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

NCT06543719 | Completed

• 1 other identifier: HEPRU-2024-07
• Study Type: interventional
• Target: 21
• Locations: 1 country
• Sites: 1

Brief Summary

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.

Conditions

Core Temperature; Heat Stress, Exertional; Heat Fatigue; Exercise

Keywords

Heat exposure; Heat strain; Body temperature; Thermoregulation; Occupational heat stress; Uncompensable heat stress; Aging

Outcome Measures

Primary Outcomes (6)

• 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 morning day 1

• 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 of day 2

Secondary Outcomes (54)

• Heart rate at initial stay time

  End of first (morning day 1) continuous work bout

• Heart rate at initial stay time

  End of first (afternoon day 1) continuous work bout

• Heart rate at initial stay time

  End of first (morning day 2) continuous work bout

• Average heart rate

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

• Average heart rate

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

Study Arms (2)

Younger adults.

EXPERIMENTAL

Adults aged 18-30 years with no pre-existing health conditions

Other: Simulated work in the heat

Older adults. Adults aged 50-69 with no pre-existing health conditions

EXPERIMENTAL

Adults aged 50-69 with no pre-existing health conditions

Other: Simulated work in the heat

Interventions

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).

Older adults. Adults aged 50-69 with no pre-existing health conditions; Younger adults.

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, K1N 6N5, Canada

Location

MeSH Terms

Conditions

Heat Stress Disorders; Motor Activity

Condition Hierarchy (Ancestors)

Wounds and Injuries; Behavior

Study Design

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

Study Record Dates

• First Submitted: August 4, 2024
• First Posted: August 9, 2024
• Study Start: July 25, 2024
• Primary Completion: November 28, 2024
• Study Completion: November 28, 2024
• Last Updated: June 18, 2025

Record last verified: 2025-06

Locations

CA (1)