Leonidas G. Ioannou

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.

Introduction: Work‐rest allocations recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) limit heat strain and are typically implemented from the onset of work. Whether delaying implementation until after the initial stay time (IST; the duration of continuous work before core temperature reaches 38.0°C) improves total work performed is unknown. Methods: Young males performed moderate‐ (200 W·m ⁻² ) and heavy‐ (260 W·m ⁻² ) intensity work at a wet‐bulb globe temperature 26°C (34.1°C, 35% relative humidity). Two work protocols were compared using independent cohorts (moderate: n = 15 per cohort; heavy: n = 10 per cohort): Protocol A, IST‐guided delayed work‐rest implementation and Protocol B, standard ACGIH work‐rest implementation consisting of moderate: 45‐min work, 15‐min rest; heavy: 30‐min work, 30‐min rest. Trials consisted of 240‐min morning and afternoon work periods separated by a 1‐h lunch break. Results: During moderate work, total work time was greater with Protocol A in the morning (+24 min, 95% CI: 13–36, p < 0.001), with no difference in the afternoon (0 min, 95% CI: −12 to 12, p = 0.994). During heavy work, morning work time was also greater with Protocol A (+30 min, 95% CI: 22–38, p < 0.001), with no afternoon difference (+7 min, 95% CI: −1 to 15, p = 0.081). Time above 38.3°C did not differ during moderate work ( p = 0.252) and was lower during heavy work (−24 min, 95% CI: −44 to −4, p = 0.017). Conclusion: Delaying work‐rest cycles until after IST increases morning work output by ~10%–13% without increasing time above critical core temperature thresholds.

Global warming poses major public health challenges, with pregnant individuals particularly vulnerable to heat. We aim to synthesise evidence on the effects of ambient heat on pregnancy and birth outcomes and identify heat thresholds associated with increased risk of these adverse outcomes. We searched Medline, Web of Science, Scopus, GreenFile and Cinahl (June 2024) for peer-reviewed studies reporting thresholds for ambient heat; heat from medical devices, saunas, pools or studies on conception, IVF, postnatal health were excluded. Participants within studies are limited to individuals capable of becoming pregnant, including women and those identifying differently. Risk of bias is assessed using the Office of Health Assessment and Translation tool. Findings are narratively synthesised according to outcome and exposure metric, and a random effects meta-regression is used to estimate heat thresholds for acute heat effects on adverse outcomes. Evidence is limited by heterogeneity and methodological variability among studies included, and through review process limitations, including no independent dual data extraction and single person performing quality assessment. Meta-analyses determine Tmax thresholds 30.55 °C, 34.11 °C and 39.07 °C for 5%, 10% and 20% risk increase (21 studies, sample=5,589,862), suggesting increasing risk of adverse pregnancy and birth outcomes with increasing heat. The narrative syntheses (103 studies, sample=41,951,950) identifies heterogeneous indicators and a lack of comprehensive heat stress indices. Funding was received through the European Union’s Horizon Framework Programme and UKRI Innovate. PROSPERO registration: CRD42024610438.

Introduction: Accurate assessment of heat strain at initial stay time (IST; core temperature 38.0°C or Δ1.0°C above resting), when heat mitigation decisions are required, is critical for managing occupational heat exposure. We evaluated the relationship and agreement between physiological (Physiological Strain Index [PSI] and adaptive PSI [aPSI]) and perceptual (Perceptual Strain Index [PeSI]) indices during simulated occupational work performed until IST. Methods: Ninety‐seven healthy adults completed treadmill walking to IST at either moderate‐intensity (200 W·m ⁻² ; 24 young [12 females] and 25 older adults [13 females]) or heavy‐intensity (260 W·m ⁻² ; 24 young [12 females] and 24 older adults [12 females]) at wet‐bulb globe temperatures (WBGT) of 26°C, 29°C, or 32°C. Core and skin temperatures, heart rate, thermal sensation, and ratings of perceived exertion were measured to calculate PSI, aPSI, and PeSI. Relationships between indices were assessed using linear mixed‐effects models, and agreement was evaluated using Bland−Altman analyses. Results: PeSI increased with PSI and aPSI during moderate‐ and heavy‐intensity work (all p < 0.01). These relationships were not modified by group, environmental condition, body surface area, or aerobic fitness. However, agreement between perceptual and physiological indices was limited, showing wide limits of agreement between PSI and PeSI during moderate‐ (−2.3 to 4.2 AU) and heavy‐intensity (−3.0 to 4.0 AU) work. The agreement between PeSI and aPSI was modestly improved but remained variable. Conclusion: We show that although perceptual responses increase with physiological heat strain, they do not reliably reflect physiological burden at IST, when decisions regarding the implementation of heat mitigation strategies are required.

Rationale: Industry guidelines recommend work‐rest allocations to limit increases in core temperature (Tcore; > 38.0°C or > Δ1.0°C above resting) during work in the heat. However, the initial duration of permissible work before implementing these allocations is not specified (initial stay time [IST]) and individual factors such as sex and age affecting thermoregulation are not considered. Objectives: The objective of this study was to assess whether factors of sex and age influence IST. Methods: We examined the permissible work duration before reaching Tcore of 38.0°C or Δ1.0°C above resting in young (18–30 years) and older (50–69 years) males and females performing moderate‐intensity (200 W/m ² ) simulated work at ambient conditions of 26, 29, and 32°C wet‐bulb globe temperature (WBGT). Findings: IST significantly decreased as ambient conditions intensified ( p < 0.0001). Median (95% CI) durations to 38°C across groups were 59 (43–87) minutes at 26°C, 40 (34–65) minutes at 29°C, and 35 (31–46) minutes at 32°C WBGT. Likewise, median times to achieve Δ1.0°C above resting temperature significantly differed by environmental conditions ( p < 0.0001): 66 (56–128) minutes at 26°C, 68 (54–80) minutes at 29°C, and 44 (41–53) minutes at 32°C WBGT. Older females were significantly more likely to experience Tcore elevations of 38.0°C (hazard ratio [95% CI]: (3.1 [1.6, 5.8], p < 0.001) and Δ1.0°C above resting (1.9 [1.1, 3.0], p = 0.007) compared to young males. In contrast, IST for young females and older males were similar to young males. Impact: Our findings underscore the impact of hotter ambient conditions on IST and identify older females as particularly susceptible during moderate‐intensity work in heat emphasizing the need for tailored occupational heat exposure guidelines.

Rationale: Industry guidelines recommend work‐rest schedules to prevent unsafe core temperature elevations (> 38.0°C or > Δ1.0°C above baseline resting) in an “average” worker exposed to occupational heat stress. While permissible initial work durations (initial stay times [IST]) before implementing rest schedules have been suggested, existing data are limited to moderate‐intensity efforts. Objectives: The objective of this study was to assess whether factors of sex and age influence IST during heavy‐intensity work. Methods: We assessed work durations before reaching unsafe core temperature thresholds (i.e., > 38.0°C or > Δ1.0°C above baseline resting) among young (18–30 years) and older (50–69 years) males and females during simulated heavy‐intensity work at 26°C, 29°C, and 32°C wet‐bulb globe temperatures (WBGT). Findings: The likelihood of surpassing a core temperature of 38.0°C significantly increased at 32°C WBGT (hazard ratio [95% confidence interval]: 2.0 [1.5, 2.7], p = 0.001), but not at 29°C WBGT (1.1 [0.9, 1.4], p = 0.342), compared to 26°C WBGT. Median (interquartile range) durations to reach 38.0°C across groups were 32 (23–52), 29 (23–47), and 26 (18–35) minutes at 26°C, 29°C, and 32°C WBGT, respectively. The likelihood of reaching Δ1.0°C above baseline resting significantly increased at both 32°C (3.8 [2.6, 5.4], p < 0.001) and 29°C WBGT (1.5 [1.1, 2.0], p < 0.001), with median durations being 55 (40–82), 42 (35–56), and 33 (29–42) min, respectively. Older females were more likely to reach 38.0°C (2.4 [1.3, 4.3], p = 0.004) and Δ1.0°C above baseline resting (2.5 [1.4, 4.7], p = 0.003) compared to young males. No other sex‐ or age‐related differences were detected ( p > 0.145). Impact: These findings highlight the heightened vulnerability of older females during heavy‐intensity occupational heat stress, emphasizing the need for tailored guidelines to ensure equitable workforce protection.

Objective: We investigated the impact of workplace heat and cold on work time loss. Methods: Field experiments in different industrial sectors were conducted in multiple countries across all seasons between 2016 and 2024. Hundreds of workers were video-recorded and their full shifts (n = 603) were analyzed on a second-by-second basis (n = 16,065,501 sec). Environmental data were recorded using portable weather stations. The Workplace Environmental Labor Loss (WELL) functions were developed to describe work time loss due to workplace temperature. Results: The WELL functions revealed a U-shaped relationship whereby the least work time loss is observed at 18 °C (64 °F), and increases for every degree above or below this optimal temperature. Conclusions: The WELL functions quantify the impact of workplace temperature on work time loss, extending to temperatures previously believed to be unaffected.

Sleep deprivation has been associated with impaired thermoregulatory function. However, whether these impairments translate to changes in whole-body heat exchange during exercise-heat stress remains unknown. Therefore, following either a night of normal sleep or 24 h of sleep deprivation, 10 young men (mean (SD): 23 (3) years) completed three 30-min bouts of semi-recumbent cycling at increasing fixed rates of metabolic heat production (150, 200, 250 W/m²), each separated by a 15-min rest in dry heat (40 °C, ~ 13% relative humidity). Rates (W/m²) of whole-body total heat exchange (dry + evaporative) were measured continuously and expressed as peak responses [mean of the final 5-min of exercise at the highest metabolic heat production (250 W/m²)]. Body heat storage was quantified as the temporal summation of heat production and loss. Core temperature, indexed by rectal temperature, was measured continuously. Relative to normal sleep, sleep deprivation did not modify whole-body heat exchange (evaporative (−6 [−18, 5] W/m²; P = 0.245), or dry (7 [−5, 19] W/m²; P = 0.209; sleep deprivation—normal sleep mean difference [95%CIs]) and therefore total heat loss (1 [−14, 15] W/m²; P = 0.917). There were no differences in either the change in body heat storage (−9 [−67, 49] kJ; P = 0.732) or change in core temperature (0.1 [−0.1, 0.3] °C; P = 0.186) between conditions. Overall, we showed that 24-h sleep deprivation did not influence whole-body dry or evaporative heat exchange, resulting in no differences in total whole-body heat exchange or body heat storage in young adults during exercise under hot-dry conditions.