Consecutive Occupational Heat Exposure With and Without Overnight Cooling

Time to core temperature of 38.0°C

Time, in minutes, from the start of continuous walking to the point at which core temperature reaches 38.0°C. Participants who do not reach 38.0°C before voluntary termination, reaching the applicable stopping criterion, or completing 240 minutes of continuous walking will have their time recorded at termination or completion.

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, in minutes, from the start of continuous walking to the point at which rectal temperature increases by 1.0°C above baseline. For participant safety, continuous walking will not continue beyond a rectal temperature of 38.5°C. Therefore, for participants who reach 38.5°C before attaining a 1.0°C increase in rectal temperature, the time at which 38.5°C is reached will be recorded. Participants who voluntarily terminate exercise or complete 240 minutes of continuous walking before reaching either endpoint will have their time recorded at termination or completion.

Time frame: During each 240-minute morning and afternoon work bout on Day 1 and Day 2.

Average core temperature during work/rest cycles

Average core temperature measured during the intermittent work-rest cycles following the initial continuous work period.

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

Heart rate measured at the time a participant reaches a rectal temperature of 38.0°C during the initial continuous walking period. For participants who do not reach a rectal temperature of 38.0°C before voluntary termination, reaching the applicable stopping criterion, or completing 240 minutes of continuous walking, heart rate at termination or completion will be recorded.

Time frame: Each morning and afternoon work bout on Day 1 and Day 2.

Heart rate at 1.0°C increase in core temperature

Heart rate measured at the time a participant achieves a 1.0°C increase in core temperature above baseline during the initial continuous walking period. For participant safety, continuous walking will not continue beyond a core temperature of 38.5°C. Therefore, for participants who reach a core temperature of 38.5°C before attaining a 1.0°C increase above baseline, the heart rate at 38.5°C will be recorded. Participants who voluntarily terminate exercise or complete 240 minutes of continuous walking before reaching either endpoint will have their final recorded heart rate used.

Time frame: Each morning and afternoon work bout on Day 1 and Day 2.

Average heart rate during work/rest cycles

Average heart rate measured during the intermittent work-rest cycles following the initial continuous work period.

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

Mean skin temperature measured at the time a participant reaches a core temperature of 38.0°C during the initial continuous walking period. For participants who do not reach a core temperature of 38.0°C before voluntary termination, reaching the applicable stopping criterion, or completing 240 minutes of continuous walking, mean skin temperature at termination or completion will be recorded.

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

Mean skin temperature measured at the time a participant achieves a 1.0°C increase in core temperature above baseline during the initial continuous walking period. For participant safety, continuous walking will not continue beyond a core temperature of 38.5°C. Therefore, for participants who reach a core temperature of 38.5°C before attaining a 1.0°C increase above baseline, the mean skin temperature at 38.5°C will be recorded. Participants who voluntarily terminate exercise or complete 240 minutes of continuous walking before reaching either endpoint will have their final recorded mean skin temperature used.

Time frame: Each morning and afternoon work bout on Day 1 and Day 2.

Average mean skin temperature during work/rest cycles

Average mean skin temperature measured during the intermittent work-rest cycles following the initial continuous work period.

Time frame: During each 240-minute morning and afternoon work bout on Day 1 and Day 2.

Overnight heart rate

Average heart rate during the overnight recovery period. Heart rate will be measured continuously throughout each overnight exposure.

Time frame: Overnight recovery periods (18:30-08:00) on Night 1 and Night 2.

Overnight mean skin temperature

Average mean skin temperature during the overnight recovery period. Mean skin temperature will be measured continuously throughout each overnight exposure.

Time frame: Overnight recovery periods (18:30-08:00) on Night 1 and Night 2.

Overnight core temperature

Average core temperature during the overnight recovery period. Core temperature will be measured continuously throughout each overnight exposure.

Time frame: Overnight recovery periods (18:30-08:00) on Night 1 and Night 2.

Metabolic workload

Metabolic workload during simulated occupational work will be assessed using indirect calorimetry. Oxygen consumption (VO₂) and respiratory exchange ratio (RER) will be measured and used to calculate metabolic workload, expressed in watts per square meter (W/m²). Higher values indicate greater metabolic heat production and physiological work demand.

Time frame: During morning and afternoon work bouts on Day 1 and Day 2.

Whole-body evaporative heat loss

Change in whole-body evaporative heat loss from baseline to post-exposure, assessed using direct air calorimetry during standardized exercise at three work intensities.

Time frame: Baseline approximately 7 days before exposure and post-exposure on the morning of Day 3.

Whole-body dry heat exchange

Change in whole-body dry heat exchange from baseline to post-exposure, assessed using direct air calorimetry during standardized exercise at three work intensities.

Time frame: Baseline approximately 7 days before exposure and post-exposure on the morning of Day 3.

Whole-body total heat loss

Change in whole-body total heat loss from baseline to post-exposure, assessed using direct air calorimetry during standardized exercise at three work intensities.

Time frame: Baseline approximately 7 days before exposure and post-exposure on the morning of Day 3.

Urine specific gravity

Urine specific gravity will be measured to assess hydration status. Higher values indicate greater urine concentration and therefore poorer hydration status, whereas lower values indicate lower urine concentration and therefore better hydration status.

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

Fluid loss from body mass change

Fluid loss calculated from changes in body mass during each daytime work exposure, corrected for food and fluid intake.

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

Changes in plasma volume

Change in plasma volume from baseline values calculated from duplicate measurements of hemoglobin and hematocrit at the start and end of each exposure using the technique by Dill and Costill

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

Systolic blood pressure

Systolic blood pressure measured in triplicate using automated oscillometry.

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

Diastolic blood pressure

Diastolic blood pressure measured in triplicate using automated oscillometry.

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

Orthostatic intolerance symptoms

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"

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

Thermal sensation scale

Thermal sensation will be assessed using a visual analog ASHRAE-based scale in response to the question "How hot do you feel?" Scores range from 0 (neutral) to 7 (extremely hot). Higher scores indicate a greater sensation of heat and therefore a worse outcome, whereas lower scores indicate less perceived heat and therefore a better outcome.

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

Thermal comfort

Thermal comfort will be assessed in response to the question "How comfortable does your body temperature feel?" using a visual analog scale ranging from 1 (comfortable) to 4 (very uncomfortable). Higher scores indicate lower thermal comfort and therefore a worse outcome, whereas lower scores indicate greater thermal comfort and therefore a better outcome.

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

Thirst sensation

Thirst sensation will be assessed in response to the question "How thirsty are you?" using a visual analog scale ranging from 1 (not thirsty at all) to 9 (very, very thirsty). Higher scores indicate greater thirst and therefore a worse outcome, whereas lower scores indicate less thirst and therefore a better outcome.

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

Rating of Perceived Exertion (Borg 6-20 Scale)

Perceived exertion will be assessed using a visual analog scale in response to the question "How hard are you working?" Scores range from 6 (no exertion at all) to 20 (maximal exertion). Higher scores indicate greater perceived effort and therefore a worse outcome, whereas lower scores indicate less perceived effort and therefore a better outcome.

Time frame: Each morning and afternoon work bout on Day 1 and Day 2.

Feel-good scale

Feelings of well-being will be assessed using a visual analog scale in response to the question "How good do you feel?" Scores range from -5 (very bad) to +5 (very good), with 0 representing neutral. Higher scores indicate more positive feelings and therefore a better outcome, whereas lower scores indicate more negative feelings and therefore a worse outcome.

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

Sleep duration

Sleep quantity will be assessed each morning using a sleep diary. Participants will report their bedtime, wake time, and total sleep duration during the previous night. Sleep duration will be recorded in hours, with higher values indicating longer sleep duration.

Time frame: Assessed on the morning of Day 1, Day 2, and Day 3

Sleep quality rating

Sleep quality will be assessed each morning using a sleep diary. Participants will rate the quality of their sleep during the previous night on a 5-point scale ranging from 1 (very poor) to 5 (very good). Higher scores indicate better perceived sleep quality, whereas lower scores indicate poorer perceived sleep quality.

Time frame: Assessed on the morning of Day 1, Day 2, and Day 3

Overnight thermal symptom score

Thermal symptoms experienced during sleep will be assessed each morning using the Sleep Sensation Questionnaire. Participants will rate the severity of heat- and sweat-related symptoms experienced overnight, including feeling too hot, feeling sweaty, and sweat accumulation on bedding. Responses range from 0 (not at all) to 4 (extremely) for each symptom and are summed to generate a total symptom score. Total scores range from 0 to 32, with higher scores indicating greater overnight thermal discomfort.

Time frame: Assessed on the morning of Day 1, Day 2, and Day 3

Profiles of Mood States (POMS-40) total mood disturbance score

Mood state will be assessed using the 40-item Profiles of Mood States (POMS-40) questionnaire. Participants will rate how they feel at the time of assessment using a 5-point Likert scale ranging from 0 (not at all) to 4 (extremely). Responses will be summed to generate a total score. Higher scores indicate greater overall mood disturbance and therefore a worse outcome, whereas lower scores indicate lower mood disturbance and therefore a better outcome.

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

Environmental Symptoms Questionnaire-IV (ESQ-IV) total symptom score

Environmental symptomatology associated with prolonged heat exposure will be assessed using the Environmental Symptoms Questionnaire-IV (ESQ-IV). Participants will rate 68 symptoms using a 6-point Likert scale ranging from 0 (not at all) to 5 (extreme). Responses are summed, with positive items reverse scored as appropriate, to generate a Total Symptom Score ranging from 0 to 340. Higher scores indicate greater symptom burden and therefore a worse outcome, whereas lower scores indicate fewer symptoms and therefore a better outcome.

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

State hostility scale total score

Hostility and aggression will be assessed using the State Hostility Scale, a self-report questionnaire evaluating feelings of hostility, irritability, anger, and aggressive affect. Responses are summed to generate a total score after reverse scoring of applicable items. Total scores range from 35 to 175, with higher scores indicating greater hostility and aggression and therefore a worse outcome, whereas lower scores indicate less hostility and aggression and therefore a better outcome.

Time frame: Assessed at baseline and throughout Day 1, Day 2, and Day 3.

Cued Stroop Test average reaction time (Sway Medical)

Executive function will be assessed using the Cued Stroop Test administered through the Sway Medical platform. Participants will complete a series of congruent, neutral, and incongruent color-word tasks requiring response inhibition and cognitive flexibility. Performance will be quantified using average reaction time, measured in milliseconds (ms). Lower values indicate faster responses and therefore better executive function, whereas higher values indicate slower responses and therefore poorer executive function.

Time frame: Each morning and evening on Day 1 and Day 2, as well as the morning of Day 3.

Cued Stroop Test error count (Sway Medical)

Executive function will be assessed using the Cued Stroop Test administered through the Sway Medical platform. Participants will complete a series of congruent, neutral, and incongruent color-word tasks requiring response inhibition and cognitive flexibility. Performance will be quantified using the total number of errors recorded during the test. Lower values indicate better executive function, whereas higher values indicate poorer executive function.

Time frame: Each morning and evening on Day 1 and Day 2, as well as the morning of Day 3.

Impulse Control Test reaction time (Sway Medical)

Participants will be provided with a tablet device running the Sway Medical testing platform for the assessment of impulse control. During the Impulse Control Test, participants will be presented with a series of visual "go" and "no-go" cues. Participants will be instructed to initiate a movement of the device as quickly as possible in response to "go" cues and to withhold movement in response to "no-go" cues. Performance will be quantified using reaction time, measured in milliseconds (ms). Lower values indicate faster responses and therefore better impulse control, whereas higher values indicate slower responses and therefore poorer impulse control.

Time frame: Each morning and evening on Day 1 and Day 2, as well as the morning of Day 3.

Impulse Control Test error count (Sway Medical)

Participants will be provided with a tablet device running the Sway Medical testing platform for the assessment of impulse control. During the Impulse Control Test, participants will be presented with a series of visual "go" and "no-go" cues. Participants will be instructed to initiate a movement of the device as quickly as possible in response to "go" cues and to withhold movement in response to "no-go" cues. Performance will be quantified using the total number of errors recorded during the test, including failures to respond appropriately to the presented cue. Lower values indicate better impulse control, whereas higher values indicate poorer impulse control.

Time frame: Each morning and evening on Day 1 and Day 2, as well as the morning of Day 3.

Reaction Time Test reaction time (Sway Medical)

Participants will be provided with a tablet device running the Sway Medical testing platform for the assessment of reaction time. During the Reaction Time Test, participants will be presented with a visual cue and instructed to initiate a movement of the device as quickly as possible in response. Performance will be quantified using reaction time, measured in milliseconds (ms). Lower values indicate faster responses and therefore better reaction time performance, whereas higher values indicate slower responses and therefore poorer reaction time performance.

Time frame: Each morning and evening on Day 1 and Day 2, as well as the morning of Day 3.

Memory Test working memory score (Sway Medical)

Memory will be assessed using the Memory Test administered through the Sway Medical platform. Participants will be presented with a three-letter code to memorize, followed by a series of increasingly complex spatial memory tasks involving the recall of patterns displayed on a 2 × 3 grid. Performance will be quantified using the Working Memory Score, defined as the number of spatial patterns correctly recalled before an error is made. Higher scores indicate better working memory performance, whereas lower scores indicate poorer working memory performance.

Time frame: Each morning and evening on Day 1 and Day 2, as well as the morning of Day 3.

Memory Test delayed recall score (Sway Medical)

Memory will be assessed using the Memory Test administered through the Sway Medical platform. Participants will be presented with a three-letter code to memorize, followed by a series of increasingly complex spatial memory tasks involving the recall of patterns displayed on a 2 × 3 grid. Following completion of the spatial memory task, participants will be asked to recall the original three-letter code. Performance will be quantified using the Delayed Recall Score, which ranges from 0 to 3 and reflects the number of letters correctly recalled. Higher scores indicate better delayed recall performance, whereas lower scores indicate poorer delayed recall performance.

Time frame: Each morning and evening on Day 1 and Day 2, as well as the morning of Day 3.

CDC 4-Stage Balance Test composite score (Sway Medical)

Balance will be assessed using the CDC 4-Stage Balance Test administered through the Sway Medical platform. Participants will hold a tablet device against their chest while auditory cues guide them through four consecutive stances: feet side-by-side, semi-tandem stance, tandem stance, and single-leg stance. Body sway will be measured using the tablet's integrated accelerometer and quantified using a composite balance score generated by the Sway Medical platform. Scores range from 0 to 100, with higher scores indicating better balance performance and lower scores indicating poorer balance performance.

Time frame: Each morning and evening on Day 1 and Day 2, as well as the morning of Day 3.

Postural sway total path excursion length (BTrackS Force Plate)

Balance will be assessed using a BTrackS force plate. Participants will stand with their feet shoulder-width apart, hands on their hips, and eyes closed. The assessment will consist of four 20-second trials, including one practice trial, separated by 20 seconds of rest. Center of pressure (COP) displacement along the mediolateral (x) and anteroposterior (y) axes will be used to calculate total path excursion length, measured in centimeters (cm). Lower values indicate less postural sway and therefore better balance performance, whereas higher values indicate greater postural sway and therefore poorer balance performance.

Time frame: Each morning and evening on Day 1 and Day 2, as well as the morning of Day 3.

Heart rate variability: standard deviation of normal R-R intervals (SDNN)

Cardiac autonomic modulation will be assessed using a 5-lead Holter electrocardiogram (ECG) monitor (DigiTrak XT, Philips Zymed, Andover, MA, USA). Consecutive R-R intervals will be derived from the ECG recording to calculate the standard deviation of normal R-R intervals (SDNN), measured in milliseconds (ms). Higher values indicate greater heart rate variability and cardiac autonomic modulation, whereas lower values indicate reduced heart rate variability and cardiac autonomic modulation.

Time frame: Throughout Day 1, Day 2, and Day 3.

Heart rate variability: root mean square of successive differences (RMSSD)

Cardiac autonomic modulation will be assessed using a 5-lead Holter electrocardiogram (ECG) monitor (DigiTrak XT, Philips Zymed, Andover, MA, USA). Consecutive R-R intervals will be derived from the ECG recording to calculate the root mean square of successive differences between adjacent normal R-R intervals (RMSSD), measured in milliseconds (ms). Higher values indicate greater parasympathetic modulation and heart rate variability, whereas lower values indicate reduced parasympathetic modulation and heart rate variability.

Time frame: Throughout Day 1, Day 2, and Day 3.