Core Temperature, Exercise, Heat Fatigue, Heat Stress, Exertional
Conditions
Keywords
Heat exposure, Heat strain, Body temperature, Thermoregulation, Occupational heat stress, Uncompensable heat stress, Aging, Sex
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 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, this information is limited to a single work bout and does not consider a second work bout preceded by an extended rest period (e.g., lunch) or next day effects. This is a key consideration, as prolonged work in the heat has been shown to cause next-day impairments in heat dissipation in older men. Further, it remains unclear if the application of the prescribed ACGIH 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). This project will address these knowledge gaps by determining if refinements in initial stay times for moderate-intensity work (represents the average work effort of physically demanding occupations) in the heat (26°C wet-bulb globe temperature) may be required for young and older adults for i) a second work bout that is preceded by an extended rest period such as a lunch break, and ii) a work bout performed on the next day. This includes assessing the efficacy of the prescribed ACGIH work-rest allocations to mitigate increases in core temperature beyond safe limits (\>38.0°C, equivalent to a \>1°C increase in body core temperature above resting levels) during these work periods. Given the known sex-differences in heat loss that can modulate core temperature regulation during an exercise-heat stress, the investigators will conduct separate analysis to identify modulating effects of biological sex on the initial stay times and effectiveness of the work-rest allocation as a heat-alleviation control.
Interventions
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 3:1 work-rest allocation, starting with a 15 min rest break followed by a 45 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).
Sponsors
University of Ottawa
Study design
Allocation
NON_RANDOMIZED
Intervention model
PARALLEL
Primary purpose
BASIC_SCIENCE
Masking
SINGLE (Outcomes Assessor)
Masking description
Data will be blinded prior to analysis
Eligibility
Sex/Gender
ALL
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 first (morning day 1) continuous work bout | 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 | Period of work-rest allocations following initial stay time for morning day 1 | Average core temperature over final 2-hours of each work bout |
Secondary
| Measure | Time frame | Description |
|---|---|---|
| Average skin temperature | Period of work-rest allocations following initial stay time for morning day 1 | Average skin temperatures following initial stay time |
| Heart rate at initial stay time | End of first (morning day 1) continuous work bout | Heart rate at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C) |
| Average heart rate | Period of work-rest allocations following initial stay time for morning day 1 | Average heart rate following initial stay time |
| Skin temperature at initial stay time | End of first (morning day 1) continuous work bout | Skin temperature (4 sites) measured at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C) |
| Mean arterial blood pressure | End of first (morning day 1) work period | Calculated as 1/3 \* systolic blood pressure + 2/3 \* diastolic blood pressure |
| Rate pressure product | End of first (morning day 1) work period | Calculated as systolic blood pressure \* heart rate |
| Systolic blood pressure | End of first (morning day 1) work period | Arterial blood pressure measured via automated oscillometry |
| Fluid loss | End of first (morning day 1) work period | Fluid loss calculated as the change in body mass (corrected for food/fluid consumption). |
| Change in plasma volume | End of first (morning day 1) work period | Determined from venous blood samples |
| Thermal comfort scale | End of first (morning day 1) continuous work bout | Thermal comfort assessed via a visual analog scale (How comfortable does your body temperature feel?) (4: very uncomfortable to 1: comfortable) |
| Thirst sensation scale | End of first (morning day 1) continuous work bout | Assessed via a visual analog scale (How thirsty are you?) (9: very, very thirsty to 1: Not thirsty at all) |
| Rating of Perceived Exertion | End of first (morning day 1) continuous work bout | Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion). |
| Orthostatic Intolerance Symptoms Assessment | End of first (morning day 1) continuous work bout | 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. |
| Fluid consumption | End of first (morning day 1) work period | Cumulative fluid consumption calculated by weighing the participant's water intake |
| Diastolic blood pressure | End of first (morning day 1) work period | Arterial blood pressure measured via automated oscillometry |
Countries
Canada
Outcome results
None listed