Heat Loss Is Impaired in Older Men on the Day after Prolonged Work in the Heat.

PURPOSE: Prolonged work in the heat may exacerbate the rise in core temperature on the next work day, especially in older workers who display impairments in whole-body heat loss that increase body heat storage and core temperature relative to young adults during heat stress. We therefore evaluated whether whole-body heat loss in older adults was impaired on the day after prolonged work in the heat.

METHODS: Whole-body heat exchange and heat storage were assessed in nine older (53-64 yr) men during three 30-min bouts of semirecumbent cycling at fixed rates of metabolic heat production (150 [Ex1], 200 [Ex2], 250 W·m [Ex3]), each separated by 15-min recovery, in hot-dry conditions (40°C, 20% relative humidity), immediately before (day 1), and on the day after (day 2) a prolonged, work simulation (~7.5 h) involving moderate-intensity intermittent exercise in hot-dry conditions (38°C, 34% relative humidity). Total heat loss (evaporative ± dry heat exchange) and metabolic heat production were measured using direct and indirect calorimetry, respectively. Body heat storage was quantified as the temporal summation of heat production and loss.

RESULTS: Total heat loss (mean ± SD) during Ex1 did not differ between days 1 and 2 (151 ± 15 and 147 ± 14 W·m, respectively; P = 0.27), but was attenuated on day 2 during Ex2 (181 ± 15 W·m) and Ex3 (218 ± 16 W·m) relative to day 1 (192 ± 14 and 230 ± 19 W·m, respectively; both P < 0.01). Consequently, body heat storage throughout the protocol on day 2 (276 ± 114 kJ) was 31% greater than on day 1 (191 ± 87 kJ; P < 0.01).

CONCLUSIONS: Prolonged work in the heat causes next-day impairments in whole-body heat loss, which exacerbate heat storage and may elevate the risk of heat injury on the following day in older workers.