Heat Fatigue

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.

Extreme heat events are becoming more frequent, intense, and prolonged, increasing the risk of heat-related illness among workers exposed to hot environments. Current occupational heat-stress guidelines are designed to limit excessive increases in core body temperature and physiological strain during work in the heat through the use of work-rest schedules. However, these guidelines are largely based on single-day heat exposures and assume that workers recover in cool indoor environments between shifts. During heat events, many workers may instead recover in homes or accommodations that remain overheated, particularly in the absence of air conditioning. Inadequate overnight cooling may impair the body's ability to dissipate heat, increase physiological strain, and reduce tolerance to heat exposure on subsequent workdays. Older adults may be especially vulnerable due to age-related impairments in heat-loss capacity. To date, the effects of recovering in an overheated indoor environment on physiological strain and thermoregulatory function during repeated occupational heat exposure remain poorly understood, limiting the development of evidence-based recommendations to protect workers during prolonged heat events. The primary objective of this study is to determine whether overnight recovery in an overheated indoor environment increases physiological strain during repeated occupational heat exposure in older adults. Specifically, investigators will assess whether recovery at the recently proposed upper indoor temperature limit of 26°C and 45% relative humidity (RH) (PMID: 38329752) is sufficient to attenuate elevations in core temperature during subsequent work in the heat compared with recovery in an overheated indoor environment (31°C, 45% RH) representative of conditions commonly experienced during extreme heat events. Healthy older adults will complete two consecutive days and nights of simulated moderate-intensity occupational heat exposure, with overnight recovery occurring in either the 26°C or 31°C condition. Measures of core temperature, cardiovascular strain, heat-loss capacity, hydration status, sleep quality, cognitive function, mood, and perceptual responses will be assessed to determine the impact of overnight recovery conditions on responses to repeated heat exposure. Whole-body heat-loss capacity will also be evaluated before and after the exposure protocol as assessed during incremental, intermittent exercise protocol performed in an air calorimeter.