Consecutive Occupational Heat Exposure With and Without Overnight Cooling

Official title

Physiological Responses to Consecutive Days of Occupational Heat Exposure With and Without Overnight Access to Cooling

Overview

Extreme heat events are becoming more frequent, intense, and prolonged, increasing the risk of heat-related illness among workers exposed to hot environments. Current occupational heat-stress guidelines are designed to limit excessive increases in core body temperature and physiological strain during work in the heat through the use of work-rest schedules. However, these guidelines are largely based on single-day heat exposures and assume that workers recover in cool indoor environments between shifts. During heat events, many workers may instead recover in homes or accommodations that remain overheated, particularly in the absence of air conditioning. Inadequate overnight cooling may impair the body's ability to dissipate heat, increase physiological strain, and reduce tolerance to heat exposure on subsequent workdays. Older adults may be especially vulnerable due to age-related impairments in heat-loss capacity. To date, the effects of recovering in an overheated indoor environment on physiological strain and thermoregulatory function during repeated occupational heat exposure remain poorly understood, limiting the development of evidence-based recommendations to protect workers during prolonged heat events. The primary objective of this study is to determine whether overnight recovery in an overheated indoor environment increases physiological strain during repeated occupational heat exposure in older adults. Specifically, investigators will assess whether recovery at the recently proposed upper indoor temperature limit of 26°C and 45% relative humidity (RH) (PMID: 38329752) is sufficient to attenuate elevations in core temperature during subsequent work in the heat compared with recovery in an overheated indoor environment (31°C, 45% RH) representative of conditions commonly experienced during extreme heat events. Healthy older adults will complete two consecutive days and nights of simulated moderate-intensity occupational heat exposure, with overnight recovery occurring in either the 26°C or 31°C condition. Measures of core temperature, cardiovascular strain, heat-loss capacity, hydration status, sleep quality, cognitive function, mood, and perceptual responses will be assessed to determine the impact of overnight recovery conditions on responses to repeated heat exposure. Whole-body heat-loss capacity will also be evaluated before and after the exposure protocol as assessed during incremental, intermittent exercise protocol performed in an air calorimeter.

Interventions

• Other Simulated occupational heat exposure with recovery at recommended indoor temperature limit
  Participants complete a 2-day, 2-night simulated occupational heat exposure protocol involving daytime work in a hot environment (37.5°C, 35% RH) and overnight recovery (18:30-08:00) at the recommended upper indoor temperature limit (26°C, 45% RH).
• Other Simulated occupational heat exposure with recovery in an overheated indoor environment
  Participants complete a 2-day, 2-night simulated occupational heat exposure protocol involving daytime work in a hot environment (37.5°C, 35% RH) and overnight recovery (18:30-08:00) in an overheated indoor environment (31°C, 45% RH).

Primary outcome measures

• Total work volume. [Time frame: During each 240-minute morning and afternoon work bout on Day 1 and Day 2.]

Secondary outcome measures (12)

• Time to core temperature of 38.0°C [Time frame: During each 240-minute morning and afternoon work bout on Day 1 and Day 2.]
• Time to 1.0°C increase in rectal temperature [Time frame: During each 240-minute morning and afternoon work bout on Day 1 and Day 2.]
• Average core temperature during work/rest cycles [Time frame: During each 240-minute morning and afternoon work bout on Day 1 and Day 2.]
• Heart rate at core temperature of 38.0°C [Time frame: Each morning and afternoon work bout on Day 1 and Day 2.]
• Heart rate at 1.0°C increase in core temperature [Time frame: Each morning and afternoon work bout on Day 1 and Day 2.]
• Average heart rate during work/rest cycles [Time frame: During each 240-minute morning and afternoon work bout on Day 1 and Day 2.]
• Mean skin temperature at core temperature of 38.0°C [Time frame: Each morning and afternoon work bout on Day 1 and Day 2.]
• Mean skin temperature at 1.0°C increase in core temperature [Time frame: Each morning and afternoon work bout on Day 1 and Day 2.]
• Average mean skin temperature during work/rest cycles [Time frame: During each 240-minute morning and afternoon work bout on Day 1 and Day 2.]
• Overnight heart rate [Time frame: Overnight recovery periods (18:30-08:00) on Night 1 and Night 2.]
• Overnight mean skin temperature [Time frame: Overnight recovery periods (18:30-08:00) on Night 1 and Night 2.]
• Overnight core temperature [Time frame: Overnight recovery periods (18:30-08:00) on Night 1 and Night 2.]

Eligibility criteria

Inclusion criteria

• 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)

Exclusion criteria

• 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

Criteria are shown verbatim from the registry (in English). Final eligibility is always assessed by the study center.

Healthy volunteers: Yes

Study design

Allocation

Randomized

Model

Crossover

Masking

Single blind

Primary purpose

Basic science

Study locations

Canada · 1 center

• University of Ottawa — Ottawa

Publications

• Meade RD, Akerman AP, Notley SR, Kirby NV, Sigal RJ, Kenny GP. Effects of Daylong Exposure to Indoor Overheating on Thermal and Cardiovascular Strain in Older Adults: A Randomized Crossover Trial. Environ Health Perspect. 2024 Feb;132(2):27003. doi: 10.1289/EHP13159. Epub 2024 Feb 8. PMID 38329752

Identifiers

NCT: NCT07656519 · HEPRU-2026-06-A

Primary sources (government registries)

• ClinicalTrials.gov (US NIH)

View this study on ClinicalTrials.gov ↗