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. 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.

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