Working in the Heat at Simulated Altitude: Effects of Normobaric Hypoxia on Physiological Strain

Effects of Normobaric Hypoxia on Initial Stay Time and Physiological Strain During Moderate-Intensity Work in the Heat in Young and Older Males

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. Current occupational heat-stress guidelines aim to limit excessive increases in core body temperature and physiological strain through the use of work-rest schedules. However, these guidelines do not provide clear direction on the safe, maximum duration of continuous work that can be performed before heat-mitigation controls should be implemented, 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, but these estimates were developed under normoxic conditions. In many occupational settings, workers may be exposed to additional environmental stressors such as hypoxia, including work at altitude or in oxygen-reduced environments. Hypoxia can increase cardiovascular strain, reduce arterial oxygen saturation, and alter exercise tolerance, potentially compromising the effectiveness of heat-stress guidance developed under normal oxygen conditions. Older workers may be especially vulnerable due to age-related reductions in thermoregulatory and cardiovascular function. To date, it remains unclear whether hypoxia alters 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 exposure to hypoxia increases physiological strain during moderate-intensity work in the heat in young and older males. Specifically, investigators will assess whether normoxia-derived initial stay times and recovery recommendations for work at 29°C wet-bulb globe (37.5°C, 35% RH) temperature remain effective when workers are exposed to hypoxia equivalent to 14% inspired oxygen (simulating ~3000 m in altitude). Healthy young and older males will complete simulated occupational heat exposure under two conditions: heat alone and combined 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. Measures of core temperature, cardiovascular strain, oxygen saturation, skin temperature, hydration status, perceptual responses, mood, symptoms of environmental stress, and cognitive function will be assessed to determine whether hypoxia modifies responses to work in the heat. This study will provide important evidence on whether occupational heat-stress guidance requires refinement when workers are exposed to combined heat and hypoxic stress.

Study Overview

• Status: Recruiting
• Conditions: Body Temperature Changes; Work Related Stress; Physiological Stress; Heat Stress; Heat Exposure; Hypoxia, Altitude; Heat Fatigue
• Intervention / Treatment: Other: Simulated Occupational heat exposure under normoxic conditions; Other: Simulated Occupational heat exposure under hypoxic conditions

• Study Type: Interventional
• Enrollment (Estimated): 20
• Phase: Not Applicable

Contacts and Locations

• Study Contact: Name: Glen P Kenny, PhD; Phone Number: 4282 6135625800; Email: gkenny@uottawa.ca

Study Locations

• Canada
  • Ontario
    • Ottawa, Ontario, Canada, K1N 6N5
      • Recruiting
      • University of Ottawa
      • Contact: Glen P Kenny, PhD; Phone Number: 4282 613-562-5800; Email: gkenny@uottawa.ca

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: No

Description

Inclusion Criteria:

• Are male.
• Do not smoke or have been smoke free for at least 5 years.
• English or French speaking.
• Ability to provide informed consent.
• Are physically active (engage in exercise at least 2 times per week for ≥30 minutes)

Exclusion Criteria:

• Are regularly exposed to hot environments (e.g., occupational heat exposure)
• Are regularly exposed to hypoxic environments (e.g., high altitudes)
• Experience claustrophobia or discomfort in enclosed environments
• Episode(s) of severe hypoglycemia (requiring the assistance of another person) within the previous year, or inability to sense hypoglycemia (hypoglycemia unawareness).
• Serious complications related to diabetes (gastroparesis, renal disease, uncontrolled hypertension, severe autonomic neuropathy).
• Uncontrolled hypertension - BP >150 mmHg systolic or >95 mmHg diastolic in a sitting position.
• Restrictions in physical activity due to disease (e.g. intermittent claudication, renal impairment, active proliferative retinopathy, unstable cardiac or pulmonary disease, disabling stroke, severe arthritis, etc.).
• Use of or changes in medication judged by the patient or investigators to make participation in this study inadvisable.
• Cardiac abnormalities identified during screening

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Basic Science
• Allocation: Randomized
• Interventional Model: Crossover Assignment
• Masking: Single

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Normoxic exposure; Participants complete a simulated moderate-intensity occupational heat exposure protocol in a hot environment (37.5°C, 35% RH) under normoxic conditions. Participants walk on a treadmill until they reach their initial stay time, followed by prescribed work-rest cycles and seated recovery.	Other: Simulated Occupational heat exposure under normoxic conditions; 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.
Experimental: Hypoxic exposure; Participants complete a simulated moderate-intensity occupational heat exposure protocol in a hot environment (37.5°C, 35% RH) with an added hypoxic exposure equivalent to 14% inspired oxygen. Participants walk on a treadmill until they reach their initial stay time, followed by prescribed work-rest cycles and seated recovery.	Other: Simulated Occupational heat exposure under hypoxic conditions; 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.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Total 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.	During the 240-minute work period

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
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.	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.	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.	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.	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.	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.	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.	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.	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.	During the 240-minute work period
Recovery core temperature	Average core temperature during the recovery period. Core temperature will be measured continuously throughout recovery.	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.	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.	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	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	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.	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.	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	Assessed at baseline and after recovery
Systolic blood pressure	Systolic blood pressure measured in triplicate using automated oscillometry.	Assessed at baseline and throughout protocol
Diastolic blood pressure	Diastolic blood pressure measured in triplicate using automated oscillometry.	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"	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.	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.	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.	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.	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.	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.	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.	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.	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.	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.	During 240-minute work bout and subsequent 1 hour recovery

Collaborators and Investigators

• Sponsor: University of Ottawa

Study record dates

Study Major Dates

• Study Start (Estimated): June 17, 2026
• Primary Completion (Estimated): December 31, 2026
• Study Completion (Estimated): February 28, 2027

Study Registration Dates

• First Submitted: June 17, 2026
• First Submitted That Met QC Criteria: June 17, 2026
• First Posted (Actual): June 24, 2026

Study Record Updates

• Last Update Posted (Actual): June 24, 2026
• Last Update Submitted That Met QC Criteria: June 17, 2026
• Last Verified: June 1, 2026

More Information

Terms related to this study

• Keywords: Exercise; Hypoxia; Aging; Thermoregulation; Simulated altitude; Hyperthermia; Heat Strain; Core temperature; Occupational Heat Stress
• Additional Relevant MeSH Terms: Wounds and Injuries; Behavioral Symptoms; Respiratory Tract Diseases; Respiration Disorders; Signs and Symptoms, Respiratory; Stress, Psychological; Occupational Diseases; Pathological Conditions, Signs and Symptoms; Behavior; Signs and Symptoms; Occupational Stress; Hyperthermia; Hypoxia; Heat Stress Disorders; Body Temperature Changes; Altitude Sickness; Motor Activity
• Other Study ID Numbers: HEPRU-2026-06-B

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No