Initial Stay Times and Heat Mitigation Controls for Uncompensable Occupational Heat Stress

Safe Maximum Work Times and the Effectiveness of Work-rest Allocations in Mitigating Increases in Core Temperature During and on the Day Following Prolonged Work in the Heat in Young and Older Workers

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.

Heat exposure, Heat strain, Body temperature, Thermoregulation, Occupational heat stress, Uncompensable heat stress, Aging, Sex

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).

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).

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).

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).

Heart rate at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)

Heart rate at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)

Heart rate at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)

Skin temperature (4 sites) measured at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)

Skin temperature (4 sites) measured at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)

Skin temperature (4 sites) measured at the initial increase in absolute core temperature of 38°C (or relative increase of 1°C)

Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)

Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)

Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)

Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)

Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)

Thermal comfort assessed via a visual analog scale ("How comfortable does your body temperature feel?") (4: very uncomfortable to 1: comfortable)

Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)

Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)

Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)

Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)

Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)

Assessed via a visual analog scale ("How thirsty are you?") (9: very, very thirsty to 1: Not thirsty at all)

Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).

Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).

Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).

Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).

Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).

Perceived exertion assessed via a self-report questionnaire upon verbal prompting (6: no exertion at all to 20: maximal exertion).

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".

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".

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".

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".

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".

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".

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)

Deidentified participant data will be made available with approved analysis plan and signed access agreement