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

Plain-language summary

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.

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