Initial Stay Time and Work‐Rest Scheduling During Consecutive Heavy‐Intensity Workdays in Hot Environments in Young and Older Males

ABSTRACT

Introduction

This study examined initial stay times (IST), defined as the duration of uninterrupted work before core temperature approaches recommended safety limits (38.0°C, ~1.0°C above baseline), and the efficacy of work‐rest cycles during repeated heavy work across consecutive workdays in young and older males.

Methods

Twenty‐one active, non‐heat‐acclimated males (10 young: 22 [20–25] y; 11 older: 67 [63–68] y) completed a 1.5‐day work protocol wearing coveralls (~1 Clo). Participants performed treadmill walking at ~260 W·m⁻² in 26°C WBGT during morning (AM1) and afternoon (PM1) bouts on Day 1, separated by a 1‐h lunch break, followed by a morning bout (AM2) on Day 2.

Results

After reaching IST, ACGIH‐recommended work‐rest cycles (30‐min work/30‐min rest) were implemented for up to 240 min while core temperature was monitored. IST was shorter during PM1 than AM1, reflecting elevated baseline core temperatures entering the afternoon work period (median [Q1, Q3]: AM1: 61 [44, 75] min; PM1: 29 [24, 34] min). Following overnight recovery, IST returned to morning baseline values on Day 2 (AM2: 62 [47, 111] min). Age did not significantly influence IST across work periods. After reaching IST, prescribed work‐rest cycles maintained core temperature near safety limits (≤ 38.1°C) across work periods, although older males exhibited slightly higher peak values.

Conclusion

We show that the duration of the initial work bout before reaching recommended safety limits is reduced during afternoon work but returns to baseline after overnight recovery. Once implemented, prescribed work‐rest cycles effectively regulate thermal strain across work periods in young and older males.

1. Introduction

Occupational heat stress is an increasing concern for workers performing prolonged physical labor, particularly as climate change intensifies environmental heat exposure [1] and the workforce ages [2]. Heat strain can impair health, safety, and productivity, making effective management of occupational heat exposure a growing priority across many industries [3]. Preventing excessive physiological strain requires understanding how long continuous work can be safely performed before recovery periods are implemented. This duration, termed the initial stay time (IST), represents the period of uninterrupted work before core temperature approaches recommended safety thresholds (i.e., 38.0°C, ~1.0°C above baseline resting). Defining safe IST is therefore critical for limiting cumulative thermal and cardiovascular strain during prolonged work shifts and repeated heat exposures.

Workplace heat management strategies commonly rely on exposure limits provided by the American Conference of Governmental Industrial Hygienists (ACGIH) [4]. These guidelines aim to restrict increases in core temperature to ≤ 1.0°C above resting values (~37.5°C) and provide environmental exposure limits for non‐heat‐acclimatized workers (Action Limits) and heat‐acclimatized workers (Threshold Limit Values). When environmental conditions, clothing, and metabolic workload exceed these limits, prescribed work‐rest allocations are recommended to maintain safe core temperatures. However, the guidelines provide limited direction regarding how long workers can safely perform continuous work before recovery periods should begin. In many occupations, workers must travel to worksites and perform uninterrupted tasks before structured recovery is feasible. Until recently, empirical evidence defining safe durations of continuous work prior to implementing work‐rest cycles has been limited.

Recent laboratory investigations have begun to characterize IST under controlled occupational heat stress conditions [5, 6]. During moderate‐intensity work (metabolic rate of ~200 W·m⁻²), the duration of continuous work before core temperature increased by 1.0°C above baseline decreased with environmental heat stress, with median durations of ~82, 60, and 53 min at 26, 29, and 32°C WBGT, respectively [5]. Under heavy‐intensity work (~260 W·m⁻²), permissible continuous work durations were substantially shorter (~55, 42, and 33 min) [6]. These findings demonstrate how rapidly thermal strain can accumulate during continuous work in the heat. However, these investigations evaluated only a single uninterrupted work bout at the start of the workday, leaving it unclear whether tolerance to continuous heavy work changes when work is repeated later in the workday or across consecutive days.

Building on this work, a recent study examining repeated moderate‐intensity work (~200 W·m⁻²) in young and older males and females demonstrated that IST declines during an afternoon work bout following a morning exposure, reflecting incomplete recovery from accumulated thermal strain under similar environmental conditions (26°C WBGT) [7]. In that study, age‐related differences were primarily evident in older females, who exhibited shorter IST compared with young males, whereas older males and young females showed responses comparable to young males. Importantly, once work‐rest cycles were implemented after reaching IST, core temperature remained within recommended safety thresholds throughout prolonged work exposures. Following overnight recovery, IST returned to baseline values the next morning, indicating restoration of work tolerance before the subsequent work shift. While these findings suggest that tolerance to continuous moderate‐intensity work is largely preserved across age groups, it remains unknown whether similar responses occur during heavy‐intensity work, where greater metabolic heat production and physiological fatigue may impose additional strain sufficient to reveal age‐related limitations.

Heavy labor imposes substantially greater physiological demands than moderate work. Increased metabolic heat production accelerates body heat storage and elevates cardiovascular strain as greater skin blood flow and sweating are required for heat dissipation [8, 9]. Heavy work also increases physiological stress and neuromuscular fatigue, which can impair sustained performance and physiological stability [10]. When repeated across prolonged shifts or consecutive days, incomplete recovery of fluid balance, cardiovascular function, and metabolic substrates may further reduce tolerance to heat stress [9, 11]. Age‐related changes in thermoregulatory [12] and cardiovascular function [13] may further influence these responses. Advancing age is associated with reduced heat‐loss capacity, including attenuated sweating responses and diminished cutaneous vasodilation, which impair heat dissipation during exercise [12]. Although previous investigations during single heavy work bouts have reported relatively small differences between young and older males [14], these differences may become more apparent when work is repeated across prolonged exposures [15, 16].

Accordingly, this study determined whether tolerance to continuous heavy‐intensity work varies across repeated work periods within a workday and across consecutive days in young and older males. Participants performed simulated heavy‐intensity work (~260 W·m⁻²) at 26°C WBGT. IST, defined as the duration of continuous work before core temperature reached 38.0°C, was assessed during a morning work bout, a second bout following a 1‐h lunch recovery, and during the subsequent morning work period on the following day. We also evaluated whether prescribed work‐rest cycles implemented after reaching IST would limit further increases in core temperature during prolonged work exposures. We hypothesized that IST would decline during the afternoon work bout due to residual physiological strain and that this reduction would be greater in older males. We further hypothesized that, despite overnight recovery, IST on the subsequent morning would remain lower in older males relative to younger males. Finally, we hypothesized that despite a shorter IST in older males, implementation of prescribed work‐rest cycles would maintain core temperature within recommended safety limits (i.e., ~38.0°C) across all work periods.

2. Methods

2.1. Ethical Approval and Study Design

The study was approved by the University of Ottawa Health Sciences and Science Research Ethics Board (H‐11‐22‐8660) and conducted in accordance with the Declaration of Helsinki (excluding database registration). All participants provided written informed consent prior to participation. This single‐site, laboratory‐based study (NCT06543719) was designed to examine the effects of age and work period on thermoregulatory and cardiovascular responses during prolonged heavy‐intensity work in the heat over consecutive days.

2.2. Participants

Twenty‐two volunteers who met eligibility criteria were enrolled; however, one older male withdrew due to illness unrelated to the study. The final sample therefore included 10 young males and 11 older males. Participant characteristics are presented in Table 1. Participants were habitually active, non‐smokers, and free from cardiovascular, metabolic, or respiratory conditions that could contraindicate heat exposure or exercise testing. All participants were non‐heat‐acclimated and reported no regular occupational or recreational heat exposure.

Table 1.

Physical characteristics of enrolled participants.

Variable	Young Males (n = 10)	Older Males (n = 11)
Age, years	22 (20–25)	67 (63–68)
Height, cm	180 (179–183)	177 (175–183)
Mass, kg	75.2 (70.5–91.0)	88.6 (80.7–92.8)
Body surface area, m 2	1.9 (1.9–2.1)	2.1 (2.0–2.1)
Smoking status
Never	10 (100%)	10 (91%)
Past	0 (0%)	1 (9%)
Habitual physical activity, min/week	203 (165–285)	360 (175–380)
Types of physical activity No. (%)
Walking	4 (40%)	9 (82%)
Jogging, biking, swimming, rowing	10 (100%)	5 (45%)
Organized sports	5 (50%)	5 (45%)
Peak aerobic capacity (V̇O2peak), mL/kg/min	51.5 (45.9–56.5)	36.7 (34.9–39.0)
ACSM VO2peak percentile, %	68 (45–81)	85 (85–90)
Using prescribed medications, No. (%)	1 (10%)	7 (64%)

Note: Values are median (interquartile range) or number of participants (%).

Anthropometrics, including height, body mass, and body composition, were measured during screening. Body surface area (BSA) was estimated using the Du Bois equation (0.20247 × height in meters^0.725 × mass in kg^0.425) [17]. Habitual physical activity levels were evaluated using the Canadian Society for Exercise Physiology Get Active Questionnaire [18] and activity types detailed using the Kohl Physical Activity Questionnaire [19]. Peak aerobic capacity (V̇O₂peak) was assessed using an incremental treadmill test to volitional fatigue.

Medication use was self‐reported during screening. Reported medications included antihypertensives (calcium channel blockers and/or ACE inhibitors, n = 2), statins (n = 4), attention‐deficit/hyperactivity disorder medications (n = 1), anticonvulsants (sodium channel blocker, n = 1), acid reflux inhibitors (n = 1), non‐steroidal anti‐inflammatory drugs for chronic pain (n = 1), medication for overactive bladder (n = 1), and ophthalmic medications (eye drops, n = 1).

2.3. Experimental Protocol

All participants completed one preliminary session and one experimental session conducted over two consecutive days. Participants were instructed to avoid strenuous exercise, alcohol, caffeine, and non‐steroidal anti‐inflammatory drugs for at least 24 h prior to testing and to consume a light meal and approximately 500 mL of water 2 h before arriving at the laboratory.

2.4. Preliminary Screening

During the screening visit, participants completed the health and physical activity questionnaires described above and were familiarized with all experimental procedures, including treadmill walking and instrumentation. Body height and mass were measured via a stadiometer (model 2391, Detecto) and a digital weighing terminal (model CBU150X, Mettler Toledo Inc.) and used to calculate body surface area [17].

Participants then completed a treadmill walking test (Desmo Evo, Woodway, Waukesha, WI) to determine the speed and incline to achieve a fixed metabolic rate of 260 W·m⁻², corresponding to a heavy‐intensity work effort [4] as quantified using indirect calorimetry (MCD Medgraphics Ultima Series, MGC Diagnostics, MN, USA). Following a brief rest (~5 min), V̇O₂peak was measured using a progressive, incremental treadmill running protocol to volitional fatigue [20].

2.5. Experimental Sessions

Experimental sessions were conducted over two consecutive days, beginning at 07:00 to simulate a typical workday. Upon arrival, participants’ hydration status was assessed via urine specific gravity (USG ≤ 1.025) [21]. Participants who did not meet this criterion consumed approximately 500 mL of water during instrumentation, after which USG was reassessed prior to heat exposure.

Following 15 min of seated rest, baseline measurements of core temperature, heart rate, arterial blood pressure, and perceptual responses were recorded. Participants then entered the climate‐controlled chamber maintained at 34.1°C and 35% relative humidity with minimal airflow (< 0.2 m·s⁻¹). Participants performed treadmill walking at the individualized speed and grade until core (rectal) temperature reached 38.0°C (IST). Subsequent work‐rest cycles followed an ACGIH‐recommended 1:1 allocation [4] (30 min seated rest, 30 min work) until 4 h had elapsed or volitional fatigue occurred. Water maintained at ambient chamber temperature (~34°C) was available ad libitum during all seated rest periods.

Following the morning exposure, participants exited the chamber for a 1‐h lunch break in a temperate environment (~22°C), during which they remained seated and consumed a light self‐selected meal consistent with their usual lunch. Nude and clothed body mass were measured before and after lunch, and hydration status was reassessed.

Participants then returned to the chamber for the afternoon work session, which followed the same protocol as the morning exposure. Participants completed a 15‐min seated habituation period followed by continuous heavy‐intensity treadmill walking until core temperature reached 38.0°C (IST). Work‐rest cycles were then implemented for a total exposure duration of 4 h. The session concluded with a final 15‐min seated rest period in the heat, after which nude and clothed body mass and a urine sample were collected. Participants typically departed the laboratory at approximately 19:00.

The following morning (07:00), participants completed a repeat of the morning session (AM2), identical in structure to Day 1. The experimental protocol concluded following completion of this final exposure.

For clarity, “rest” refers to the 30‐min seated pauses within work‐rest cycles during heat exposure, whereas “recovery” refers to the longer intervals between work bouts or days, such as the 1‐h lunch break and overnight period.

2.6. Measurements

2.6.1. Core and Skin Temperatures

Rectal temperature, used as an index of core temperature, was continuously measured using a flexible thermocouple inserted ≥ 12 cm beyond the anal sphincter (Mon‐a‐Therm™ General Purpose Temperature Probe; Mallinckrodt Medical Inc., St. Louis, MO, USA). Skin temperature was recorded continuously at eight standard sites using miniature data loggers (DS1922L Thermochron; OnSolution Pty Ltd). Mean skin temperature was calculated using ISO 9886 weighting factors as follows: forehead (7%), right scapula (17.5%), upper right chest (17.5%), upper right arm (7%), right forearm (7%), right hand (5%), right anterior thigh (19%), and right calf (20%) [22].

2.6.2. Cardiovascular Responses

Heart rate was recorded continuously using a telemetry system (Polar H7 WearLink transmitter with M400 receiver; Polar Electro Oy, Kempele, Finland). Systolic and diastolic blood pressures were measured after 10 min of seated rest at baseline (prior to heat exposure) and following the final 15‐min seated rest at the end of each session. Measurements were obtained at the brachial artery using an automated oscillometric device (Digital Blood Pressure Monitor UM‐211; A&D Medical, Tokyo, Japan). Three measurements were obtained at each time point, separated by 1‐min intervals, and averaged for analysis.

Heart rate responses were expressed as absolute values (beats·min⁻¹) and as percentage heart rate reserve (%HRR) calculated as:

$$\%\text{HRR}=({\text{HR}}_{\text{work}}-{\text{HR}}_{\text{rest}})/({\text{HR}}_{\max }-{\text{HR}}_{\text{rest}})\times 100$$

where HR_work represents heart rate at a given time point, HR_rest represents resting heart rate, and HR_max was determined during the incremental exercise test.

2.6.3. Hydration

Urine specific gravity (USG) was measured at the start of each day and immediately following the afternoon trial on Day 1 and the morning trial on Day 2 using a handheld optical refractometer (Reichert TS 400; Reichert Inc., Depew, NY, USA). Venous blood samples were collected at the same time points to determine hemoglobin and hematocrit (DxH, Beckman Coulter, Brea, CA, USA), enabling estimation of relative plasma volume changes during exercise and recovery [23].

2.6.4. Perceptual Responses

Participants’ perceptions of heat and exertion were monitored throughout each exercise bout. Thermal sensation using a modified ASHRAE scale (0 = neutral, 7 = extremely hot) and rating of perceived exertion (RPE) using the Borg 6–20 scale [24] were assessed in the final minute of each bout.

2.7. Statistical Analysis

To assess the influence of work period and group on the time required to reach a core temperature of 38.0°C (IST), work times were compared between work periods (AM1 [reference condition], PM1, AM2) and group (Young Males [reference group], Older Males) using Cox regression models accounting for repeated measures within individuals using the “survival” [25] package in R [26]. Differences in survival distributions across work periods and groups were evaluated using log‐rank tests. Additional analyses were conducted to examine potential effects of physical fitness and medication use on IST by incorporating V̇O₂peak (continuous covariate) and medication use (yes/no; categorical predictor) into the models.

Secondary analyses examined whether heart rate (and heart rate reserve), mean skin temperature, and perceptual responses (RPE and thermal sensation) at the completion of IST and during work‐rest cycles were influenced by group or work period using linear mixed‐effects models with participant included as a random effect (lme4 package [27] in R).

Descriptive analyses of core temperature responses during work periods were conducted to determine mean and peak core temperatures following IST and the duration during which core temperature exceeded 38.0°C. To evaluate whether the likelihood of exceeding 38.0°C differed by group, a generalized linear mixed‐effects model with binomial distribution and logit link was used. The binary outcome variable indicated whether core temperature exceeded 38.0°C (1 = yes, 0 = no) at each time point. The model included group as a fixed effect and participant ID as a random effect to account for repeated measures.

Analyses were conducted in R (version 4.5.0), using the lme4 package [27]. Only work periods in which participants completed a full 30‐min continuous bout of work were included in these analyses. Data were excluded when rest periods exceeded the prescribed 30‐min duration.

3. Results

3.1. Baseline Physiological Responses

Baseline resting physiological variables before each work period are presented in Table 2.

Table 2.

Mean (standard deviation) resting body temperatures and heart rate recorded during 15‐min habituation period prior to each period of simulated work.

	Work Period	Young Males	Older Males
Tcore (°C)	AM1	37.0 (0.2)	37.0 (0.3)
	PM1	37.5 (0.2)	37.4 (0.3)
	AM2	36.9 (0.2)	36.9 (0.3)
Tskin (°C)	AM1	33.8 (0.8)	33.4 (0.8)
	PM1	34.4 (0.8)	33.5 (0.8)
	AM2	33.6 (0.8)	33.4 (0.8)
Heart Rate (beats/min)	AM1	76 (11)	71 (14)
	PM1	91 (11)	84 (14)
	AM2	70 (11)	67 (14)

Abbreviations: Tcore, core temperature; Tskin, mean skin temperature; AM1, morning workday; 1, PM1, afternoon workday 1; AM2, morning of workday 2.

3.2. Initial Stay Time (IST)

The median (interquartile range) time to reach a core temperature of 38.0°C in each work period is shown in Figure 1. For all participants combined, median IST was 61 (44, 75) min during AM1, 29 (24, 34) min during PM1, and 62 (47, 111) min during AM2.

Figure 1.

Median (interquartile range) time to reach initial stay time (IST) defined by a core (rectal) temperature of 38°C across groups and work periods.

3.3. Effect of Age on the Time Taken to Reach a Core Temperature of 38.0°C

Age did not significantly influence the time required to reach a core temperature of 38.0°C across all work periods (log‐rank test: p = 0.113). In the model examining the effect of age, adjustment for aerobic fitness indicated that individuals with higher V̇O₂peak were less likely to reach 38.0°C (hazard ratio: 0.96 [95% CI: 0.92, 1.00], p = 0.031). Medication was not associated with the likelihood of reaching 38.0°C (hazard ratio: 1.24 [0.7, 2.2], p = 0.471).

3.4. Effect of Work Period on the Time Taken to Reach a Core Temperature of 38.0°C

The time required to reach a core temperature of 38.0°C differed significantly among work periods (log‐rank test, p < 0.001). Compared with AM1, participants reached 38.0°C more rapidly during PM1 (hazard ratio: 4.5 [95% confidence interval: 2.0, 10.1], p < 0.001), whereas the rate of reaching 38.0°C was lower during AM2 (0.6 [0.4, 0.9], p = 0.007). Accordingly, median (Q1, Q3) IST across all participants decreased from 61 (44, 75) min during AM1 to 29 (24, 34) min during PM1 before returning to 62 (47, 111) min during AM2. In the model examining work period, individuals with higher V̇O₂peak were less likely to reach a core temperature of 38.0°C, independent of group and work period (0.90 [0.90, 1.00], p < 0.001).

3.5. Core Temperature Responses During Work‐Rest Cycles

During work‐rest cycles, core temperature was higher in Older than Young Males (Table 3, p = 0.003). Core temperatures during work‐rest cycles were higher in PM1 than AM1 (mean difference: 0.2 [95% CI: 0.2, 0.2]°C, p < 0.001). Core temperature during work‐rest cycles was marginally lower in AM2 than AM1 (mean difference: −0.03 [95% CI: −0.05 to −0.02]°C, p < 0.001); however, the magnitude of this difference was physiologically trivial. The median (Q1, Q3) and mean (SD) duration of time with core temperature > 38.0°C during each work‐rest cycle is shown in Table 4. The duration of time with core temperature > 38.0°C did not differ between groups (p = 0.078). Descriptive analysis indicated that implementation of work‐rest cycles following IST maintained median core temperature ≤ 38.1°C. Despite this, eight individual instances occurred (1 in AM1, 4 in PM1, and 3 in AM2) where peak core temperature in Older Males exceeded 38.5°C. No Young Males exceeded 38.5°C, although three instances > 38.3°C occurred during PM1. Median (p = 0.038) and peak (p = 0.016) core temperatures were higher in PM1 than AM1. Older Males also exhibited higher median (p = 0.006) and peak (p = 0.013) core temperatures than Young Males. These differences were not influenced by medication use (both p > 0.709).

Table 3.

Median and peak (interquartile range) core temperatures recorded during work‐rest cycles for each work period in young and older males.

Work Period	Group	Median (Q1, Q3)	Peak (Median [Q1, Q3])
AM1	Young Males	37.9 (37.7, 37.9)	38.1 (37.9, 38.2)
	Older Males	38.0 (37.9, 38.0)	38.3 (38.1, 38.4)
PM1	Young Males	37.9 (37.8, 38.0)	38.2 (38.1, 38.3)
	Older Males	38.1 (38.0, 38.4)	38.3 (38.2, 38.6)
AM2	Young Males	37.7 (37.7, 37.9)	38.0 (37.9, 38.1)
	Older Males	38.0 (37.9, 38.1)	38.3 (38.1, 38.4)

Abbreviations: AM1, morning workday 1, PM1, afternoon workday 1; AM2, morning of workday 2.

Table 4.

Duration (minutes; interquartile range) of work‐rest cycles where core temperature exceeded 38.0°C.

Work Period	W:R Cycle	Median (Q1, Q3)	Mean (SD)
AM1	1	0 (0, 7)	4 (6)
AM1	2	5 (0, 12)	7 (9)
AM1	3	6 (1, 16)	10 (11)
PM1	1	16 (4, 24)	15 (11)
PM1	2	18 (0, 24)	15 (12)
PM1	3	11 (0, 17)	11 (11)
AM2	1	0 (0, 9)	5 (6)
AM2	2	0 (0, 8)	6 (8)
AM2	3	10 (0, 13)	9 (9)

Abbreviations: W, work; R, rest. SD, standard deviation. AM1, morning workday 1, PM1, afternoon workday 1; AM2, morning of workday 2.

3.6. Heart Rate and Skin Temperature Responses at the Completion of IST

Heart rate at IST was influenced by both work period and group (Table 5). Heart rate was higher during PM1 than AM1 (p = 0.001) but did not differ between AM2 and AM1 (p = 0.756). Young Males also exhibited higher heart rates than Older Males (15 [2, 29] beats·min⁻¹, p = 0.038). A significant group x work period interaction existed (p = 0.040). Heart rate reserve was higher during PM1 than AM1 (p < 0.001) but did not differ between AM2 and AM1 (p = 0.638) or between Young and Older Males (3 [−5, 12]%, p = 0.465). However, heart rate reserve demonstrated a significant group × work period interaction (p = 0.014), with Older Males exhibiting higher heart rate reserve than Young Males during PM1 (7 [1, 13]%, p = 0.035). Mean skin temperature at IST was lower during AM2 than AM1 (p = 0.004) but did not differ between PM1 and AM1 (p = 0.823). Mean skin temperature also did not differ between Young and Older Males (p = 0.214). No significant group x work period interaction existed (p = 0.382).

Table 5.

Physiological and perceptual responses at the completion of initial stay times.

					P values
	Condition	Young Males	Older Males	Older Males ‐ young Males	Group	Work Period	Interaction
Mean Skin Temperature (°C)	AM1	34.4 (0.7)	34.2 (0.8)	−0.2 (−0.8, 0.4)	0.214	0.003	0.382
	PM1	34.8 (0.5)	34.4 (0.6)	−0.3 (−0.9, 0.3)
	AM2	34.5 (0.7)	34.0 (0.6)	−0.5 (−1.1, 0.1)
Heart Rate (beats/min)	AM1	129 (16)	109 (16)	−17 (−32, −2)	0.038	< 0.001	0.040
	PM1	129 (19)	119 (19)	−10 (−25, 5)
	AM2	125 (13)	107 (16)	−20 (−35, 5)
Heart Rate Reserve (%)	AM1	49 (10)	44 (11)	−5 (−15, 5)	0.465	< 0.001	0.014
	PM1	52 (11)	54 (13)	2 (−8, 12)
	AM2	49 (8)	42 (13)	−7 (−17, 3)
Thermal Sensation (scale; 0‐7)	AM1	2 (1)	4 (1)	1 (0, 2)	0.033	0.772	0.168
	PM1	2 (1)	3 (1)	1 (0, 2)
	AM2	3 (1)	3 (1)	1 (0, 2)
RPE (scale; 6‐20)	AM1	11 (2)	12 (1)	1 (−1, 2)	0.355	0.079	0.233
	PM1	11 (2)	12 (1)	1 (−1, 2)
	AM2	12 (2)	12 (2)	0 (−2, 2)

Note: Values are mean (standard deviation). AM1, morning workday 1, PM1, afternoon workday 1; AM2, morning of workday 2.

3.7. Heart Rate and Skin Temperature Responses During Work‐Rest Cycles

Heart rate, heart rate reserve, and mean skin temperature differed by work period (all p < 0.001) (Table 6). For each variable, values were higher during PM1 than AM1 and lower during AM2 than AM1 (all p < 0.001). Neither heart rate nor heart rate reserve differed between Young and Older Males during work‐rest cycles (both p ≥ 0.141). Mean skin temperature also did not differ between groups (p = 0.132). However, a significant group × work period interaction was present for mean skin temperature (p < 0.001).

Table 6.

Physiological and perceptual responses during work‐rest cycles.

					P values
	Condition	Young Males	Older Males	Older Males ‐ young Males	Group	Work Period	Interaction
Mean Skin Temperature (°C)	AM1	34.4 (0.8)	33.9 (0.9)	−0.4 (−1.1, 0.2)	0.132	< 0.001	< 0.001
	PM1	34.5 (0.7)	34.0 (0.8)	−0.4 (−1.1, 0.2)
	AM2	34.3 (0.8)	33.6 (0.7)	−0.6 (−1.3, 0.0)
Heart Rate (beats/min)	AM1	111 (24)	99 (20)	−13 (−27, 1)	0.141	< 0.001	< 0.001
	PM1	112 (23)	102 (20)	−7 (−21, 6)
	AM2	108 (22)	98 (19)	−10 (−24, 4)
Heart Rate Reserve (%)	AM1	38 (17)	34 (16)	−4 (−13, 4)	0.789	< 0.001	< 0.001
	PM1	38 (16)	38 (16)	1 (−7, 10)
	AM2	34 (19)	33 (16)	−1 (−9, 8)
Thermal Sensation (scale; 0–7)	AM1	3 (1)	4 (1)	1 (0, 2)	0.003	0.020	0.335
	PM1	3 (1)	4 (1)	1 (0, 2)
	AM2	3 (1)	4 (1)	1 (0, 2)
RPE (scale; 6–20)	AM1	12 (2)	12 (1)	1 (0, 2)	0.060	< 0.001	0.030
	PM1	12 (2)	14 (2)	1 (0, 2)
	AM2	12 (2)	13 (2)	0 (−2, 2)

Note: Values are mean (standard deviation). AM1, morning workday 1, PM1, afternoon workday 1; AM2, morning of workday 2. RPE, rate of perceived exertion.

3.8. Perceptual Responses at IST and During Work‐Rest Cycles

RPE at IST was higher during AM2 than AM1 (p = 0.026) but did not differ between PM1 and AM1 (p = 0.186). No group effect (p = 0.355) or group × work period interaction (p = 0.233) was observed. Thermal sensation at IST also did not differ between work periods (PM1: p = 0.569; AM2: p = 0.928). However, Older Males reported higher thermal sensation than Young Males (p = 0.033), with no group × work period interaction (p = 0.168). During work‐rest cycles, RPE was higher during PM1 (1 [95% CI: 1, 1] AU, p < 0.001) and AM2 (1 [0, 1] AU, p = 0.011) compared with AM1. No overall group effect was observed (p = 0.060), although a group × work period interaction was present (p = 0.030), with Older Males reporting higher RPE than Young Males during PM1 (1 [0, 2] AU, p = 0.010) but not AM2 (0 [−1, 1] AU, p = 0.461).

Thermal sensation during work‐rest cycles also differed by work period (p = 0.020), with higher values during PM1 than AM1 (p = 0.009) but no difference between AM2 and AM1 (p = 0.663). Older Males reported higher thermal sensation overall, with no group × work period interaction (p = 0.335).

3.9. Markers of Hydration Across Work Periods

Changes in plasma volume following each 4‐h work period did not differ between Young and Older Males (p = 0.737) and were not influenced by work period (p = 0.127). Urine specific gravity (USG) also did not differ between groups (p = 0.662) but varied over time, with higher values following PM1 than at baseline on Day 1 (p < 0.001) (Table 7). No other time‐dependent differences in USG were observed (all p > 0.114).

Table 7.

Urine specific gravity in young and older males across work periods.

Group	Time	N	USG
Young Males	AM1 Baseline	10	1.021 (1.015, 1.025)
Older Males	AM1 Baseline	11	1.016 (1.014, 1.022)
Young Males	Lunch 1	10	1.023 (1.020, 1.026)
Older Males	Lunch 1	11	1.021 (1.016, 1.024)
Young Males	Post PM1	10	1.027 (1.025, 1.030)
Older Males	Post PM1	10	1.026 (1.025, 1.027)
Young Males	AM2 Baseline	10	1.018 (1.014, 1.025)
Older Males	AM2 Baseline	11	1.024 (1.019, 1.025)
Young Males	Post AM2	9	1.022 (1.017, 1.026)
Older Males	Post AM2	11	1.023 (1.013, 1.026)

Note: Data are presented as median (Q1, Q3). N, number of participants. USG, urine specific gravity. AM1, morning workday 1, PM1, afternoon workday 1; AM2, morning of workday 2.

4. Discussion

This study examined tolerance to continuous heavy‐intensity work across repeated work periods within a day and across consecutive days in young and older males. We hypothesized that IST would be shorter during the afternoon work period due to residual physiological strain, that this decline would be greater in older males, and that the implementation of prescribed work‐rest cycles following IST would maintain core temperature within recommended safety limits. Consistent with our first hypothesis, IST was markedly reduced during the afternoon work period compared with the morning work period. Contrary to our second hypothesis, age did not significantly influence IST, although older males exhibited slightly greater thermal strain during subsequent work‐rest cycles. Supporting our final hypothesis, the implementation of ACGIH‐recommended work‐rest cycles [4] effectively maintained core temperature near recommended safety limits (≤ 38°C, ~Δ1.0°C above baseline resting) across all work periods.

Across groups, median IST values were 61 min (44–75) during the initial morning work bout (AM1), 29 min (24–34) during the afternoon work bout (PM1), and 62 min (47–111) during the subsequent morning exposure (AM2). Young and older males demonstrated comparable tolerance to uninterrupted heavy work across these work periods, indicating that age did not significantly influence the duration of continuous work required to reach recommended thermal safety limits. Both groups exhibited a similar reduction in IST during the afternoon work bout, with work tolerance returning to baseline the following morning. Despite comparable ISTs, as discussed below, older males exhibited slightly higher core temperatures during subsequent work‐rest cycles, suggesting modest age‐related differences in the regulation of thermal strain once work‐rest allocations were implemented.

These comparable responses between young and older males were somewhat unexpected given well‐documented age‐related changes in thermoregulatory function. Advancing age is associated with reductions in sweating capacity and cutaneous vasodilation that can impair heat dissipation during exercise [28, 29]. Aerobic fitness can also influence thermophysiological responses during work in the heat, as higher aerobic fitness improves cardiovascular stability and enhances heat dissipation during exercise [30]. However, including V̇O₂peak as a covariate did not alter the observed relationships between age and IST, indicating that differences in aerobic fitness do not account for the comparable work tolerance observed between young and older males in the present study.

Previous work has defined IST during single bouts of continuous heavy‐intensity work (~260 W·m⁻²) across environmental conditions ranging from 26°C to 32°C WBGT [6]. In that investigation, older females exhibited shorter IST compared with young males, whereas young females and older males demonstrated responses comparable to young males, indicating that sex‐related differences in heat tolerance may emerge under conditions of elevated thermal strain. Similar patterns have been reported during moderate‐intensity work (~200 W·m⁻²) performed under comparable environmental conditions, although ISTs were longer due to the lower metabolic heat production associated with moderate workloads [5]. Collectively, these investigations demonstrate that tolerance to the initial continuous work bout is strongly influenced by the combined heat stress imposed by environmental conditions and metabolic workload. However, these earlier investigations defined IST during a single work bout performed at the start of the workday and therefore did not evaluate how tolerance to continuous work changes across a work shift or across consecutive workdays.

Evidence that tolerance to continuous work can vary across the workday has recently been demonstrated during repeated moderate‐intensity work (~200 W·m⁻²) performed under similar environmental conditions (26°C WBGT) in young and older males and females [7]. In that investigation, IST declined during an afternoon work bout following a morning exposure, reflecting incomplete recovery from accumulated thermal strain despite a lunch recovery period. Importantly, this daylong pattern was consistent across sex and age groups, and tolerance to continuous work returned to baseline values the next morning after overnight recovery. The present study extends these observations by demonstrating that similar daylong and next‐day responses occur during heavy‐intensity work in young and older males.

The marked reduction in IST during the afternoon work period indicates that tolerance to uninterrupted heavy work is strongly influenced by the physiological state at the start of the work bout. Baseline core temperatures prior to the afternoon work bout were approximately 0.4–0.5°C higher than those recorded prior to the morning work period in both young and older males, indicating that participants entered the afternoon work bout with a similarly elevated thermal load regardless of age. Because IST was defined by reaching a core temperature threshold of 38.0°C, the elevated baseline core temperature reduced the available thermal reserve before this limit was reached, resulting in a shorter duration of continuous work during the afternoon period.

The elevated baseline temperatures further suggest that the 1‐h lunch recovery period was insufficient to fully dissipate heat stored during the morning work exposure. Experimental studies examining passive recovery following exercise in the heat indicate that core temperature declines at an average rate of approximately 0.012°C·min⁻¹ [31], with cooling rates progressively slowing as core temperature approaches resting levels. Consequently, full restoration of baseline thermal state following prolonged heavy work may require recovery periods longer than those typically available during standard work breaks, allowing residual heat storage to persist entering subsequent work bouts.

In addition to incomplete thermal recovery, several physiological mechanisms likely contributed to the reduced tolerance to continuous work observed during the afternoon work period. First, the elevation in baseline core temperature likely reflects both residual thermal strain from the morning work bout and the normal circadian rise in core temperature across daytime hours. Circadian rhythms produce predictable fluctuations in core temperature throughout the day, with values typically increasing during daytime hours and peaking in the late afternoon [32]. Although aging may modestly reduce the amplitude of circadian temperature rhythms, the daytime rise in core temperature is generally preserved in healthy older adults [33]. Consistent with this, both young and older males exhibited comparable elevations in baseline core temperature prior to the afternoon work bout. Second, participants exhibited higher heart rate and heart rate reserve during the afternoon work period despite identical metabolic workloads. These responses were observed in both young and older males and are consistent with cardiovascular drift during prolonged exercise in the heat, whereby reductions in stroke volume are compensated for by increases in heart rate to maintain cardiac output [34, 35, 36]. As cardiovascular strain increases, the capacity to simultaneously support skeletal muscle perfusion and cutaneous blood flow required for heat dissipation may become compromised, thereby contributing to a more rapid rise in core temperature [34, 35, 36, 37, 38].

Beyond these cardiovascular responses, cumulative physiological fatigue following the initial prolonged work bout at the start of the work shift may further reduce tolerance to uninterrupted work later in the day. Heavy‐intensity exercise imposes substantial thermoregulatory [39] and cardiovascular demands [40], and repeated exposures can increase physiological strain even when workload and environmental conditions remain constant. In the present study, participants entered the afternoon work bout with elevated baseline core temperatures and greater cardiovascular strain, suggesting that residual physiological strain from the morning exposure may have reduced the capacity to dissipate heat and maintain cardiovascular stability during the subsequent bout. Evidence for cumulative heat strain across days, however, remains limited. A recent systematic review examining prolonged physical work performed across consecutive days reported minimal evidence for cumulative increases in core temperature or cardiovascular strain, although only a small number of studies used standardized environmental and work conditions [41]. Consistent with these observations, our findings did not indicate a progressive increase in heat strain on the subsequent day. The present findings, together with our recent investigation of repeated moderate‐intensity work under comparable environmental conditions [7], help address this gap by providing controlled laboratory evidence describing changes in tolerance to continuous work across the workday and across consecutive workdays in the heat.

In contrast to the incomplete recovery observed during the lunch break, overnight recovery appeared sufficient to restore work tolerance. ISTs during the subsequent morning work bout were comparable to those observed at the start of the first day, indicating that the extended recovery period between workdays allowed restoration of baseline physiological state even following heavy‐intensity work. Resting heart rate and hydration indices measured prior to the second day were also similar to values recorded prior to the first morning work bout in both young and older males, suggesting that participants began the second day in a comparable physiological condition. These findings align with our recent investigation examining repeated moderate‐intensity work performed under comparable environmental conditions [7], in which IST during the subsequent morning work period was likewise similar to that observed at the start of the first day despite prior daylong heat exposure. Collectively, these observations suggest that when adequate overnight recovery is provided, tolerance to continuous work may be restored by the following day even after prolonged occupational heat exposure, provided workers begin the subsequent workday adequately hydrated.

However, the present findings differ somewhat from several field investigations in which workers performing prolonged duties in hot environments exhibited greater increases in core temperature on the second day of deployment, including studies of firefighters [42] and electrical utility workers [42, 43]. These observations have been interpreted as evidence that cumulative heat strain across successive workdays may progressively challenge thermoregulatory capacity. In contrast, controlled laboratory studies examining repeated heat exposures have produced inconsistent results. Some investigations report no detectable reduction in heat tolerance on the following day [44, 45], whereas others have documented greater thermal strain during repeated heat exposures in young adults [46]. Evidence in older individuals is limited; however, one laboratory study reported a reduction in heat‐loss capacity on the day following prior heat exposure in older males [47], a response not observed in young adults under a comparable protocol [44]. Importantly, participants in the present study began the second day well hydrated, whereas field investigations consistently report that workers often arrive for subsequent work shifts mildly dehydrated following previous workdays in the heat [48, 49], a factor that may increase physiological strain and reduce heat tolerance, thereby influencing both IST and the effectiveness of work‐rest cycles, as discussed in the following section.

Despite the marked reduction in IST during the afternoon work period, the implementation of ACGIH‐recommended work‐rest cycles following IST effectively regulated core temperature across all work periods. Core temperature during work‐rest cycles remained ≤ 38.1°C, indicating that once structured recovery periods were introduced, thermal strain was generally maintained within recommended safety limits. Although most responses remained within this range, several individual instances exceeded this level. Older males experienced a small number of instances where peak core temperature exceeded 38.5°C, whereas no young males exceeded this threshold. Importantly, core temperatures during the subsequent morning work period returned to values comparable to those observed during the first morning exposure, indicating that overnight recovery restored the effectiveness of the work‐rest allocations. Collectively, these findings indicate that although tolerance to uninterrupted heavy work declines across the workday, the implementation of structured work‐rest cycles following IST can effectively limit further increases in core temperature and allow prolonged work to be performed while maintaining thermal strain near recommended safety limits.

Notably, hydration status was maintained across work periods and did not differ between young and older males, indicating that fluid balance was unlikely to have influenced the observed changes in IST or thermophysiological responses during the subsequent work‐rest cycles. These findings should therefore be interpreted within the context of occupational field observations, where workers performing repeated days of heavy labor in hot environments often begin subsequent work shifts mildly dehydrated due to incomplete fluid replacement following the previous workday [48, 49]. Dehydration reduces plasma volume, increases cardiovascular strain, and impairs sweating and skin blood flow, thereby accelerating heat storage during physical work [9, 50]. Both aging [28] and hypohydration [9, 50] independently attenuate heat loss during exercise, which can increase physiological strain during work in the heat [51]. Recent work further suggests that hypohydration may influence thermophysiological responses differently across age groups [52]. For example, hypohydration has been shown to impair evaporative heat dissipation in young adults, whereas in middle‐aged men evaporative heat loss was largely maintained but cardiovascular strain increased to a greater extent compared with younger individuals. Although hydration status was controlled in the present study, these findings suggest that workers who begin subsequent work shifts dehydrated may experience greater physiological strain, reduced heat tolerance, potentially shorter ISTs, and diminished effectiveness of work‐rest cycles in maintaining core temperature within recommended safety limits.

In occupational settings, workers and supervisors often rely on perceptual cues such as perceived exertion or thermal sensation to judge heat strain [53]. However, the present findings indicate that these subjective responses did not consistently reflect the physiological responses observed across work periods. Ratings of perceived exertion at the completion of IST were modestly higher during AM2 compared with AM1 but did not differ between AM1 and PM1 despite the substantially shorter IST observed during the afternoon work bout. Similarly, thermal sensation at the completion of IST did not differ across work periods, although older males reported slightly higher thermal sensation overall than younger males. During subsequent work‐rest cycles, ratings of perceived exertion were higher during PM1 and AM2 compared with AM1, whereas thermal sensation increased only modestly during the afternoon work period. These perceptual responses did not closely mirror the physiological responses observed across work periods, during which tolerance to continuous work was markedly reduced during PM1 but restored the following morning. Similar patterns were observed in our recent investigation examining repeated moderate‐intensity work performed under comparable environmental conditions [7], in which perceptual responses changed only modestly despite clear reductions in IST during the afternoon work bout. Experimental studies examining perceptual indices of heat strain provide mixed evidence regarding their ability to reflect physiological strain. While some investigations report strong correlations between perceptual and physiological strain indices under controlled laboratory conditions, suggesting that perceptual responses may provide a general indication of heat strain [53], others demonstrate that perceptual measures can underestimate physiological strain during sustained exercise‐heat stress when core temperature and cardiovascular strain progressively increase [54]. Together with the present findings during heavy‐intensity work, these observations indicate that perceptual cues do not consistently track the physiological responses that determine tolerance to continuous work. Consequently, relying on subjective perceptions of heat strain alone may be insufficient for managing occupational heat exposure, reinforcing the importance of physiologically derived exposure limits and structured work‐rest schedules for protecting workers performing prolonged labor in hot environments.

4.1. Limitations

Several limitations should be considered when interpreting these findings. First, a number of participants reported using prescription medications. Although medication use did not influence the observed core temperature responses across work periods in the present study, certain medications can influence thermoregulation, cardiovascular responses, or fluid balance and may therefore alter heat tolerance under occupational conditions [55]. Second, the present investigation examined responses in males only and therefore these findings may not fully generalize to female workers. Finally, the study was conducted under controlled laboratory conditions in which hydration status and environmental exposures were carefully regulated, whereas occupational environments often involve greater variability in hydration status and environmental heat exposure. Accordingly, caution is warranted when extrapolating these findings directly to field settings.

In conclusion, tolerance to continuous heavy work in the heat is strongly influenced by the timing of work within the day and accumulated physiological strain from prior activity. Continuous work tolerance was markedly reduced during the afternoon work period but returned to baseline values following overnight recovery. Importantly, once prescribed work‐rest schedules were implemented following IST, core temperature remained near recommended safety limits across work periods and age groups. These findings demonstrate that combining IST‐based exposure limits with structured recovery periods provides an effective strategy for maintaining worker safety during repeated heavy work in hot environments.

Author Contributions

G.P.K. conceived and designed the study. F.K.O. and G.P.K. performed initial literature searches. G.B. and S.M.T. collected data. F.K.O. performed statistical analysis and prepared tables and figures. G.P.K. and F.K.O. drafted the manuscript, and all authors revised the manuscript for important intellectual content, approved the final version, and agree to be accountable for all aspects of the work. All persons designated as authors qualify for authorship, and all those who qualify are listed. All authors had full access to and accept responsibility for the data presented in the study.

Ethics Statement

The study was approved by the University of Ottawa Health Sciences and Science Research Ethics Board (H‐11‐22‐8660) and conformed to the Declaration of Helsinki, except for registration in a database. Written and informed consent was obtained from all volunteers prior to their participation. All work was performed at the University of Ottawa.

Kenny G. P., Bourgois G., Taggart S. M., et al., “Initial Stay Time and Work‐Rest Scheduling During Consecutive Heavy‐Intensity Workdays in Hot Environments in Young and Older Males,” American Journal of Industrial Medicine 69 (2026): 1093‐1105. 10.1002/ajim.70138.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.