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

NCT06630832 | Completed

• 1 other identifier: HEPRU-2024-09-B
• Study Type: interventional
• Target: 23
• Locations: 1 country
• Sites: 1

Brief Summary

Occupational heat stress directly threatens workers' ability to live healthy and productive lives. Heat exposed workers are at an elevated risk of experiencing impaired work performance and cognitive function leading to a greater risk of work-related injuries which includes traumatic injury and a myriad of pathophysiological conditions (e.g., heat stroke, acute kidney injury, adverse cardiovascular events). To mitigate the adverse health effects of occupational heat stress, safety organizations recommend upper limits for heat stress, typically defined by a worker's metabolic rate and the prevailing wet-bulb globe temperature (WBGT). In instances where the heat load created by the combination of work intensity, environment, and clothing worn exceed the upper heat stress limits (uncompensable heat stress), controls such as rest breaks are prescribed to limit increases in core temperature beyond recommended limits. While workers are encouraged to find shelter from the heat during a rest break, it is not always possible or feasible. Typically, workers may rest while remaining exposed to the heat, recover in a shaded area or rest in an air-conditioned room or vehicle. However, the effectiveness of these cooling strategies in mitigating the level of physiological strain experienced by the worker during prolonged work in a hot environment remains unclear. In this project, the investigators will assess the efficacy of the different cooling strategies in preventing excursions in core temperature beyond recommended limits (38.0°C) following the initial stay time for moderate-intensity work in hot ambient conditions (WBGT of 29°C; represents hot outdoor conditions experienced by workers in summers in Ontario, Canada) in context of the prescribed American Conference of Governmental Industrial Hygienists (ACGIH) work-to-rest allocation for unacclimated adults. On three separate days, participants will walk on a treadmill at a fixed metabolic rate of 200 W/m2 until core temperature reaches and/or exceeds 38.0°C or until volitional fatigue. Thereafter, participants will complete an additional 180 min work bout employing the recommended ACGIH work-to-rest allocation of 1:3 (starting with a 45 min rest break followed by a 15 min work bout, with the cycle repeated three times over the 180 min work simulation bout) without (Control) or with cooling mitigation during each 15-min break consisting of either: i) partial cooling equivalent to sitting in a shaded space (WBGT 24°C; 31.7°C and 35% RH) such as under a tree with a light breeze (simulated with pedestal fan fixed at \~2 m/s) or ii) full cooling equivalent to sitting in air-conditioned space (e.g., room or vehicle) maintained at 22°C and 35% RH (equivalent WBGT of 16°C).

Conditions

Work Related Stress; Body Temperature Changes; Heat Stress; Heat Exposure; Heat Fatigue; Hyperthermia

Keywords

Heat strain; Thermoregulation; Exercise; Core temperature; Occupational heat stress; Uncompensable heat stress; Aging

Outcome Measures

Primary Outcomes (13)

• 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 continuous work bout as defined when core temperature reaches 38.0°C or when the maximum of 240 minutes of continuous exercise is achieved

• Average core temperature during work-rest allocations

  Average core temperature following initial stay time

  Period of work-rest allocations over a 180 minute following the initial stay time

• 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 continuous work bout as defined when core temperature reaches 38.0°C or when the maximum of 240 minutes of continuous exercise is achieved

• Average heart rate during work-rest allocations

  Average heart rate following initial stay time

  Period of work-rest allocations over a 180 minute following the initial stay time

• Skin temperature at initial stay time

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

  End of continuous work bout as defined when core temperature reaches 38.0°C or when the maximum of 240 minutes of continuous exercise is achieved

• Average skin temperature during work-rest allocations

  Average heart rate following initial stay time

  Period of work-rest allocations over a 180 minute following the initial stay time

• Thermal comfort scale at initial stay time

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

  End of continuous work bout as defined when core temperature reaches 38.0°C or when the maximum of 240 minutes of continuous exercise is achieved

• Thermal comfort scale during work-rest allocations

  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 over a 180 minute following the initial stay time

• Thirst sensation scale at initial stay time

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

  End of continuous work bout as defined when core temperature reaches 38.0°C or when the maximum of 240 minutes of continuous exercise is achieved

• Thirst sensation scale during work-rest allocations

  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 over a 180 minute following the initial stay time

• Rating of Perceived Exertion at initial stay time

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

  End of continuous work bout as defined when core temperature reaches 38.0°C or when the maximum of 240 minutes of continuous exercise is achieved

• Rating of Perceived Exertion during work-rest allocations

  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 over a 180 minute following the initial stay time

• 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"

  At the end of the 180 minute work-rest allocations

Study Arms (3)

No cooling

ACTIVE COMPARATOR

Participants perform a continuous moderate-intensity work bout (metabolic rate of \~200 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 1:3 work-rest allocation, starting with a 45 min rest break followed by a 15 min work bout for an additional 180-min of work without cooling.

Other: Simulated work in the heat with no cooling during rest breaks

Partial cooling

EXPERIMENTAL

Participants perform a continuous heavy-intensity work bout (metabolic rate of \~200 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 1:3 work-rest allocation, starting with a 45 min rest break followed by a 15 min work bout for an additional 180-min of work with partial cooling equivalent to sitting in a shaded space (WBGT 24°C; 31.7°C and 35% RH) such as under a tree with a light breeze (simulated with pedestal fan fixed at \~2 m/s).

Other: Simulated work in the heat with partial cooling during rest breaks

Full cooling

EXPERIMENTAL

Participants perform a continuous heavy-intensity work bout (metabolic rate of \~200 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 1:3 work-rest allocation, starting with a 45 min rest break followed by a 15 min work bout for an additional 180-min of work with full cooling equivalent to sitting in air-conditioned space (e.g., room or vehicle) maintained at 22°C and 35% RH (equivalent WBGT of 16°C).

Other: Simulated work in the heat with full cooling during rest breaks

Interventions

Simulated work in the heat with no cooling during rest breaks OTHER

Young and older adults will not undergo cooling during rest breaks applied during the implementation of work-rest allocations over the duration of a 180-min work period following a continuous work bout to 38.0°C (equivalent to a 1°C increase in body core temperature above resting levels).

No cooling

Simulated work in the heat with partial cooling during rest breaks OTHER

Young and older adults will not undergo partial cooling during rest breaks applied during the implementation of work-rest allocations over the duration of a 180-min work period following a continuous work bout to 38.0°C (equivalent to a 1°C increase in body core temperature above resting levels).

Partial cooling

Simulated work in the heat with full cooling during rest breaks OTHER

Young and older adults will not undergo full cooling during rest breaks applied during the implementation of work-rest allocations over the duration of a 180-min work period following a continuous work bout to 38.0°C (equivalent to a 1°C increase in body core temperature above resting levels).

Full cooling

Eligibility Criteria

• Age: 18 Years - 69 Years
• Sex: male
• Healthy Volunteers: Yes
• Age Groups: Adult (18-64), Older Adult (65+)

You may qualify if:

• 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

You may not qualify if:

• 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)

Sponsors & Collaborators

• University of Ottawa: lead

Study Sites (1)

University of Ottawa

Ottawa, Ontario, K1N1A2, Canada

Location

MeSH Terms

Conditions

Body Temperature Changes; Heat Stress Disorders; Occupational Stress; Hyperthermia; Motor Activity

Condition Hierarchy (Ancestors)

Signs and Symptoms; Pathological Conditions, Signs and Symptoms; Wounds and Injuries; Occupational Diseases; Stress, Psychological; Behavioral Symptoms; Behavior

Study Design

• Study Type: interventional
• Phase: not applicable
• Allocation: RANDOMIZED
• Masking: SINGLE
• Who Masked: OUTCOMES ASSESSOR
• Purpose: BASIC SCIENCE
• Intervention Model: PARALLEL
• Sponsor Type: OTHER
• Responsible Party: PRINCIPAL INVESTIGATOR
• PI Title: Full Professor

Study Record Dates

• First Submitted: October 4, 2024
• First Posted: October 8, 2024
• Study Start: September 27, 2024
• Primary Completion: March 29, 2025
• Study Completion: March 29, 2025
• Last Updated: June 17, 2025

Record last verified: 2025-06

Locations

CA (1)