Safe Work Times For Men Performing Consecutive Workdays In The Heat: A Preliminary Assessment

To mitigate the adverse health effects of occupational heat stress, regulatory agencies provide guidance on the use of heat-alleviation controls (work/rest cycles) to limit core temperature from reaching potentially unsafe levels, typically defined as sustained elevations above 38 °C. However, no recommendations are given for the initial work duration before controls should be employed (termed initial stay time). Although a recent study provided initial stay times for moderate-intensity work in varying ambient conditions, this assessment was limited to a single simulated work shift. A day of work in the heat can exacerbate increases in core temperature during subsequent work shifts, particularly in older workers, placing them at a greater risk of heat-related injuries. However, no study has evaluated the effect of work in the heat on safe initial stay times during subsequent work bouts. PURPOSE: To determine if refinements in initial stay times for young and older males performing moderate work in very warm conditions are required for i) a second work bout on the same day that is preceded by an extended rest period (e.g., a lunch break), and ii) a work bout performed on the next day. METHODS: Six young (median (IQR) age: 23 (21 - 24) yrs) and 8 older (66 (64 - 67) yrs) males performed a 4-h moderate-intensity treadmill walk (200 W/m2 metabolic heat production) in 26 °C wet-bulb globe temperature. Work continued until core (rectal) temperature reached 38 °C. Thereafter, the recommended work-to-rest allocation of 3:1 was employed for the remainder of the work period, starting with 15 min rest. The protocol was completed again in the afternoon after a 1-hour lunch break in a temperate environment (22 °C) and on the following morning. RESULTS: Initial stay time was 63% [95% CI: 47, 74] lower in the afternoon (26 (17 - 40) min) compared to the morning of day 1 (74 (53 - 127) min; P < 0.01). By contrast, morning stay times were not different between day 1 and 2 (63 (38 - 119) min; P = 0.89). Stay times were similar between young and older men (P = 0.59). CONCLUSION: Initial stay time decreased in the afternoon following a morning work period. However, there was no difference in morning stay time over consecutive days. Our findings suggest that refinements in safe initial stay times are required for afternoon work but not for the morning of the next workday. Workplace Safety and Insurance Board (Ontario) and Mitacs