An interventional study of Constant Heat Exposure and Progressively Increasing Heat Exposure in Body Temperature Changes, Heat Stress and Heat Exposure, sponsored by University of Ottawa. Not yet recruiting at 1 site in Canada. Open to male participants aged 45 Years to 65 Years, including healthy volunteers. Per ClinicalTrials.gov, last updated 2026-09-29.
Sponsored by University of Ottawa · Not applicable, Interventional, and Basic science
Occupational heat-stress guidelines are commonly based on the level of environmental heat exposure and are largely derived from studies conducted under constant environmental conditions. However, outdoor workers are typically exposed to dynamic conditions, with environmental heat often increasing progressively from morning to afternoon. Consequently, workers may experience substantially different patterns of heat exposure across the workday even when average daily heat exposure is equivalent. It remains unclear whether the temporal pattern of heat exposure influences the amount of work that can be performed or the physiological strain experienced during a workday. Understanding these responses is important for determining whether occupational heat-stress guidelines developed under constant laboratory conditions adequately protect workers exposed to progressively changing environmental conditions.
The primary objective of this study is to determine whether the temporal pattern of heat exposure influences work performed and physiological heat strain during a simulated occupational workday in healthy middle-aged and older men. Participants will complete two 9-hour simulated workdays consisting of a 4-hour morning work period, a 1-hour lunch break, and a 4-hour afternoon work period. During one trial, wet-bulb globe temperature (WBGT) will remain constant at 28°C throughout the workday, whereas during the other trial, WBGT will progressively increase from morning to afternoon to simulate the changing environmental conditions of a typical hot summer workday. Both trials will have the same mean daily WBGT of 28°C. Participants will perform moderate-intensity treadmill exercise, and measures including total work performed, core temperature, heart rate, and time to the initial stay-time threshold will be assessed to determine whether constant and progressively increasing heat exposures produce different physiological and work-performance responses across the workday.
Exclusion Criteria:
Participants will complete a 9-hour simulated occupational workday consisting of a 4-hour morning work period, a 1-hour seated lunch-recovery period, and a 4-hour afternoon work period. Wet-bulb globe temperature (WBGT) will remain constant at 28°C throughout the workday. During each work period, participants will perform moderate-intensity treadmill walking at a metabolic heat production of approximately 200 W·m-2. Participants will initially work continuously until reaching the initial stay-time threshold, defined as a core temperature of 38.0°C or an increase of 1.0°C above pre-work baseline, whichever occurs first. Thereafter, participants will complete 45-min work and 15-min seated rest cycles for the remainder of the work period.
Other: Constant Heat Exposure
Participants will complete a 9-hour simulated occupational workday consisting of a 4-hour morning work period, a 1-hour seated lunch-recovery period, and a 4-hour afternoon work period. WBGT will progressively increase from morning to afternoon to simulate the changing environmental conditions of a hot summer workday, while maintaining a mean daily WBGT of 28°C. During each work period, participants will perform moderate-intensity treadmill walking at a metabolic heat production of approximately 200 W·m-2. Participants will initially work continuously until reaching the initial stay-time threshold, defined as a core temperature of 38.0°C or an increase of 1.0°C above pre-work baseline, whichever occurs first. Thereafter, participants will complete 45-min work and 15-min seated rest cycles for the remainder of the work period.
Other: Progressively Increasing Heat Exposure
Participants complete a simulated occupational workday under constant environmental conditions, with WBGT maintained at 28°C throughout the morning and afternoon work periods.
Participants complete a simulated occupational workday in which WBGT progressively increases from morning to afternoon while maintaining a mean daily WBGT of 28°C.
Total work performed
Total work completed during each 240-minute morning and afternoon work period, expressed as cumulative minutes of treadmill walking. Each work period consists of an initial continuous walking phase until core temperature reaches 38.0°C or increases by 1.0°C above the pre-work baseline, whichever occurs first, followed by intermittent 45-min work and 15-min seated rest cycles.
Time frame: From the start to the end of the morning and afternoon work periods during each experimental trial (0-4 hours and 5-9 hours after the start of the trial, respectively).
Initial stay time
Time, in minutes, from the start of continuous treadmill walking until the initial stay-time threshold is reached, defined as a core temperature of 38.0°C or an increase of 1.0°C above the pre-work baseline, whichever occurs first. Participants who do not reach the threshold during the 240-minute work period will be identified as not having reached the threshold within 240 min.
Time frame: During the initial continuous walking phase of each morning and afternoon work period.
Core temperature
Core temperature measured continuously using a rectal thermocouple temperature probe to assess thermoregulatory strain throughout the simulated workday.
Time frame: Continuously throughout each experimental trial, including the morning work period, lunch recovery, and afternoon work period.
Average core temperature during work-rest cycles
Average core temperature during the intermittent 45-min work and 15-min seated rest cycles following attainment of the initial stay-time threshold.
Time frame: During the work-rest cycles of each morning and afternoon work period.
Mean skin temperature
Mean skin temperature calculated from temperatures measured at eight body sites using ISO 9886 weighting factors.
Time frame: Assessed throughout each experimental trial, including the morning work period, lunch recovery, and afternoon work period.
Heart rate
Heart rate measured continuously using a chest-strap heart rate monitor and expressed as absolute heart rate.
Time frame: Continuously throughout each experimental trial, including the morning work period, lunch recovery, and afternoon work period.
Percentage of heart rate reserve
Heart rate expressed as a percentage of the participant's heart rate reserve to assess relative cardiovascular strain during heat exposure.
Time frame: Throughout each experimental trial, including the morning work period, lunch recovery, and afternoon work period.
Systolic blood pressure
Systolic blood pressure measured at the brachial artery using automated oscillometry. Three measurements obtained at approximately 1-min intervals will be averaged for analysis.
Time frame: Assessed at predetermined intervals throughout each experimental trial.
Diastolic blood pressure
Diastolic blood pressure measured at the brachial artery using automated oscillometry. Three measurements obtained at approximately 1-min intervals will be averaged for analysis.
Time frame: Assessed at predetermined intervals throughout each experimental trial.
Heart rate variability: standard deviation of normal R-R intervals (SDNN)
Cardiac autonomic modulation assessed from consecutive R-R intervals obtained using a 5-lead Holter electrocardiogram to calculate the standard deviation of normal R-R intervals (SDNN), expressed in milliseconds.
Time frame: Throughout each experimental trial.
Heart rate variability: root mean square of successive differences (RMSSD)
Cardiac autonomic modulation assessed from consecutive R-R intervals obtained using a 5-lead Holter electrocardiogram to calculate the root mean square of successive differences between adjacent normal R-R intervals (RMSSD), expressed in milliseconds.
Time frame: Throughout each experimental trial.
Urine specific gravity
Urine specific gravity measured to assess hydration status upon arrival at the laboratory. Higher values indicate greater urine concentration, whereas lower values indicate lower urine concentration.
Time frame: Assessed upon arrival before each experimental trial.
Fluid consumption
Cumulative fluid consumption determined from all fluid consumed during each experimental session.
Time frame: Throughout each experimental trial.
Fluid loss from body mass change
Fluid loss calculated from changes in body mass during each daytime work exposure, corrected for food and fluid intake.
Time frame: Assessed at the end of each 240-minute morning and afternoon work period of each experimental trial.
Thermal sensation
Thermal sensation assessed using an 8-point scale ranging from 0 (neutral) to 7 (extremely hot). Higher scores indicate a greater sensation of heat.
Time frame: Assessed at baseline, every 10 min during initial continuous work, at attainment of the initial stay-time threshold, at the end of the work and recovery components of each work-rest cycle, and at the beginning and end of lunch recovery.
Thermal comfort
Thermal comfort assessed using the ISO 10551 scale ranging from 1 (comfortable) to 4 (very uncomfortable). Higher scores indicate greater thermal discomfort.
Time frame: Assessed at baseline, every 10 min during initial continuous work, at attainment of the initial stay-time threshold, at the end of the work and recovery components of each work-rest cycle, and at the beginning and end of lunch recovery.
Thirst sensation
Thirst sensation assessed using a 9-point scale ranging from 1 (not thirsty at all) to 9 (very, very thirsty). Higher scores indicate greater perceived thirst
Time frame: Assessed at baseline, every 10 min during initial continuous work, at attainment of the initial stay-time threshold, at the end of the work and recovery components of each work-rest cycle, and at the beginning and end of lunch recovery.
Affective state
Affective state will be assessed using the 11-point Feeling Scale ranging from -5 (very bad) to +5 (very good), with 0 representing neutral. Higher scores indicate more positive affective state.
Time frame: Assessed at baseline, every 10 min during initial continuous work, at attainment of the initial stay-time threshold, at the end of the work and recovery components of each work-rest cycle, and at the beginning and end of lunch recovery.
Perceived arousal
Perceived arousal assessed using a 7-point scale ranging from 1 (low arousal) to 7 (high arousal). Higher scores indicate greater perceived arousal.
Time frame: Assessed at baseline, every 10 min during initial continuous work, at attainment of the initial stay-time threshold, at the end of the work and recovery components of each work-rest cycle, and at the beginning and end of lunch recovery.
Rating of perceived exertion
Perceived exertion assessed using the Borg Rating of Perceived Exertion scale ranging from 6 (no exertion at all) to 20 (maximal exertion). Higher scores indicate greater perceived exertion.
Time frame: Assessed at baseline, every 10 min during initial continuous work, at attainment of the initial stay-time threshold, and at the end of the work and recovery components of each work-rest cycle.
Environmental Symptoms Questionnaire-IV (ESQ-IV) total symptom score
Symptoms associated with heat exposure will be assessed using the Environmental Symptoms Questionnaire-IV (ESQ-IV). Responses will be combined to generate a total symptom score, with higher scores indicating greater environmental symptom burden.
Time frame: Assessed at baseline and following completion of each experimental trial.
Profile of Mood States-40 (POMS-40) total mood disturbance score
Mood state will be assessed using the abbreviated 40-item Profile of Mood States questionnaire (POMS-40). Responses will be used to calculate a total mood disturbance score, with higher scores indicating greater overall mood disturbance.
Time frame: Assessed at baseline and following completion of each experimental trial.
Changes in plasma volume
Change in plasma volume from baseline values calculated from duplicate measurements of hemoglobin and hematocrit at the start and end of each exposure using the technique by Dill and Costill.
Time frame: Assessed as baseline, and following each 240-minute morning and afternoon work period.
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University of Ottawa