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Completed

NCT Number: NCT06543719

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

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.

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Key information

Conditions

Core Temperature Behavior Exercise Heat Fatigue Heat Stress Disorders Heat Stress, Exertional Motor Activity Wounds and Injuries

Age range

18 year–69 year

Sex eligibility

Male

Study type

Interventional

Phase

Not applicable

Primary location

University of Ottawa

Ottawa, Ontario, K1N 6N5, Canada

Who can participate

Healthy volunteers accepted: Yes

Only the study team can determine whether someone qualifies for participation.

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)

Treatment and study plan

Simulated work in the heat

Other

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).

Primary outcomes

  1. Initial stay time

    Time frame: End of first (morning day 1) continuous work bout

    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).

  2. Initial stay time

    Time frame: End of second (afternoon day 1) continuous work bout

    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).

  3. Initial stay time

    Time frame: End of third (morning day 2) continuous work bout

    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).

  4. Average core temperature

    Time frame: Period of work-rest allocations following initial stay time for morning day 1

    Average core temperature following initial stay time

  5. Average core temperature

    Time frame: Period of work-rest allocations following initial stay time for afternoon day 1

    Average core temperature following initial stay time

  6. Average core temperature

    Time frame: Period of work-rest allocations following initial stay time for morning of day 2

    Average core temperature following initial stay time

Secondary outcomes

  1. Heart rate at initial stay time

    Time frame: End of first (morning day 1) continuous work bout

    Heart rate at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)

  2. Heart rate at initial stay time

    Time frame: End of first (afternoon day 1) continuous work bout

    Heart rate at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)

  3. Heart rate at initial stay time

    Time frame: End of first (morning day 2) continuous work bout

    Heart rate at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)

  4. Average heart rate

    Time frame: Period of work-rest allocations following initial stay time for morning of day 1

    Average heart rate following initial stay time

  5. Average heart rate

    Time frame: Period of work-rest allocations following initial stay time for afternoon of day 1

    Average heart rate following initial stay time

  6. Average heart rate

    Time frame: Period of work-rest allocations following initial stay time for morning of day 2

    Average heart rate following initial stay time

  7. Skin temperature at initial stay time

    Time frame: End of first (morning day 1) continuous work bout

    Skin temperature (4 sites) measured at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)

  8. Skin temperature at initial stay time

    Time frame: End of first (afternoon day 1) continuous work bout

    Skin temperature (4 sites) measured at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)

  9. Skin temperature at initial stay time

    Time frame: End of first (morning day 2) continuous work bout

    Skin temperature (4 sites) measured at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)

  10. Average skin temperature

    Time frame: Period of work-rest allocations following initial stay time for morning of day 1

    Average heart rate following initial stay time

  11. Average skin temperature

    Time frame: Period of work-rest allocations following initial stay time for afternoon of day 1

    Average heart rate following initial stay time

  12. Average skin temperature

    Time frame: Period of work-rest allocations following initial stay time for morning of day 2

    Average heart rate following initial stay time

  13. Systolic blood pressure

    Time frame: End of first (morning day 1) work period

    Arterial blood pressure measured via automated oscillometry

  14. Systolic blood pressure

    Time frame: End of second (afternoon day 1) work period

    Arterial blood pressure measured via automated oscillometry

  15. Systolic blood pressure

    Time frame: End of third (morning day 2) work period

    Arterial blood pressure measured via automated oscillometry

  16. Diastolic blood pressure

    Time frame: End of first (morning day 1) work period

    Arterial blood pressure measured via automated oscillometry

  17. Diastolic blood pressure

    Time frame: End of second (afternoon day 1) work period

    Arterial blood pressure measured via automated oscillometry

  18. Diastolic blood pressure

    Time frame: End of third (morning day 1) work period

    Arterial blood pressure measured via automated oscillometry

  19. Mean arterial pressure

    Time frame: End of first (morning day 1) work period

    Calculated as 1/3 * systolic blood pressure + 2/3 * diastolic blood pressure

  20. Mean arterial pressure

    Time frame: End of second (afternoon day 1) work period

    Calculated as 1/3 * systolic blood pressure + 2/3 * diastolic blood pressure

  21. Mean arterial pressure

    Time frame: End of third (morning day 2) work period

    Calculated as 1/3 * systolic blood pressure + 2/3 * diastolic blood pressure

  22. Rate pressure product

    Time frame: End of first (morning day 1) work period

    Calculated as systolic blood pressure * heart rate

  23. Rate pressure product

    Time frame: End of second (afternoon day 1) work period

    Calculated as systolic blood pressure * heart rate

  24. Rate pressure product

    Time frame: End of third (morning day 2) work period

    Calculated as systolic blood pressure * heart rate

  25. Fluid consumption

    Time frame: End of first (morning day 1) work period

    Cumulative fluid consumption calculated by weighing the participant's water intake

  26. Fluid consumption

    Time frame: End of second (afternoon day 1) work period

    Cumulative fluid consumption calculated by weighing the participant's water intake

  27. Fluid consumption

    Time frame: End of third (morning day 2) work period

    Cumulative fluid consumption calculated by weighing the participant's water intake

  28. Fluid loss

    Time frame: End of first (morning day 1) work period

    Fluid loss calculated as the change in body mass (corrected for food/fluid consumption).

  29. Fluid loss

    Time frame: End of second (afternoon day 1) work period

    Fluid loss calculated as the change in body mass (corrected for food/fluid consumption).

  30. Fluid loss

    Time frame: End of third (morning day 2) work period

    Fluid loss calculated as the change in body mass (corrected for food/fluid consumption).

  31. Change in plasma volume

    Time frame: End of first (morning day 1) work period

    Determined from venous blood samples

  32. Change in plasma volume

    Time frame: End of second (afternoon day 1) work period

    Determined from venous blood samples

  33. Change in plasma volume

    Time frame: End of third (morning day 2) work period

    Determined from venous blood samples

  34. Thermal comfort scale

    Time frame: End of first (morning day 1) continuous work bout

    Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)

  35. Thermal comfort scale

    Time frame: End of second (afternoon day 1) continuous work bout

    Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)

  36. Thermal comfort scale

    Time frame: End of third (morning day 2) continuous work bout

    Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)

  37. Thermal comfort scale

    Time frame: Period of work-rest allocations following initial stay time for morning of day 1

    Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)

  38. Thermal comfort scale

    Time frame: Period of work-rest allocations following initial stay time for afternoon of day 1

    Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)

  39. Thermal comfort scale

    Time frame: Period of work-rest allocations following initial stay time for morning of day 2

    Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)

  40. Thirst sensation scale

    Time frame: End of first (morning day 1) continuous work bout

    Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)

  41. Thirst sensation scale

    Time frame: End of second (afternoon day 1) continuous work period

    Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)

  42. Thirst sensation scale

    Time frame: End of third (morning day 2) continuous work period

    Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)

  43. Thirst sensation scale

    Time frame: Period of work-rest allocations following initial stay time for morning of day 1

    Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)

  44. Thirst sensation scale

    Time frame: Period of work-rest allocations following initial stay time for afternoon day 2

    Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)

  45. Thirst sensation scale

    Time frame: Period of work-rest allocations following initial stay time for morning of day 2

    Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)

  46. Rating of Perceived Exertion

    Time frame: End of first (morning day 1) continuous work bout

    Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).

  47. Rating of Perceived Exertion

    Time frame: End of second (afternoon day 1) continuous work bout

    Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).

  48. Rating of Perceived Exertion

    Time frame: End of third (morning day 2) continuous work bout

    Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).

  49. Rating of Perceived Exertion

    Time frame: Period of work-rest allocations following initial stay time for morning of day 1

    Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).

  50. Rating of Perceived Exertion

    Time frame: Period of work-rest allocations following initial stay time for afternoon of day 1

    Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).

  51. Rating of Perceived Exertion

    Time frame: Period of work-rest allocations following initial stay time for morning of day 2

    Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).

  52. Orthostatic Intolerance Symptoms Assessment

    Time frame: End of first (morning day 1) continuous work bout

    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"

  53. Orthostatic Intolerance Symptoms Assessment

    Time frame: End of second (afternoon day 1) continuous work bout

    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"

  54. Orthostatic Intolerance Symptoms Assessment

    Time frame: End of third (afternoon day 1) continuous work bout

    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"

Sponsors and collaborators

Lead sponsor

University of Ottawa

Other

Registry information

Official study title

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

Important dates

Study start
2024
Primary completion
2024
Study completion
2024
First posted
Aug 9, 2024
Registry last updated
Jun 18, 2025

OpenTrials presents study information sourced from ClinicalTrials.gov. The official registry record should be consulted for the latest information.

View the official ClinicalTrials.gov record (opens in a new tab)

This listing is for discovery and informational purposes only. It is not medical advice, does not guarantee that a study is recruiting, and does not determine eligibility. Contact the study team and a qualified healthcare professional when considering participation.

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