Workplaces rely on upper heat stress limits provided by the American Conference of Governmental Industrial Hygienists (ACGIH) to manage the health and safety of workers in hot environments. This is primarily achieved by interspersing work with rest periods, the length of which is dictated by environmental conditions and work intensity, to maintain core temperature at or below 38.0°C (equivalent to a 1°C increase in body core temperature above resting levels). However, these guidelines employ a "one size fits all" approach to exposure limits that does not consider individual variation (e.g., age) between workers. Moreover, they fail to provide direction on the safe, initial stay times before these heat-mitigation controls should be employed (i.e., rest breaks) in conditions exceeding upper heat stress limits. While recent work has generated estimates of the initial stay times for young to older men before heat-mitigation controls are required for moderate-intensity work, information on initial stay times for heavy-intensity work remains to be assessed. This project will assess the initial stay times for heavy-intensity work for a single work bout as well as for a second work bout that is preceded by an extended rest period such as a lunch break and a work bout performed on the next day to determine if refinements in initial stay times across these periods may be required. Further, the investigators will evaluate if the application of recommended work-rest allocations thereafter would alleviate increases in core temperature for the duration of the work period (e.g., start of shift versus post-lunch period). Given the known age-differences in heat loss that can modulate core temperature regulation during an exercise-heat stress, the investigators will assess responses response in young and older adults.
Study Type
INTERVENTIONAL
Allocation
NON_RANDOMIZED
Purpose
BASIC_SCIENCE
Masking
SINGLE
Enrollment
21
Participants perform a continuous heavy-intensity work bout (metabolic rate of \~260 W/m2) until core temperature reaches 38.0°C (equivalent to a 1°C increase in body core temperature above resting levels), which is immediately followed by intermittent work using a 1:1 work-rest allocation, starting with a 30 min rest break followed by a 30 min work bout for a total work duration of \~240 min. The work protocol is performed in the morning of day 1, and repeated in a post-lunch work period (i.e., afternoon of day 1) and on the morning of the next day (i.e., morning of day 2).
University of Ottawa
Ottawa, Ontario, Canada
Initial stay time
Total continuous work time to achieve an initial absolute increase in core temperature of 38°C (equivalent to a 1°C increase in body core temperature above resting levels) (note: in instances where a participant voluntarily terminates work prematurely before the absolute core temperature of ≥38°C (or relative increase of ≥1°C) is achieved or the absolute core temperature is \<38°C (or relative increase of \<1°C), the time of termination or end of the work bout will be taken as the initial stay time respectively).
Time frame: End of first (morning day 1) continuous work bout
Initial stay time
Total continuous work time to achieve an initial absolute increase in core temperature of 38°C (equivalent to a 1°C increase in body core temperature above resting levels) (note: in instances where a participant voluntarily terminates work prematurely before the absolute core temperature of ≥38°C (or relative increase of ≥1°C) is achieved or the absolute core temperature is \<38°C (or relative increase of \<1°C), the time of termination or end of the work bout will be taken as the initial stay time respectively).
Time frame: End of second (afternoon day 1) continuous work bout
Initial stay time
Total continuous work time to achieve an initial absolute increase in core temperature of 38°C (equivalent to a 1°C increase in body core temperature above resting levels) (note: in instances where a participant voluntarily terminates work prematurely before the absolute core temperature of ≥38°C (or relative increase of ≥1°C) is achieved or the absolute core temperature is \<38°C (or relative increase of \<1°C), the time of termination or end of the work bout will be taken as the initial stay time respectively).
Time frame: End of third (morning day 2) continuous work bout
Average core temperature
Average core temperature following initial stay time
Time frame: Period of work-rest allocations following initial stay time for morning day 1
Average core temperature
Average core temperature following initial stay time
Time frame: Period of work-rest allocations following initial stay time for afternoon day 1
Average core temperature
Average core temperature following initial stay time
Time frame: Period of work-rest allocations following initial stay time for morning of day 2
Heart rate at initial stay time
Heart rate at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)
Time frame: End of first (morning day 1) continuous work bout
Heart rate at initial stay time
Heart rate at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)
Time frame: End of first (afternoon day 1) continuous work bout
Heart rate at initial stay time
Heart rate at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)
Time frame: End of first (morning day 2) continuous work bout
Average heart rate
Average heart rate following initial stay time
Time frame: Period of work-rest allocations following initial stay time for morning of day 1
Average heart rate
Average heart rate following initial stay time
Time frame: Period of work-rest allocations following initial stay time for afternoon of day 1
Average heart rate
Average heart rate following initial stay time
Time frame: Period of work-rest allocations following initial stay time for morning of day 2
Skin temperature at initial stay time
Skin temperature (4 sites) measured at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)
Time frame: End of first (morning day 1) continuous work bout
Skin temperature at initial stay time
Skin temperature (4 sites) measured at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)
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Time frame: End of first (afternoon day 1) continuous work bout
Skin temperature at initial stay time
Skin temperature (4 sites) measured at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)
Time frame: End of first (morning day 2) continuous work bout
Average skin temperature
Average heart rate following initial stay time
Time frame: Period of work-rest allocations following initial stay time for morning of day 1
Average skin temperature
Average heart rate following initial stay time
Time frame: Period of work-rest allocations following initial stay time for afternoon of day 1
Average skin temperature
Average heart rate following initial stay time
Time frame: Period of work-rest allocations following initial stay time for morning of day 2
Systolic blood pressure
Arterial blood pressure measured via automated oscillometry
Time frame: End of first (morning day 1) work period
Systolic blood pressure
Arterial blood pressure measured via automated oscillometry
Time frame: End of second (afternoon day 1) work period
Systolic blood pressure
Arterial blood pressure measured via automated oscillometry
Time frame: End of third (morning day 2) work period
Diastolic blood pressure
Arterial blood pressure measured via automated oscillometry
Time frame: End of first (morning day 1) work period
Diastolic blood pressure
Arterial blood pressure measured via automated oscillometry
Time frame: End of second (afternoon day 1) work period
Diastolic blood pressure
Arterial blood pressure measured via automated oscillometry
Time frame: End of third (morning day 1) work period
Mean arterial pressure
Calculated as 1/3 \* systolic blood pressure + 2/3 \* diastolic blood pressure
Time frame: End of first (morning day 1) work period
Mean arterial pressure
Calculated as 1/3 \* systolic blood pressure + 2/3 \* diastolic blood pressure
Time frame: End of second (afternoon day 1) work period
Mean arterial pressure
Calculated as 1/3 \* systolic blood pressure + 2/3 \* diastolic blood pressure
Time frame: End of third (morning day 2) work period
Rate pressure product
Calculated as systolic blood pressure \* heart rate
Time frame: End of first (morning day 1) work period
Rate pressure product
Calculated as systolic blood pressure \* heart rate
Time frame: End of second (afternoon day 1) work period
Rate pressure product
Calculated as systolic blood pressure \* heart rate
Time frame: End of third (morning day 2) work period
Fluid consumption
Cumulative fluid consumption calculated by weighing the participant's water intake
Time frame: End of first (morning day 1) work period
Fluid consumption
Cumulative fluid consumption calculated by weighing the participant's water intake
Time frame: End of second (afternoon day 1) work period
Fluid consumption
Cumulative fluid consumption calculated by weighing the participant's water intake
Time frame: End of third (morning day 2) work period
Fluid loss
Fluid loss calculated as the change in body mass (corrected for food/fluid consumption).
Time frame: End of first (morning day 1) work period
Fluid loss
Fluid loss calculated as the change in body mass (corrected for food/fluid consumption).
Time frame: End of second (afternoon day 1) work period
Fluid loss
Fluid loss calculated as the change in body mass (corrected for food/fluid consumption).
Time frame: End of third (morning day 2) work period
Change in plasma volume
Determined from venous blood samples
Time frame: End of first (morning day 1) work period
Change in plasma volume
Determined from venous blood samples
Time frame: End of second (afternoon day 1) work period
Change in plasma volume
Determined from venous blood samples
Time frame: End of third (morning day 2) work period
Thermal comfort scale
Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)
Time frame: End of first (morning day 1) continuous work bout
Thermal comfort scale
Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)
Time frame: End of second (afternoon day 1) continuous work bout
Thermal comfort scale
Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)
Time frame: End of third (morning day 2) continuous work bout
Thermal comfort scale
Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)
Time frame: Period of work-rest allocations following initial stay time for morning of day 1
Thermal comfort scale
Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)
Time frame: Period of work-rest allocations following initial stay time for afternoon of day 1
Thermal comfort scale
Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)
Time frame: Period of work-rest allocations following initial stay time for morning of day 2
Thirst sensation scale
Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)
Time frame: End of first (morning day 1) continuous work bout
Thirst sensation scale
Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)
Time frame: End of second (afternoon day 1) continuous work period
Thirst sensation scale
Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)
Time frame: End of third (morning day 2) continuous work period
Thirst sensation scale
Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)
Time frame: Period of work-rest allocations following initial stay time for morning of day 1
Thirst sensation scale
Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)
Time frame: Period of work-rest allocations following initial stay time for afternoon day 2
Thirst sensation scale
Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)
Time frame: Period of work-rest allocations following initial stay time for morning of day 2
Rating of Perceived Exertion
Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).
Time frame: End of first (morning day 1) continuous work bout
Rating of Perceived Exertion
Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).
Time frame: End of second (afternoon day 1) continuous work bout
Rating of Perceived Exertion
Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).
Time frame: End of third (morning day 2) continuous work bout
Rating of Perceived Exertion
Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).
Time frame: Period of work-rest allocations following initial stay time for morning of day 1
Rating of Perceived Exertion
Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).
Time frame: Period of work-rest allocations following initial stay time for afternoon of day 1
Rating of Perceived Exertion
Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).
Time frame: Period of work-rest allocations following initial stay time for morning of day 2
Orthostatic Intolerance Symptoms Assessment
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"
Time frame: End of first (morning day 1) continuous work bout
Orthostatic Intolerance Symptoms Assessment
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"
Time frame: End of second (afternoon day 1) continuous work bout
Orthostatic Intolerance Symptoms Assessment
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"
Time frame: End of third (afternoon day 1) continuous work bout