Initial Stay Times and Heat Mitigation Controls for Uncompensable Occupational Heat Stress - Part II

Safe Maximum Work Times and the Effectiveness of Work-rest Allocations in Mitigating Increases in Core Temperature During and on the Day Following Prolonged Heavy-Intensity Work in the Heat in Young and Older Workers

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 Overview

• Status: Completed
• Conditions: Exercise; Core Temperature; Heat Stress, Exertional; Heat Fatigue
• Intervention / Treatment: Other: Simulated work in the heat

• Study Type: Interventional
• Enrollment (Actual): 21
• Phase: Not Applicable

Contacts and Locations

Study Locations

• Canada
  • Ontario
    • Ottawa, Ontario, Canada, K1N 6N5
      • University of Ottawa

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: Yes

Description

Inclusion Criteria:

• young (18-30 years) and older adults (50-69 years)
• habitually active, not endurance trained (<2 sessions per week, <150 minutes per week)
• non-smoking
• English or French speaking
• ability to provide informed consent

Exclusion Criteria:

• pre-existing health conditions (e.g., diabetes, hypertension)
• use of medication deemed to significantly modulate thermoregulatory function and heat tolerance (e.g., antidepressants, antihistamines, diuretics)
• engaged in jobs and/or activities that involve frequent exposure to hot environments (e.g., regular sauna use)

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Basic Science
• Allocation: Non-Randomized
• Interventional Model: Parallel Assignment
• Masking: Single

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Younger adults.; Adults aged 18-30 years with no pre-existing health conditions	Other: Simulated work in the heat; 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).
Experimental: Older adults. Adults aged 50-69 with no pre-existing health conditions; Adults aged 50-69 with no pre-existing health conditions	Other: Simulated work in the heat; 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).

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
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).	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).	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).	End of third (morning day 2) continuous work bout
Average core temperature	Average core temperature following initial stay time	Period of work-rest allocations following initial stay time for afternoon day 1
Average core temperature	Average core temperature following initial stay time	Period of work-rest allocations following initial stay time for morning day 1
Average core temperature	Average core temperature following initial stay time	Period of work-rest allocations following initial stay time for morning of day 2

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Systolic blood pressure	Arterial blood pressure measured via automated oscillometry	End of first (morning day 1) work period
Systolic blood pressure	Arterial blood pressure measured via automated oscillometry	End of second (afternoon day 1) work period
Systolic blood pressure	Arterial blood pressure measured via automated oscillometry	End of third (morning day 2) work period
Diastolic blood pressure	Arterial blood pressure measured via automated oscillometry	End of first (morning day 1) work period
Diastolic blood pressure	Arterial blood pressure measured via automated oscillometry	End of second (afternoon day 1) work period
Rate pressure product	Calculated as systolic blood pressure * heart rate	End of first (morning day 1) work period
Rate pressure product	Calculated as systolic blood pressure * heart rate	End of second (afternoon day 1) work period
Rate pressure product	Calculated as systolic blood pressure * heart rate	End of third (morning day 2) work period
Fluid consumption	Cumulative fluid consumption calculated by weighing the participant's water intake	End of first (morning day 1) work period
Fluid consumption	Cumulative fluid consumption calculated by weighing the participant's water intake	End of second (afternoon day 1) work period
Fluid consumption	Cumulative fluid consumption calculated by weighing the participant's water intake	End of third (morning day 2) work period
Fluid loss	Fluid loss calculated as the change in body mass (corrected for food/fluid consumption).	End of first (morning day 1) work period
Fluid loss	Fluid loss calculated as the change in body mass (corrected for food/fluid consumption).	End of second (afternoon day 1) work period
Fluid loss	Fluid loss calculated as the change in body mass (corrected for food/fluid consumption).	End of third (morning day 2) work period
Change in plasma volume	Determined from venous blood samples	End of first (morning day 1) work period
Change in plasma volume	Determined from venous blood samples	End of second (afternoon day 1) work period
Change in plasma volume	Determined from venous blood samples	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)	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)	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)	End of third (morning day 2) 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)	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).	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).	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).	End of third (morning day 2) 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)	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)	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)	Period of work-rest allocations following initial stay time for afternoon 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)	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)	End of first (morning day 2) continuous work bout
Average heart rate	Average heart rate following initial stay time	Period of work-rest allocations following initial stay time for morning of day 1
Average heart rate	Average heart rate following initial stay time	Period of work-rest allocations following initial stay time for afternoon of day 1
Average heart rate	Average heart rate following initial stay time	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)	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)	End of first (morning day 2) continuous work bout
Average skin temperature	Average heart rate following initial stay time	Period of work-rest allocations following initial stay time for morning of day 1
Average skin temperature	Average heart rate following initial stay time	Period of work-rest allocations following initial stay time for afternoon of day 1
Average skin temperature	Average heart rate following initial stay time	Period of work-rest allocations following initial stay time for morning of day 2
Diastolic blood pressure	Arterial blood pressure measured via automated oscillometry	End of third (morning day 1) work period
Mean arterial pressure	Calculated as 1/3 * systolic blood pressure + 2/3 * diastolic blood pressure	End of first (morning day 1) work period
Mean arterial pressure	Calculated as 1/3 * systolic blood pressure + 2/3 * diastolic blood pressure	End of second (afternoon day 1) work period
Mean arterial pressure	Calculated as 1/3 * systolic blood pressure + 2/3 * diastolic blood pressure	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)	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)	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)	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)	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)	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)	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)	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).	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).	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).	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"	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"	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"	End of third (afternoon day 1) continuous work bout

Collaborators and Investigators

• Sponsor: University of Ottawa

Study record dates

Study Major Dates

• Study Start (Actual): July 25, 2024
• Primary Completion (Actual): November 28, 2024
• Study Completion (Actual): November 28, 2024

Study Registration Dates

• First Submitted: August 4, 2024
• First Submitted That Met QC Criteria: August 4, 2024
• First Posted (Actual): August 9, 2024

Study Record Updates

• Last Update Posted (Actual): June 18, 2025
• Last Update Submitted That Met QC Criteria: June 13, 2025
• Last Verified: June 1, 2025

More Information

Terms related to this study

• Keywords: Aging; Thermoregulation; Body temperature; Heat strain; Heat exposure; Occupational heat stress; Uncompensable heat stress
• Additional Relevant MeSH Terms: Wounds and Injuries; Heat Stress Disorders
• Other Study ID Numbers: HEPRU-2024-07

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No
• product manufactured in and exported from the U.S.: No