Abstract
Introduction: To reduce the health risks associated with occupational heat stress, workplaces rely on upper heat stress limits recommended by regulatory agencies to mitigate increases in core temperature above safe limits of 38°C for unacclimatized workers. However, current guidelines fail to consider individual factors (e.g., age, sex) which have been shown to influence heat tolerance1. Further, no guidance is provided on how long an individual can perform continuous work (initial stay time; IST) before work-rest allocations should be implemented. This lack of guidance could compromise worker safety and productivity. While preliminary estimates for ISTs for moderate-intensity work have been introduced for young (18-30 years) and older males (50-70 years) with and without chronic health conditions2 and young-to-middle-aged females (18-45 years)3 across various ambient conditions, larger studies are needed to validate and extend these estimates. This includes generating estimates for heavy-intensity work and older females who represent a growing segment of the workforce.
Methods: On separate days, young (n=48 (24 female); 18-30 years) and older (n=49 (25 female), 50-69 years) males and females completed either moderate- (fixed metabolic rate of 200 W/m2) or heavy- (260 W/m2) intensity work at a wet-bulb globe temperature (WBGT) of 26°C (34.1°C, 35% relative humidity (RH)), 29°C (37.5°C, 35% RH) and 32°C (41.3°C, 35% RH). Participants walked on a treadmill until core (rectal) temperature reached 38°C (defined as the IST). Cox regression analysis was used to assess the influence of work intensity, environmental conditions, and group on ISTs.
Results: Across groups, ISTs were reduced during heavy compared to moderate intensity work (P<0.001). Further, ISTs were reduced as WBGT increased for both moderate (P<0.002) and heavy (P<0.001) intensity work. For moderate-intensity work in 26°C WBGT, the average IST was 70-min and as WBGT increased, IST decreased to 59- and 43-min in 29°C and 32°C WBGT, respectively. For heavy-intensity work, IST was 44-min in 26°C WBGT and decreased to 41- and 28-min in 29°C and 32°C WBGT, respectively. Further in line with prior work2,3, ISTs did not differ between young and older males or between young males and females during moderate-intensity work. We extend upon prior findings by showing a similar response for heavy-intensity work. However, older females had reduced ISTs regardless of work intensity or ambient condition (both P≤0.017).
Conclusion: As WBGT and the intensity of work increased, ISTs declined. Further, older females had reduced ISTs compared to the other groups across both work intensities and ambient conditions.
Practical Implications: We provide estimates for the duration continuous work can be safely performed before work-rest allocations should be implemented. However, it is important to note that these times should only be employed for an initial work bout typically performed at the start of the workday. As we present in a companion presentation4, adjustments in ISTs are required when a second prolonged work bout is performed in the afternoon but not the morning of the next workday.
Funding: Workplace Safety and Insurance Board (Ontario).
References: 1)MSSE 2020, 52: 2242; 2)MSSE 2021, 53: 2196; 3)APNM 2022, 47: 711; 4)CIM 2025