Initial Stay Time and Work‐Rest Scheduling During Consecutive Heavy‐Intensity Workdays in Hot Environments in Young and Older Males
8 affiliations
- 1Human and Environmental Physiology Research Unit, School of Human Kinetics University of Ottawa Ottawa Ontario Canada
- 2Institute of the Environment University of Ottawa Ottawa Ontario Canada
- 3Interdisciplinary School of Health Sciences, Faculty of Health Sciences University of Ottawa Ottawa Ontario Canada
- 4Clinical Epidemiological Program Ottawa Hospital Research Institute Ottawa Ontario Canada
- 5Department of Automatics, Biocybernetics and Robotics Jožef Stefan Institute Ljubljana Slovenia
- 6Medical School University of Cyprus Nicosia Cyprus
- 7School of Allied Health and Social Work Griffith University Southport Queensland Australia
- 8Australian Centre for Precision Health and Technology Griffith University Southport Queensland Australia
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.
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BibTeX
@article{kenny2026initial,
title = {{Initial Stay Time and Work‐Rest Scheduling During Consecutive Heavy‐Intensity Workdays in Hot Environments in Young and Older Males}},
author = {Glen P. Kenny and Gil Bourgois and Sarah M. Taggart and Kelli E. McCormick and James J. McCormick and Leonidas G. Ioannou and Fergus K. O'Connor},
journal = {American Journal of Industrial Medicine},
year = {2026},
month = sep,
doi = {10.1002/ajim.70138},
url = {https://doi.org/10.1002/ajim.70138},
note = {CC BY-NC-ND 4.0},
}
RIS
TY - JOUR
TI - Initial Stay Time and Work‐Rest Scheduling During Consecutive Heavy‐Intensity Workdays in Hot Environments in Young and Older Males
AU - Glen P. Kenny
AU - Gil Bourgois
AU - Sarah M. Taggart
AU - Kelli E. McCormick
AU - James J. McCormick
AU - Leonidas G. Ioannou
AU - Fergus K. O'Connor
PY - 2026
DA - 2026/09/14
JO - American Journal of Industrial Medicine
DO - 10.1002/ajim.70138
UR - https://doi.org/10.1002/ajim.70138
AB - 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.
SN - 0271-3586
SN - 1097-0274
ER -