Preliminary Assessment Of Safe Initial Stay Times Over Consecutive Workdays For Younger And Older Men

On the advice of regulatory agencies, workplaces rely on the use of work-rest allocations to limit elevations in core temperature (Tco) above 38 °C. However, no direction is provided on the continuous work time that can be performed (initial stay time; IST) before work-rest cycles are implemented. We recently generated estimates for ISTs for moderate-intensity work over consecutive days and showed similar responses in younger and older males. However, responses over consecutive days have yet to be assessed for heavy-intensity work, where greater age-related differences have been observed. PURPOSE: To determine IST for heavy-intensity work in the heat and evaluate if adjustments in ISTs are required for younger and older males for i) a second work bout following a lunch break and ii) a work bout performed the next day. METHODS: Eight younger and 10 older males performed 4-h of heavy-intensity treadmill walking (260 W·m−2 metabolic rate) in 26 °C wet-bulb globe temperature. Work continued until Tco reached 38 °C after which a work-rest allocation of 1:1 (starting with 30 min rest) was applied for the remainder of the work period. The protocol was repeated in the afternoon after a 1-h lunch break (22 °C), and the next morning. Core (rectal) temperature was measured continuously. RESULTS: The IST was significantly shorter in the afternoon (30 ± 9 min: p < 0.05) compared to the morning of day 1 (62 ± 25 min) and day 2 (75 ± 39 min), while responses did not differ between morning work (p = 0.15). Further, no differences were observed between younger and older males (p = 0.10). Peak and average Tco during work-rest allocations were greater in the afternoon compared to mornings of day 1 and day 2 (p < 0.05). Relative to their younger counterparts, peak (older: 38.47 ± 0.27 vs younger: 38.12 ± 0.16 °C) and average (older: 38.20 ± 0.20 vs younger: 37.87 ± 0.18 °C) Tco in older males were greater during the afternoon work-rest allocations (p < 0.05). CONCLUSION: Although ISTs were shorter in the afternoon following morning work, peak Tco and average Tco were significantly greater in older males relative to younger during the work-rest allocations in the afternoon session. Employers should aim to schedule heavy-intensity work in the morning to avoid sustained elevations in Tco and unsafe work behavior when working in the heat during afternoon periods. Supported by: Workplace Safety and Insurance Board (Ontario)