured using the hydrostatic weighing technique and used to calculate body fat percentage (24). Indirect calorimetry was used to quantify peak aerobic power (MCD Medgraphics Ultima Series; MGC Diagnostics, MN) during a progressive incremental exercise protocol (25) on a semirecumbent cycle ergometer (Corival, Lode B.V., Groningen, Netherlands) in thermoneutral conditions (~23°C).

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Heat stress tests

For each heat stress test (7:00 AM to 10:00 AM, days 1 and 2), participants changed into shorts and sandals and were instrumented in a temperate room (~25°C). Participants then entered a direct air calorimeter regulated to a fixed ambient temperature (40°C) and relative humidity (20%) and completed a 15-min seated rest period (baseline), followed by three 30-min bouts of semirecumbent cycling at increasing, fixed rates of metabolic heat production of 150 (Ex1), 200 (Ex2), and 250 W·m−2 (Ex3), each followed by a 15-min recovery (Rec1, Rec2, Rec3) for a total duration of 150 min. Because it is known that aging modulates heat loss as a function of the net heat load (13,14), an exercise model that elicited progressively greater net heat loads was used to identify the specific net heat load at which prolonged heat strain may influence whole-body heat loss on the next day. These work rates were equivalent to approximately 38%, 47% and 57% of each individual’s peak aerobic power.

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Work simulation

After the first heat stress test (day 1), participants immediately donned a 100% cotton work uniform (coveralls, short-sleeved T-shirt, socks) and sports shoes before entering a climate chamber (38°C, 34% relative humidity; equivalent to a wet-bulb globe temperature of ~29°C) to perform a work simulation (10:00 AM to 6:00 PM, day 1). This simulation involved three work bouts (10:30 AM to 12:30 PM, 1:00 PM TO 3:00 PM and 3:30 PM to 5:30 PM) separated by morning (10:00 AM to 10:30 AM), lunch (12:30 PM to 1:00 PM), and afternoon (3:00 PM to 3:30 PM) breaks (seated rest) and a supervised recovery (5:30 PM to 6:00 PM), with water (chamber temperature) and food being consumed ad libitum throughout. Each work bout consisted of three 30-min periods of treadmill walking (3 mph, 2% grade) designed to elicit a metabolic rate equal to ~360 W (moderate-intensity work (26), separated by 10-min seated rest periods. This work-to-rest allocation (3:1) is currently recommended by the ACGIH TLV® to prevent dangerous rises in core temperature (≥38.0°C) under such conditions (21). This simulation has been shown to provide an accurate representation of the work and environmental conditions, and thus, the physiological demands experienced by workers in arduous occupations (e.g., mining, firefighting, electric utilities) (1–4). Further, the combined duration of the heat stress test (~2.5 h) and work simulation (~7.5 h) ensured that participants experienced the physiological strain associated with a prolonged work day in the heat (~10 h).

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Standardization

Before the first heat stress test (day 1), participants refrained from exercise, alcohol or caffeine and nonsteroidal anti-inflammatory drugs for >24 h and were instructed to consume ∽200 to 500 mL of water ∽2 h before arriving at the laboratory. During the recovery and sleep period (6:00 PM, day 1 to 7:00 AM, day 2) completed before the second heat stress test (7:00 AM to 10:00 AM, day 2), participants were instructed to consume food and fluid ad libitum, retain their routine sleeping patterns, and refrain from exercise, alcohol, caffeine, and medication. However, because participants may be inclined to alter their food and fluid consumption when being recorded (27), this was not monitored during the recovery and sleep period.

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Measurements

During each heat stress test, the modified Snellen direct air calorimeter was used to measure time-dependent changes in whole-body heat exchange (evaporative and dry heat loss) (28,29). The calorimeter inflow and outflow values of absolute humidity and air temperature were collected at 8-s intervals. Absolute humidity was measured using high precision dew point hygrometry (RH Systems model 373H, Albuquerque, NM), whereas air temperature was measured using high-precision resistance temperature detectors (Black Stack model 110V 50/60H2; Fluke Corporation, Everett, WA). Air mass flow through the calorimeter was measured by differential thermometry over a known heat source placed in the effluent air stream. The real-time data for absolute humidity, air temperature, and air mass flow were displayed and recorded on a personal computer with LabVIEW software (Version 7.0; National Instruments, Austin, TX). The rate of evaporative heat loss was calculated using the calorimeter outflow–inflow difference in absolute humidity, multiplied by the air mass flow (kg·s−1) and the latent heat of vaporization of sweat (2426 J·g−1). The rate of dry heat loss was calculated using the calorimeter outflow–inflow difference in air temperature, multiplied by the air mass flow and specific heat capacity of air (1005 J·kg−1·°C−1). Dry and evaporative heat losses were expressed as positive values, with a negative value for dry heat loss representing environmental heat gain (i.e., when ambient temperature exceeds skin temperature). Oxygen consumption, carbon dioxide production, and minute ventilation were derived continuously from measures of expired gases and air flows (Moxus modular metabolic system; AEI Technologies, Bastrop, TX) and used to approximate metabolic energy expenditure (29). External work rate was controlled to maintain metabolic heat production during exercise at 150 (Ex1), 200 (Ex2), and 250 W·m−2 (Ex3). To account for respiratory heat exchange, expired air was recycled back into the calorimeter.

During each heat stress test, rectal (core) temperature was measured by inserting a thermocouple probe (Mon-a-therm General Purpose Temperature Probe; Mallinckrodt Medical, St. Louis, MO) to a minimum of 12 cm past the anal sphincter. Because of technical difficulties in one trial, core temperatures are not reported for all individuals (n = 8). Skin temperature was measured continuously at four sites (bicep, chest, thigh, and calf) using T-type thermocouples (Concept Engineering, Old Saybrook, CT) attached to the skin with surgical tape. Temperature data were collected at 15-s intervals (HP Agilent data-acquisition module, model 2497A). During the work simulation, core temperature was monitored within the lower gut using a capsule thermometer (Vital Sense ingestible capsule thermometer; Mini Mitter, Bend, OR) ingested approximately 3.5 h before data collection (i.e., before the heat stress test on day 1), and recorded at 10-min intervals using a portable logger (VitalSense; Mini Mitter Company, Bend, OR). Heart rate was continuously recorded and saved using a Polar coded WearLink and transmitter, Polar M400 watch, Polar H7 wearlink, and Polar FlowSync software (Polar Electro Oy, Finland).

A urine sample was collected upon arrival to the laboratory on each day (~7:00 AM, days 1 and 2) for the measure of urine specific gravity (Reichert TS 400 total solids refractometer, Reichert, Depew, NY). Venous blood samples were collected via venipuncture after ~30 min seated rest before (n = 9) and immediately after (n = 8) each heat stress test. Blood samples were transferred directly into serum with no additive and plasma K2EDTA 5.4 mg BD Vacutainer® tubes (BD, Franklin Lakes, NJ). Nonadditive blood sat for 20 min to clot before centrifugation at approximately 3300 rpm (relative centrifugal force, 1380g) for 10 min, whereas the K2EDTA blood was mixed by inversion, used to measure blood hemoglobin and hematocrit (Beckman Coulter, Miami, FL), and centrifuged immediately. After centrifugation, serum was separated and transferred into polypropylene Eppendorf tubes, frozen at −20°C, and stored at −70°C. Pretrial serum samples were analyzed with a Micro-Osmometer (Advanced® model 3320; Norwood, MA) to determine serum osmolality. Blood hemoglobin and hematocrit values were used to estimate percent changes in plasma and blood volume (30). Nude mass was obtained (IND560, Mettler Toledo Inc., Mississauga, ON, Canada) immediately before and after each heat stress test. Clothed mass was obtained (WB-100A; Tanita Corporation, Tokyo, Japan) before and after each work bout within the work simulation and corrected for food and fluid consumption and urine production to estimate whole-body sweat rate (31). Pretrial urine specific gravity, nude body mass, serum osmolality, and hemoglobin-to-hematocrit ratio were used to evaluate hydration state.

Thermal sensation was determined using the ASHRAE 7-point scale (0 [neutral] to 7 [very, very hot]), whereas the RPE was measured using the Borg 14-point scale (6 [no exertion] to 20 [maximal exertion]) (32). These data were obtained at 15-min intervals during each heat stress test and at 10-min intervals throughout the work simulation. An 18-item fatigue questionnaire with established validity and reliability in healthy adults (33) was used to assess perceived fatigue and energy levels on a 10-point scale (100-mm visual analog scale) before each heat stress test (~7:00 AM). Fatigue severity was calculated as the mean of 13 items in the fatigue subscale (higher scores indicating greater fatigue), whereas the remaining five items (energy subscale) were averaged to obtain an energy score (higher scores indicating greater energy).

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Data analysis

Dry and evaporative heat loss, core and mean skin temperatures, and heart rate were measured throughout each heat stress test (day 1 and day 2) and expressed as minute averages, with an average of the final 5 min of each exercise and recovery period used for statistical analyses. Data for exercise and recovery were analyzed separately. Baseline values were obtained by averaging the last 5 min of data recorded during the 15-min baseline period. The net change in body heat content (storage) during each exercise and recovery period was measured as the temporal summation of total heat loss (evaporative ± dry heat exchange) and metabolic heat production. Mean skin temperature was approximated from a weighted average of the four local skin temperatures (bicep, 30%; chest, 30%; thigh, 20%; calf, 20%) (34). During the work simulation, core temperature, heart rate, thermal sensation, and perceived exertion were expressed as the average and peak response obtained during each of the three work bouts.

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Statistical analysis

Data collected or derived during each heat stress test (evaporative, dry and total heat loss, body heat content, core and mean skin temperatures, heart rate, and perceptual strain) were analyzed using a two-way, repeated measures ANOVA with the factors of test day (day 1, day 2) and either exercise (Ex1, Ex2, Ex3) or recovery time (Rec1, Rec2, Rec3). Data collected during each work bout within the work simulation (core temperature, heart rate, perceptual strain, rate of fluid consumption and whole-body sweat rate) were compared using a one-way, repeated-measures ANOVA, with the factor of time (work bout 1, 2, 3). When a significant interaction or main effect was detected, post hoc comparisons were carried out using paired t tests. For comparisons between time points, the P value was adjusted with the Bonferroni procedure as a multiple comparison. Data collected at single time points (perceived fatigue and energy scales, urine specific gravity, pretrial body mass, serum osmolality, hemoglobin-to-hematocrit ratio, changes in body mass, plasma volume and blood volume) were compared between test days using paired t-tests. An a priori power analysis indicated that based on the effect size (Cohen d = 1.33) for an approximately 20% difference in core temperature with a standard deviation of approximately 15% between test days (9), a minimum of seven subjects were required to detect between-group differences in total heat loss (primary variable of interest) of this effect size with at least 80% statistical power. Therefore, with the current sample (n = 9), these analyses were adequately powered (>80%). Alpha was set at 0.05 for all statistical comparisons, with data being reported as means ± SD. All analyses were performed using SPSS 24.0 (IBM, Armonk, NY).

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RESULTS

Work simulation

Average and peak core temperature and heart rate, as well as average thermal sensation increased throughout the work simulation (main effect of time: all P ≤ 0.05; Table 1). Although whole-body sweat rate, average and peak rating of perceived exertion, and peak thermal sensation were similar between work bouts (all P > 0.05), the rate of fluid consumption decreased over the course of the work simulation (P = 0.04; Table 1).

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Calorimetry

Metabolic heat production and total heat loss as well as the resulting changes in body heat storage are presented in Figure 1, whereas external work and evaporative and dry heat loss are shown in Table 2. Metabolic heat production and total heat loss as well as evaporative and dry heat loss were similar between days 1 and 2 at baseline (all P > 0.05). Metabolic heat production and dry heat exchange during exercise and recovery as well as external work during exercise increased over time (all P < 0.05), but did not differ between days (all P > 0.05). Metabolic heat production averaged 154 ± 6, 197 ± 8, and 249 ± 8 W·m−2 during Ex1, Ex2, and Ex3 across test days, respectively. Evaporative and total heat loss as well as the changes in body heat content during exercise and recovery also increased over time (all P < 0.01). Evaporative and total heat loss were similar between test days during all recovery bouts and Ex1 (all P > 0.05); however, those heat losses were reduced on day 2 relative to day 1 during both Ex2 and Ex3 (all P < 0.01). As a result, the change in body heat content was also similar between test days during recovery and Ex1 (all P > 0.05), but greater on day 2 relative to day 1 during Ex2 and Ex3 and across the duration of the 2.5-h protocol (i.e., total heat storage; all P ≤ 0.05).

FIGURE 1

FIGURE 1

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Thermometry

Baseline core and mean skin temperatures were similar between days (both P < 0.05). However, there was a time by test day interaction for core temperature and the change in core temperature from baseline during both exercise and recovery (all P > 0.05; Fig. 2), with core temperature being greater on day 2 relative to day 1 during Ex3 (P = 0.03) and the change in core temperature being greater on day 2 relative to day 1 during both the second and third exercise and recovery bouts (all P > 0.05). Nonetheless, mean skin temperature during both exercise and recovery did not differ over time or between test days (all P ≤ 0.05), averaging 34.9°C ± 0.4°C at baseline, 35.1°C ± 0.5°C during exercise and 34.9°C ± 0.5°C during recovery across test days.

FIGURE 2

FIGURE 2

TABLE 1

TABLE 1

TABLE 2

TABLE 2

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Heart rate, perceptual strain, and perceived fatigue

Heart rate and perceptual strain data are presented in Table 2. Baseline heart rate and thermal sensation were similar between days, averaging 62 ± 5 bpm and 2 ± 1 across days, respectively. Heart rate during both exercise and recovery, as well as thermal sensation during exercise increased over time (all P ≤ 0.05), but did not differ between days (all P > 0.38). Rating of perceived exertion also increased over time (P < 0.01) and was similar between days during Ex1 and Ex2 (both P > 0.05), but was greater on day 2 relative to day 1 during Ex3 (P = 0.03). Pretrial perceived fatigue and energy levels did not differ between day (both P > 0.05), averaging 5 ± 1 and 8 ± 1 across days, respectively.

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Hydration state

Pretrial urine specific gravity and serum osmolality increased from day 1 (1.017 ± 0.006 mOsm·kg−1 and 286 ± 4 mOsm·kg−1, respectively) to day 2 (1.021 ± 0.009 mOsm·kg−1 and 289 ± 5 mOsm·kg−1, respectively; both P ≤ 0.05) and pretrial body mass tended to be lower on day 2 (76.1 ± 5.8 kg) relative to day 1 (76.6 ± 6.0 kg; P = 0.08). However, pretrial hemoglobin-to-hematocrit ratio was similar between days (P = 0.82), averaging 3.3 ± 0.1. Changes in body mass, plasma volume and blood volume over the course of each heat stress test were also similar between test days (all P > 0.05), averaging −1.6 ± 0.2 kg, −11.9% ± 4.6% and −7.2% ± 2.7% (respectively), across days.

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DISCUSSION

In the present study, whole-body heat loss and heat storage were assessed in older men during heat stress tests involving intermittent exercise at increasing, fixed rates of metabolic heat production performed on the same day before (day 1), and on the day after (day 2) a simulated work day in the heat. With this approach, all individuals were exposed to the physiological strain associated with a prolonged (~10 h), arduous work shift before being assessed under matched work and environmental conditions on the next day. We observed two novel and important outcomes. First, as hypothesized, total heat loss was reduced on day 2 relative to day 1 (Fig. 1A), with these impairments occurring during the second and third exercise bouts (200 and 250 W·m−2, respectively). Second, these reductions in total heat loss exacerbated the change in body heat storage (Fig. 1B) and core temperature (Fig. 2B) during those exercise bouts as well as the total change in body heat storage across the 2.5 h protocol (Fig. 1B) on day 2 compared with day 1. Taken together, our findings indicate that a prolonged work day in the heat can compromise thermoregulatory function and exacerbate the risk of heat illness on the next day in older workers, particularly during work eliciting moderate-to-high net heat loads.

To elicit the physiological strain associated with a prolonged work day in the heat, participants performed a simulation involving three, 2-h work bouts at a 3:1 work-to-rest allocation, each separated by 30-min rest breaks, in hot, dry conditions (wet-bulb globe temperature: ~29°C). This simulation has been used previously to replicate the physiological demands of arduous work in young men (11), and it also elicited core temperature, heart rate, and perceptual strain responses in the current sample of older adults (Table 1) that were similar to those observed in previous field studies of electric utilities and mining throughout a work shift (2,3,10). This was an essential and unique design feature of the current study, as previous evaluations of thermoregulatory function in older adults over consecutive days have generally only evaluated those responses during short-duration exercise in the heat (≤1.5 h) that does not replicate the physiological strain of a prolonged, arduous work shift (15–18). However, even though each work bout was performed at the work-to-rest allocation recommended by the ACGIH TLV® to prevent excessive heat strain (i.e., as defined by increases in core temperature ≥38°C for extended periods) in adequately hydrated workers in these environmental conditions, peak core temperature exceeded this threshold during the second and third work bouts (Table 1). This outcome is consistent with recent reports that the TLV® do not adequately protect either young (35) or older adults (36) and confirms that these guidelines require refinement to prevent excessive heat strain during prolonged (≥4 h) work in the heat. Nonetheless, it is important to note that the elevated peak core temperature observed during work bout three was also coupled with a reduction in fluid consumption relative to work bout one, despite similar whole-body sweat rates between those work bouts (Table 1). As such, it is possible that the observed rise in peak core temperature above the upper TLV® threshold (≥38°C) during this work bout was explained, in part, by a rate of ad libitum fluid consumption that was insufficient to replace sweat losses, leading to voluntary dehydration (37). This outcome is consistent with the suggestion that older adults have reduced thirst perception relative to their young counterparts (38) and indicates that programmed fluid consumption and/or better education on the importance of adequate fluid consumption is required to prevent the additional heat strain associated with dehydration in older workers (39,40).

Although several studies have reported that occupational heat stress on the preceding day elevates the risk of exertional heat illness (6–8) and exacerbates heat strain during arduous work in the heat (9,10), a recent evaluation of these carryover effects conducted by our group in young men revealed that a prolonged work day in the heat does not significantly modify whole-body heat loss or heat storage on the next day (11). However, because older adults display reduced thermoregulatory function during exercise in the heat relative to their young counterparts (20), we anticipated that prolonged heat strain would compromise heat loss capacity on the next day. Further, given that the magnitude of age-related impairments in whole-body heat loss have been shown to increase as a function of the combined exercise and environmental (net) heat load (13,14), we also expected the magnitude of these next-day reductions in heat loss to increase at higher net heat loads. This hypothesis was accepted (Fig. 1A), with total heat loss being similar between test days during the first exercise bout, but lowered by ~5% on day 2 relative to day 1 during both exercise bouts 2 and 3. Because dry heat exchange did not differ between test days during exercise or recovery (Table 2), these reductions in total heat loss could be primarily ascribed to reductions in evaporative heat loss from day 1 to day 2 during those exercise bouts. Despite being relatively small (equal to a between-day difference in whole-body sweat rate of ~20 g·h−1 within each exercise bout), these differences may be exacerbated during more prolonged work, leading to large impairments in heat dissipation over the course of a work shift (8–12 h). Although the mechanism explaining these next-day reductions in evaporative heat loss is likely multifactorial, it is possible that mild hypohydration on day 2 lead to reduced sweat secretion at the higher heat loads employed (40). This was evidenced by an increase in pretrial urine specific gravity and serum osmolality as well as a trend for a reduction in pretrial body mass from day 1 to day 2. Given that participants were instructed to consume food and fluid ad libitum during the rest and recovery period between test days, this indicates that participants were not inclined to replace fluid losses occurring on day 1. Although additional research is needed to confirm this possibility, this outcome reinforces the need for better education on the importance of fluid replacement and/or programed fluid consumption guidelines during work as well as before and after work (40,41). Indeed, such strategies may provide a means to prevent or minimize the additional heat strain associated with performing back-to-back days of arduous work in the heat among older workers.

The impairments in total heat loss during the second and third exercise bouts on day 2 relative to day 1 caused a 15% to 27% increase in body heat storage (Fig. 1B) and core temperature change (Fig. 2B) during those exercise bouts as well as a rise (~31%) in total body heat storage across the 2.5-h protocol (Fig. 1B). These outcomes are consistent with the greater rises in core temperature (15%–36%) reported on the second of consecutive work days in recent field studies involving some older firefighters (31 ± 8 yr) (9) and electric utilities workers (38 ± 12 yr) (10), but in contrast to our recent work involving only young men (26 ± 4 yr; 11). Taken together, these findings indicate that prolonged, physically demanding work in the heat impairs thermoregulatory function on the following work day in older (≥54 yr), but not young adults (18–30 yr). Although further studies designed to compare individuals of varying age (i.e., young, middle-age, older) over consecutive work days in the heat are required to confirm this suggestion, this information markedly improves our understanding of thermoregulatory function in older adults during heat stress and indicates that modification to current workplace heat exposure guidelines (e.g., ACGIH TLV®) may be required to better protect older workers required to perform consecutive shifts in high heat stress conditions.

In our recent work on the carryover effects of prolonged work in the heat on in young men (11), participants reported greater perceived fatigue and reduced perceived energy levels before the heat stress test performed on day 2 relative to day 1, despite displaying similar whole-body heat loss and body heat storage between test days. In contrast, the older adults in the current study reported similar pretrial perceived fatigue and energy levels between test days, but showed reduced heat loss capacity and exacerbated heat storage on day 2 relative to day 1 (Fig. 1). Although it is not possible to perform a direct comparison between these groups due to differences in physical characteristics and the rates of metabolic heat production during the heat stress tests performed on days 1 and 2, these outcomes indicate that older workers may be unable to perceive their compromised ability to dissipate heat after a prolonged work day in the heat. This notion is consistent with previous reports that older adults display a lowered thirst sensitivity (38) and altered thermal perception (41) relative to their young counterparts.

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Considerations

This experiment was designed to provide a controlled, yet representative evaluation of the combined effects of prolonged work and heat exposure common to many industries on whole-body heat loss on the following day. Thus, it remains unknown if the observed next-day impairments in thermoregulatory function can be ascribed to the physiological demands of performing arduous work per se, heat exposure, excessive fluid depletion with inadequate fluid replacement (i.e., hypohydration), or a combined effect of two or more of these factors. To fully evaluate the underlying mechanisms, it would be necessary to assess the separate and combined effects of these factors on whole-body heat loss and the subsequent changes in body heat storage on the following work day. Although workers would rarely experience such effects in isolation, this research may have important mechanistic outcomes.

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Perspectives

The current study was directed to evaluating the carryover effects of a single, prolonged work day in the heat on thermoregulatory function on the next day. However, workers in arduous occupations are commonly exposed to prolonged and excessive heat strain for ≥5 consecutive days. Given that whole-body heat loss was reduced after a single prolonged work day in the heat in the current study (Fig. 1A), it is reasonable to expect that thermoregulatory function may be further compromised after two or more prolonged work days in the heat. At the same time, it is known that repeated heat strain (i.e., heat adaptation) can enhance whole-body heat loss (42) and potentially negate any next-day impairments in heat loss that exacerbate body heat storage on the second of consecutive work days. However, similar increases in the rise in body core temperature on the second of consecutive work days have been observed in field studies of firefighters (9) and electric utilities workers (10), who likely possess some form of partial heat adaptation given the nature of their daily work. It is likely, therefore, that the effects of prolonged heat strain over two or more consecutive work days may further compromise the body’s physiological capacity to dissipate heat, even in heat acclimatized workers. In view of our current study findings, this represents an important area of future research.

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CONCLUSIONS

While the physiological demands associated with performing arduous work in high heat stress conditions are well studied, we show, perhaps for the first time, that a prolonged work day in the heat can impair whole-body heat loss and exacerbate heat strain on the next work day in older adults, particularly during physically demanding work. These findings suggest that older workers employed in arduous occupations may be at greater risk of heat-related illness on the second of consecutive work days, unless current guidelines for working safely in the heat are modified to consider age-related and next-day impairments in thermoregulatory function. However, the mechanism(s) explaining these carryover effects or whether these impairments are exacerbated over two or more consecutive work days in the heat remains unclear.

The authors thank all the participants who volunteered for the present study and the members of the Human and Environmental Physiology Research Unit who assisted with data collection.

The study was conducted in the Human and Environmental Physiology Research Unit (HEPRU). This research was in part supported by the Ontario Ministry of Labour and the Natural Sciences and Engineering Research Council of Canada as well as support provided by the Electric Power Research Institute (all funds held by Dr. Glen P. Kenny). G. P. Kenny is supported by a University of Ottawa Research Chair. S. R. Notley is supported by a Postdoctoral Fellowship from the Human and Environmental Physiology Research Unit. R. D. Meade is supported by a Natural Sciences and Engineering Research Council of Canada Alexander Graham Bell Scholarship (CGS-D). A. W. D’Souza is supported by a Queen Elizabeth II Graduate Scholarship in Science and Technology. B. J. Friesen is supported by an Ontario Graduate Scholarship.

No conflict of interest, financial or otherwise, are declared by the author(s). The results of the present study do no constitute endorsement by ACSM. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.

S. R. N. and G. P.K. conceptualized and designed the research. S. R. N., R. D. M., A. W. D., and B. J. F. performed the experiments. S. R. N. analyzed the data. S. R. N., R. D. M., A. W. D., B. J. F., and G. P. K. interpreted results of experiments. S. R. N. prepared the figures. S. R. N. drafted the article. S. R. N., S. R. N., R. D. M., A. W. D., B. J. F., and G. P. K. edited and revised the article. S. R. N., R. D. M., A. W. D., B. J. F., and G. P. K. approved the final version of the article.

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    Keywords:

    CALORIMETRY; HEAT EXCHANGE; HEAT STRAIN; RECOVERY

    © 2018 American College of Sports Medicine