Progressive Heat Exposure and Physiological Strain

NCT07847437 | Not Yet Recruiting

• 1 other identifier: HEPRU-2026-09-A
• Study Type: interventional
• Target: 14
• Locations: 1 country
• Sites: 1

Brief Summary

Occupational heat-stress guidelines are commonly based on the level of environmental heat exposure and are largely derived from studies conducted under constant environmental conditions. However, outdoor workers are typically exposed to dynamic conditions, with environmental heat often increasing progressively from morning to afternoon. Consequently, workers may experience substantially different patterns of heat exposure across the workday even when average daily heat exposure is equivalent. It remains unclear whether the temporal pattern of heat exposure influences the amount of work that can be performed or the physiological strain experienced during a workday. Understanding these responses is important for determining whether occupational heat-stress guidelines developed under constant laboratory conditions adequately protect workers exposed to progressively changing environmental conditions. The primary objective of this study is to determine whether the temporal pattern of heat exposure influences work performed and physiological heat strain during a simulated occupational workday in healthy middle-aged and older men. Participants will complete two 9-hour simulated workdays consisting of a 4-hour morning work period, a 1-hour lunch break, and a 4-hour afternoon work period. During one trial, wet-bulb globe temperature (WBGT) will remain constant at 28°C throughout the workday, whereas during the other trial, WBGT will progressively increase from morning to afternoon to simulate the changing environmental conditions of a typical hot summer workday. Both trials will have the same mean daily WBGT of 28°C. Participants will perform moderate-intensity treadmill exercise, and measures including total work performed, core temperature, heart rate, and time to the initial stay-time threshold will be assessed to determine whether constant and progressively increasing heat exposures produce different physiological and work-performance responses across the workday.

Conditions

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

Keywords

Heat Strain; Thermoregulation; Core Temperature; Occupational Heat Stress; Physical Activity; Hyperthermia

Outcome Measures

Primary Outcomes (1)

• Total work performed

  Total work completed during each 240-minute morning and afternoon work period, expressed as cumulative minutes of treadmill walking. Each work period consists of an initial continuous walking phase until core temperature reaches 38.0°C or increases by 1.0°C above the pre-work baseline, whichever occurs first, followed by intermittent 45-min work and 15-min seated rest cycles.

  From the start to the end of the morning and afternoon work periods during each experimental trial (0-4 hours and 5-9 hours after the start of the trial, respectively).

Secondary Outcomes (22)

• Initial stay time

  During the initial continuous walking phase of each morning and afternoon work period.

• Core temperature

  Continuously throughout each experimental trial, including the morning work period, lunch recovery, and afternoon work period.

• Average core temperature during work-rest cycles

  During the work-rest cycles of each morning and afternoon work period.

• Mean skin temperature

  Assessed throughout each experimental trial, including the morning work period, lunch recovery, and afternoon work period.

• Heart rate

  Continuously throughout each experimental trial, including the morning work period, lunch recovery, and afternoon work period.

Study Arms (2)

Constant Heat Exposure

EXPERIMENTAL

Participants will complete a 9-hour simulated occupational workday consisting of a 4-hour morning work period, a 1-hour seated lunch-recovery period, and a 4-hour afternoon work period. Wet-bulb globe temperature (WBGT) will remain constant at 28°C throughout the workday. During each work period, participants will perform moderate-intensity treadmill walking at a metabolic heat production of approximately 200 W·m-2. Participants will initially work continuously until reaching the initial stay-time threshold, defined as a core temperature of 38.0°C or an increase of 1.0°C above pre-work baseline, whichever occurs first. Thereafter, participants will complete 45-min work and 15-min seated rest cycles for the remainder of the work period.

Other: Constant Heat Exposure

Progressively Increasing Heat Exposure

EXPERIMENTAL

Participants will complete a 9-hour simulated occupational workday consisting of a 4-hour morning work period, a 1-hour seated lunch-recovery period, and a 4-hour afternoon work period. WBGT will progressively increase from morning to afternoon to simulate the changing environmental conditions of a hot summer workday, while maintaining a mean daily WBGT of 28°C. During each work period, participants will perform moderate-intensity treadmill walking at a metabolic heat production of approximately 200 W·m-2. Participants will initially work continuously until reaching the initial stay-time threshold, defined as a core temperature of 38.0°C or an increase of 1.0°C above pre-work baseline, whichever occurs first. Thereafter, participants will complete 45-min work and 15-min seated rest cycles for the remainder of the work period.

Other: Progressively Increasing Heat Exposure

Interventions

Constant Heat Exposure OTHER

Participants complete a simulated occupational workday under constant environmental conditions, with WBGT maintained at 28°C throughout the morning and afternoon work periods.

Constant Heat Exposure

Progressively Increasing Heat Exposure OTHER

Participants complete a simulated occupational workday in which WBGT progressively increases from morning to afternoon while maintaining a mean daily WBGT of 28°C.

Progressively Increasing Heat Exposure

Eligibility Criteria

• Age: 45 Years - 65 Years
• Sex: male
• Healthy Volunteers: Yes
• Age Groups: Adult (18-64), Older Adult (65+)

You may qualify if:

• Are male.
• Do not smoke or have been smoke free for at least 5 years.
• English or French speaking.
• Ability to provide informed consent.
• Are physically active (engage in exercise at least 2 times per week for ≥30 minutes)

You may not qualify if:

• Are regularly exposed to hot environments (e.g., occupational heat exposure)
• Experience claustrophobia or discomfort in enclosed environments
• Episode(s) of severe hypoglycemia (requiring the assistance of another person) within the previous year, or inability to sense hypoglycemia (hypoglycemia unawareness).
• Serious complications related to diabetes (gastroparesis, renal disease, uncontrolled hypertension, severe autonomic neuropathy).
• Uncontrolled hypertension - BP \>150 mmHg systolic or \>95 mmHg diastolic in a sitting position.
• Restrictions in physical activity due to disease (e.g. intermittent claudication, renal impairment, active proliferative retinopathy, unstable cardiac or pulmonary disease, disabling stroke, severe arthritis, etc.).
• Use of or changes in medication judged by the patient or investigators to make participation in this study inadvisable.
• Cardiac abnormalities identified during screening

Sponsors & Collaborators

• University of Ottawa: lead

Study Sites (1)

University of Ottawa

Ottawa, Ontario, K1N 6N5, Canada

Location

MeSH Terms

Conditions

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

Condition Hierarchy (Ancestors)

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

Central Study Contacts

Glen P Kenny, PhD

CONTACT

6135625800; gkenny@uottawa.ca

Study Design

• Study Type: interventional
• Phase: not applicable
• Allocation: RANDOMIZED
• Masking: NONE
• Purpose: BASIC SCIENCE
• Intervention Model: CROSSOVER
• Sponsor Type: OTHER
• Responsible Party: PRINCIPAL INVESTIGATOR
• PI Title: Full Professor, University Research Chair

Study Record Dates

• First Submitted: September 18, 2026
• First Posted: September 29, 2026
• Study Start: October 1, 2026
• Primary Completion (Estimated): June 30, 2027
• Study Completion (Estimated): August 31, 2027
• Last Updated: September 29, 2026

Record last verified: 2026-09

Locations

CA (1)