Initial Stay Times For Heavy-Intensity Work In The Heat For Young And Older Adults

Workplaces rely on upper heat stress limits recommended by regulatory agencies to mitigate increases in core temperature above safe limits (≥1 °C above resting) during work in the heat to safeguard the health of workers. However, no guidance is provided on how much work can be performed before heat-alleviation controls (work/rest allocations) should be employed (termed the initial stay time). Current guidelines also fail to consider age-related declines in thermoregulatory function that can reduce heat tolerance. These are important knowledge gaps, as stay times that are too long may lead to core temperatures exceeding recommended limits whereas productivity may be limited if stay times are too short. While preliminary estimates for initial stay times for moderate-intensity work have been introduced, those for heavy work remain undefined. PURPOSE: To establish initial stay times for heavy-intensity work in warm to hot ambient conditions for young and older adults. METHODS: On three randomized days, 12 young adults (6 female, median (IQR) 24 (22-26) years; VO2peak: 48.9 (43.5-53.3) mL/kg/min) and 11 older adults (5 female, 65 (63-66) years; VO2peak: 33.5 (27.6-40.3) mL/kg/min) performed heavy-intensity work (fixed metabolic heat production of 260 W/m2) in wet-bulb globe temperatures (WBGT) of 26, 29 and 32 °C. Work continued until core (rectal) temperature increased +1 °C from baseline resting. RESULTS: A temperature by group interaction for stay time was observed (P < 0.01). Stay time decreased with increasing temperatures in young (P < 0.01) but not older adults (P ≥ 0.21). In 26 °C WBGT, stay time was 1.7-fold [95%CI: 1.1, 2.6] (P = 0.02) greater in young (79 (61-115) min) compared to older adults (48 (39-58) min). Conversely, stay time was not different in young versus older adults in 29 °C WGBT (50 (46-70) vs 40 (38-58) min; 1.3-fold [0.9, 1.7], P = 0.16) or 32 °C WGBT (42 (36-49) vs 42 (35-49) min; 1.0-fold [0.8, 1.2], P = 0.76). CONCLUSION: The time taken for core temperature to increase 1 °C declined with increasing WBGT in young but not older adults, such that stay times were greater in young adults in lower (26 °C WGBT) but not at higher ambient temperatures (29 °C and 32 °C WGBT). These estimates can help refine current heat stress management guidance to protect workers performing their duties in hot environments. Workplace Safety and Insurance Board (Ontario) and Mitacs