Extreme heat events are becoming more frequent, intense, and prolonged, increasing the risk of heat-related illness among workers performing physically demanding work in hot environments. Occupational heat-stress guidelines use work-rest schedules to limit increases in core temperature and physiological strain, but they do not clearly define the safe maximum duration of continuous work before heat-mitigation controls should begin, known as the initial stay time. Recent work has generated initial stay time recommendations for young and older workers performing moderate-intensity work in the heat; however, these recommendations were developed under normoxic conditions. In many workplaces, heat may occur alongside additional stressors such as hypoxia, including work at altitude or in oxygen-reduced environments. Hypoxia can increase cardiovascular strain, reduce arterial oxygen saturation, and impair exercise tolerance, potentially compromising heat-stress guidance developed under normal oxygen conditions. Older workers may be especially vulnerable because of age-related reductions in thermoregulatory and cardiovascular function. It remains unclear whether hypoxia increases physiological strain during work in the heat or whether current initial stay time and recovery recommendations remain protective under combined heat and hypoxic stress. The primary objective of this study is to determine whether hypoxia increases physiological strain during moderate-intensity work in the heat in young and older males. Specifically, the study will assess whether normoxia-derived initial stay times and recovery recommendations for work at 29°C wet-bulb globe temperature (37.5°C, 35% RH) remain effective during exposure to 14% inspired oxygen, simulating approximately 3000 m altitude. Healthy young and older males will complete two simulated occupational heat exposures: heat alone and heat plus hypoxia. Participants will perform moderate-intensity treadmill work based on age-specific initial stay times, followed by prescribed work-rest cycles and seated recovery. Core temperature, cardiovascular strain, oxygen saturation, skin temperature, hydration status, perceptual responses, mood, environmental stress symptoms, and cognitive function will be assessed to determine whether hypoxia modifies responses to work in the heat. Findings will help determine whether occupational heat-stress guidance requires refinement for workers exposed to combined heat and hypoxic stress.
Study Type
INTERVENTIONAL
Allocation
RANDOMIZED
Purpose
BASIC_SCIENCE
Masking
SINGLE
Enrollment
20
Participants complete a simulated moderate-intensity occupational heat exposure protocol involving treadmill walking in a hot environment (37.5°C, 35% RH) under normoxic conditions (\~20.9% inspired oxygen). Participants walk until they reach their initial stay time, followed by prescribed work-rest cycles and seated recovery.
Participants complete a simulated moderate-intensity occupational heat exposure protocol involving treadmill walking in a hot environment (37.5°C, 35% RH) with an added hypoxic exposure equivalent to 14% inspired oxygen. Participants walk until they reach their initial stay time, followed by prescribed work-rest cycles and seated recovery.
University of Ottawa
Ottawa, Ontario, Canada
RECRUITINGTotal work volume
Total work completed during the 240-minute work bout. The work period consists of an initial continuous walking phase to a core temperature 38.0°C or 1°C increase above baseline resting, followed by intermittent work-rest cycles.
Time frame: During the 240-minute work period
Time to core temperature of 38.0°C
Time, in minutes, from the start of continuous walking to the point at which core temperature reaches 38.0°C. Participants who do not reach 38.0°C before voluntary termination, reaching the applicable stopping criterion, or completing 240 minutes of continuous walking will have their time recorded at termination or completion.
Time frame: During the 240-minute work period
Time to 1.0°C increase in core temperature
Time, in minutes, from the start of continuous walking to the point at which rectal temperature increases by 1.0°C above baseline. For participant safety, continuous walking will not continue beyond a rectal temperature of 38.5°C. Therefore, for participants who reach 38.5°C before attaining a 1.0°C increase in rectal temperature, the time at which 38.5°C is reached will be recorded. Participants who voluntarily terminate exercise or complete 240 minutes of continuous walking before reaching either endpoint will have their time recorded at termination or completion.
Time frame: During the 240-minute work period
Average core temperature during work/rest cycles
Average core temperature measured during the intermittent work-rest cycles following the initial continuous work period.
Time frame: During the 240-minute work period
Heart rate at core temperature of 38.0°C
Heart rate measured at the time a participant reaches a rectal temperature of 38.0°C during the initial continuous walking period. For participants who do not reach a rectal temperature of 38.0°C before voluntary termination, reaching the applicable stopping criterion, or completing 240 minutes of continuous walking, heart rate at termination or completion will be recorded.
Time frame: During the 240-minute work period
Heart rate at 1.0°C increase in core temperature
Heart rate measured at the time a participant achieves a 1.0°C increase in core temperature above baseline during the initial continuous walking period. For participant safety, continuous walking will not continue beyond a core temperature of 38.5°C. Therefore, for participants who reach a core temperature of 38.5°C before attaining a 1.0°C increase above baseline, the heart rate at 38.5°C will be recorded. Participants who voluntarily terminate exercise or complete 240 minutes of continuous walking before reaching either endpoint will have their final recorded heart rate used.
Time frame: During the 240-minute work period
Average heart rate during work/rest cycles
Average heart rate measured during the intermittent work-rest cycles following the initial continuous work period.
Time frame: During the 240-minute work period
Mean skin temperature at core temperature of 38.0°C
Mean skin temperature measured at the time a participant reaches a core temperature of 38.0°C during the initial continuous walking period. For participants who do not reach a core temperature of 38.0°C before voluntary termination, reaching the applicable stopping criterion, or completing 240 minutes of continuous walking, mean skin temperature at termination or completion will be recorded.
Time frame: During the 240-minute work period
Mean skin temperature at 1.0°C increase in core temperature
Mean skin temperature measured at the time a participant achieves a 1.0°C increase in core temperature above baseline during the initial continuous walking period. For participant safety, continuous walking will not continue beyond a core temperature of 38.5°C. Therefore, for participants who reach a core temperature of 38.5°C before attaining a 1.0°C increase above baseline, the mean skin temperature at 38.5°C will be recorded. Participants who voluntarily terminate exercise or complete 240 minutes of continuous walking before reaching either endpoint will have their final recorded mean skin temperature used.
Time frame: During the 240-minute work period
Average mean skin temperature during work/rest cycles
Average mean skin temperature measured during the intermittent work-rest cycles following the initial continuous work period.
Time frame: During the 240-minute work period
Recovery core temperature
Average core temperature during the recovery period. Core temperature will be measured continuously throughout recovery.
Time frame: During 1 hour recovery following the 240-minute work period
Recovery heart rate
Average heart rate during the recovery period. Heart rate will be measured continuously throughout recovery.
Time frame: During 1 hour recovery following the 240-minute work period
Recovery mean skin temperature
Average mean skin temperature during the recovery period. Skin temperature will be measured continuously throughout recovery.
Time frame: During 1 hour recovery following the 240-minute work period
Tissue oxygenation
Functional near-infrared spectroscopy will be used to continuously assess changes in cerebral and skeletal muscle oxygenation
Time frame: During 240-minute work bout and subsequent 1 hour recovery
Peripheral oxygen saturation
Peripheral oxygen saturation will be continuously monitored using pulse oximetry to assess the level of arterial oxygen saturation during work and recovery; indicates the degree of hypoxemia and helps determine whether reduced oxygen availability contributes to physiological strain during heat exposure
Time frame: During 240-minute work bout and subsequent 1 hour recovery
Urine specific gravity
Urine specific gravity will be measured to assess hydration status. Higher values indicate greater urine concentration and therefore poorer hydration status, whereas lower values indicate lower urine concentration and therefore better hydration status.
Time frame: Assessed at baseline and after recovery
Fluid loss from body mass change
Fluid loss calculated from changes in body mass during exposure, corrected for fluid intake.
Time frame: Assessed at baseline and throughout protocol
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 exposure using the technique by Dill and Costill
Time frame: Assessed at baseline and after recovery
Systolic blood pressure
Systolic blood pressure measured in triplicate using automated oscillometry.
Time frame: Assessed at baseline and throughout protocol
Diastolic blood pressure
Diastolic blood pressure measured in triplicate using automated oscillometry.
Time frame: Assessed at baseline and throughout protocol
Orthostatic hypotension questionnaire
Cumulative sum of scores on 6 questions asking participant to rank symptoms associated with orthostatic intolerance. All symptoms scored on a scale from 0 (none) to 10 (worst possible) and include feelings of: (1) "dizziness, lightheadedness, feeling faint, or feeling like you might black out"; (2) "Problems with vision (blurring, seeing spots, tunnel vision, etc.)"; (3) "Weakness"; (4) "Fatigue"; (5) "Trouble concentrating"; and (6) "Head and neck discomfort"
Time frame: Assessed at baseline and throughout protocol
Lake Louise Mountain Sickness Questionnaire
Assess symptoms of acute mountain sickness, including headache, gastrointestinal discomfort, fatigue/weakness, dizziness/lightheadedness, and sleep disturbance. Each symptom is scored from 0 - 3 and summed for a total score, with higher scores indicating greater symptom severity and acute mountain sickness typically defined as headache plus a total score ≥3.
Time frame: Assessed at baseline and throughout protocol
Thermal sensation scale
Thermal sensation will be assessed using a visual analog ASHRAE-based scale in response to the question "How hot do you feel?" Scores range from 0 (neutral) to 7 (extremely hot). Higher scores indicate a greater sensation of heat and therefore a worse outcome, whereas lower scores indicate less perceived heat and therefore a better outcome.
Time frame: Assessed at baseline and throughout protocol
Thermal comfort
Thermal comfort will be assessed in response to the question "How comfortable does your body temperature feel?" using a visual analog scale ranging from 1 (comfortable) to 4 (very uncomfortable). Higher scores indicate lower thermal comfort and therefore a worse outcome, whereas lower scores indicate greater thermal comfort and therefore a better outcome.
Time frame: Assessed at baseline and throughout protocol
Thirst sensation
Thirst sensation will be assessed in response to the question "How thirsty are you?" using a visual analog scale ranging from 1 (not thirsty at all) to 9 (very, very thirsty). Higher scores indicate greater thirst and therefore a worse outcome, whereas lower scores indicate less thirst and therefore a better outcome.
Time frame: Assessed at baseline and throughout protocol
Rating of Perceived Exertion (Borg 6-20 Scale)
Perceived exertion will be assessed using a visual analog scale in response to the question "How hard are you working?" Scores range from 6 (no exertion at all) to 20 (maximal exertion). Higher scores indicate greater perceived effort and therefore a worse outcome, whereas lower scores indicate less perceived effort and therefore a better outcome.
Time frame: Assessed during 240-minute work period
Feel-good scale
Feelings of well-being will be assessed using a visual analog scale in response to the question "How good do you feel?" Scores range from -5 (very bad) to +5 (very good), with 0 representing neutral. Higher scores indicate more positive feelings and therefore a better outcome, whereas lower scores indicate more negative feelings and therefore a worse outcome.
Time frame: Assessed at baseline and throughout protocol
Profiles of Mood States (POMS-40) total mood disturbance score
Mood state will be assessed using the 40-item Profiles of Mood States (POMS-40) questionnaire. Participants will rate how they feel at the time of assessment using a 5-point Likert scale ranging from 0 (not at all) to 4 (extremely). Responses will be summed to generate a total score. Higher scores indicate greater overall mood disturbance and therefore a worse outcome, whereas lower scores indicate lower mood disturbance and therefore a better outcome.
Time frame: Assessed at baseline and end of recovery
Environmental Symptoms Questionnaire-IV (ESQ-IV) total symptom score
Environmental symptomatology associated with prolonged heat exposure will be assessed using the Environmental Symptoms Questionnaire-IV (ESQ-IV). Participants will rate 68 symptoms using a 6-point Likert scale ranging from 0 (not at all) to 5 (extreme). Responses are summed, with positive items reverse scored as appropriate, to generate a Total Symptom Score ranging from 0 to 340. Higher scores indicate greater symptom burden and therefore a worse outcome, whereas lower scores indicate fewer symptoms and therefore a better outcome.
Time frame: Assessed at baseline and end of recovery
Heart rate variability: standard deviation of normal R-R intervals (SDNN)
Cardiac autonomic modulation will be assessed using a 5-lead Holter electrocardiogram (ECG) monitor (DigiTrak XT, Philips Zymed, Andover, MA, USA). Consecutive R-R intervals will be derived from the ECG recording to calculate the standard deviation of normal R-R intervals (SDNN), measured in milliseconds (ms). Higher values indicate greater heart rate variability and cardiac autonomic modulation, whereas lower values indicate reduced heart rate variability and cardiac autonomic modulation.
Time frame: During 240-minute work bout and subsequent 1 hour recovery
Heart rate variability: root mean square of successive differences (RMSSD)
Cardiac autonomic modulation will be assessed using a 5-lead Holter electrocardiogram (ECG) monitor (DigiTrak XT, Philips Zymed, Andover, MA, USA). Consecutive R-R intervals will be derived from the ECG recording to calculate the root mean square of successive differences between adjacent normal R-R intervals (RMSSD), measured in milliseconds (ms). Higher values indicate greater parasympathetic modulation and heart rate variability, whereas lower values indicate reduced parasympathetic modulation and heart rate variability.
Time frame: During 240-minute work bout and subsequent 1 hour recovery
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