Preliminary Assessment Of Initial Stay Times On Work Output For Moderate-Intensity Work In The Heat

Workers performing prolonged work in the heat are at an elevated risk of experiencing heat-related injuries. To reduce these risks, workplaces are advised by regulatory agencies to implement work-rest allocations to prevent rises in core temperature above 38 °C. However, until recently no guidance was provided on the initial duration that continuous work could be performed (termed initial stay time; IST) before work-rest allocations should be employed. While general estimates for IST were introduced for moderate-intensity work, it remains unclear if the application of IST may affect total work volume completed over a prolonged workday in a hot environment. PURPOSE: To determine if total work volume differs during daylong moderate-intensity work performed with and without the application of IST prior to implementing the prescribed work-rest allocations. METHODS: On two separate days, 7 young males (age: 22.6 ± 2.6 yrs; V̇O2max: 47.0 ± 7.9 ml·min-1·kg-1) completed daylong work consisting of moderate-intensity treadmill walking (200 W·m-2 metabolic rate) at a wet-bulb globe temperature of 26 °C, performed in the morning (4-h) and afternoon (4-h), separated by a 1-h lunch break (22 °C). Participants either 1) commenced each work period with the recommended 3:1 work-rest allocation starting with a 45 min work bout followed by a 15 min rest (Trial A), or 2) performed continuous work until IST was achieved, after which the prescribed work-rest allocation of 3:1 (starting with a 15 min rest) was applied for the duration of the work period (Trial B). Core (rectal) temperature was measured continuously. RESULTS: Total work volume in the morning period was 9% higher when IST was applied (197 ± 20 min; effect size = 1.17), while only 3% in the afternoon period (186 ± 17 min; effect size = 0.53), as compared to the immediate use of work-rest cycles (i.e., 180 min). Core temperature exceeded the recommended safe upper limits of 38 °C for 42 ± 7% and 41 ± 7% of the work period for trial A and B, respectively in the morning, increasing to 55 ± 17% and 52 ± 12% in the afternoon. CONCLUSION: Performing continuous work prior to the implementation of work-rest allocations was associated with a greater work volume in the morning only, relative to commencing with work-rest allocations. IST can be safely implemented, resulting in greater work output in morning. Supported by: Workplace Safety and Insurance Board (Ontario)