Occupational heat-stress guidelines are commonly based on the level of environmental heat exposure and are largely derived from studies conducted under constant environmental conditions. However, outdoor workers are typically exposed to dynamic conditions, with environmental heat often increasing progressively from morning to afternoon. Consequently, workers may experience substantially different patterns of heat exposure across the workday even when average daily heat exposure is equivalent. It remains unclear whether the temporal pattern of heat exposure influences the amount of work that can be performed or the physiological strain experienced during a workday. Understanding these responses is important for determining whether occupational heat-stress guidelines developed under constant laboratory conditions adequately protect workers exposed to progressively changing environmental conditions.
The primary objective of this study is to determine whether the temporal pattern of heat exposure influences work performed and physiological heat strain during a simulated occupational workday in healthy middle-aged and older men. Participants will complete two 9-hour simulated workdays consisting of a 4-hour morning work period, a 1-hour lunch break, and a 4-hour afternoon work period. During one trial, wet-bulb globe temperature (WBGT) will remain constant at 28°C throughout the workday, whereas during the other trial, WBGT will progressively increase from morning to afternoon to simulate the changing environmental conditions of a typical hot summer workday. Both trials will have the same mean daily WBGT of 28°C. Participants will perform moderate-intensity treadmill exercise, and measures including total work performed, core temperature, heart rate, and time to the initial stay-time threshold will be assessed to determine whether constant and progressively increasing heat exposures produce different physiological and work-performance responses across the workday.
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 regularly exposed to hot environments (e.g., occupational heat exposure)
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).
Crossover
Constant Heat Exposure
Other interventionParticipants complete a simulated occupational workday under constant environmental conditions, with WBGT maintained at 28°C throughout the morning and afternoon work periods.
Progressively Increasing Heat Exposure
Other interventionParticipants complete a simulated occupational workday in which WBGT progressively increases from morning to afternoon while maintaining a mean daily WBGT of 28°C.
Total work performed
Total work completed during each 240-minute morning and afternoon work period, expressed as cumulative minutes of treadmill walking. Each work period consists of an initial continuous walking phase until core temperature reaches 38.0°C or increases by 1.0°C above the pre-work baseline, whichever occurs first, followed by intermittent 45-min work and 15-min seated rest cycles.
Initial stay time
Time, in minutes, from the start of continuous treadmill walking until the initial stay-time threshold is reached, defined as a core temperature of 38.0°C or an increase of 1.0°C above the pre-work baseline, whichever occurs first. Participants who do not reach the threshold during the 240-minute work period will be identified as not having reached the threshold within 240 min.
Core temperature
Core temperature measured continuously using a rectal thermocouple temperature probe to assess thermoregulatory strain throughout the simulated workday.
Average core temperature during work-rest cycles
Average core temperature during the intermittent 45-min work and 15-min seated rest cycles following attainment of the initial stay-time threshold.
Mean skin temperature
Mean skin temperature calculated from temperatures measured at eight body sites using ISO 9886 weighting factors.
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