Body Temperature Changes, Heat Exposure, Heat Fatigue, Heat Stress, Hyperthermia, Work Related Stress
Conditions
Keywords
Heat strain, Thermoregulation, Exercise, Core temperature, Occupational heat stress, Uncompensable heat stress, Aging
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).
Interventions
OTHERSimulated work in the heat with no cooling during rest breaks
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).
OTHERSimulated work in the heat with partial cooling during rest breaks
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).
OTHERSimulated work in the heat with full cooling during rest breaks
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).
Sponsors
University of Ottawa
Study design
Allocation
RANDOMIZED
Intervention model
PARALLEL
Primary purpose
BASIC_SCIENCE
Masking
SINGLE (Outcomes Assessor)
Eligibility
Sex/Gender
MALE
Age
18 Years to 69 Years
Healthy volunteers
Yes
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)
Design outcomes
Primary
| Measure | Time frame | Description |
|---|---|---|
| Initial stay time | 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 | 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). |
| Average core temperature during work-rest allocations | Period of work-rest allocations over a 180 minute following the initial stay time | Average core temperature following initial stay time |
| Heart rate at initial stay time | 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 | Heart rate at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C) |
| Average heart rate during work-rest allocations | Period of work-rest allocations over a 180 minute following the initial stay time | Average heart rate following initial stay time |
| Skin temperature at initial stay time | 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 | Skin temperature measured at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C) |
| Average skin temperature during work-rest allocations | Period of work-rest allocations over a 180 minute following the initial stay time | Average heart rate following initial stay time |
| Thermal comfort scale at initial stay time | 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 assessed via a visual analog scale (How comfortable does your body temperature feel?) (4: very uncomfortable to 1: comfortable) |
| Thermal comfort scale during work-rest allocations | Period of work-rest allocations over a 180 minute following the initial stay time | Thermal comfort assessed via a visual analog scale (How comfortable does your body temperature feel?) (4: very uncomfortable to 1: comfortable) |
| Thirst sensation scale at initial stay time | 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 | Assessed via a visual analog scale (How thirsty are you?) (9: very, very thirsty to 1: Not thirsty at all) |
| Thirst sensation scale during work-rest allocations | Period of work-rest allocations over a 180 minute following the initial stay time | Assessed via a visual analog scale (How thirsty are you?) (9: very, very thirsty to 1: Not thirsty at all) |
| Rating of Perceived Exertion at initial stay time | 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 | Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion). |
| Rating of Perceived Exertion during work-rest allocations | Period of work-rest allocations over a 180 minute following the initial stay time | Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion). |
| Orthostatic Intolerance Symptoms Assessment | At the end of the 180 minute work-rest allocations | 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 |
Countries
Canada
Outcome results
None listed