Reduced Maximum Core Temperature on a Second, but not Third, Consecutive Workday in the Heat in Both Males and Females

Abstract

Two consecutive workdays in the heat can lead to exacerbated core temperature (Tc) responses on the second day, potentially causing excessive hyperthermia. To our knowledge, no studies have investigated Tc responses across three consecutive workdays or compared responses between males and females. We hypothesized that maximum Tc would be greater on the second workday compared to the first and third day in both males and females. Fifteen participants (7 females, Age: 27±6 y, Body mass: 75.0±14.4 kg, Body fat: 16.7±9.3%) completed three consecutive days of 4 h of heavy intensity work to a 45:15 work:rest ratio in a hot environment (36.3±0.8°C, 21±5% relative humidity), abiding by national worker recommendations. Equal rest was provided between days. Tc, metabolic heat production, skin temperature (Tsk), and sweat rate were measured during work. Repeated measures analysis of variance and t-tests were used to compare responses. Maximum Tc on Day 1 (38.16±0.36°C) was greater than Day 2 (38.03±0.24°C, p=0.03), with Day 3 (38.08±0.38°C) not different from Day 1 (p=0.433) or Day 2 (p=0.70). Maximum Tc was not different between sexes on any day (all p>0.05). Resting Tc was greater on Day 1 (37.10±0.30°C) compared to Day 2 (36.96±0.32°C, p=0.02) but was not different than Day 3 (37.00±0.38°C, p=0.06). Resting Tc was not different between sexes on any day (all p>0.05). Males had a lower resting Tc on Day 2 compared to Day 1 (p=0.01) while females did not differ between these days (p=0.57). Metabolic heat production was greater on Day 1 (395±10 W) compared to Day 2 (384±12 W, p=0.01), with Day 3 (390±22 W) similar to both Day 1 (p=0.33) and Day 2 (p=0.24). There were no differences in metabolic heat production between sexes on any day (all p>0.05). Metabolic heat production was not different in males among days (all p>0.05), while females produced less metabolic heat on Day 2 (382±10) compared to Day 1 (398±8 W, p=0.02) despite completing identical work. Metabolic heat production in females on Day 2 was not different from Day 3 (394±25 W, p=0.13). Sweat rate did not differ across days (p=0.90) or between sexes (p=0.38). Resting Tsk and maximum Tsk were not different across days (p=0.28, p=0.97) or between sexes (p=0.98, p=0.19), respectively. Contrasting our hypothesis, participants experienced a lower maximum Tc on Day 2 compared to Day 1, indicating a somewhat protective effect during consecutive days of heat stress. Both males and females abiding by national worker recommendations for hot environments may see a reduced Tc on the second, but not third, day of consecutive workdays. This project was funded by the National Institute for Occupational Safety and Health (1K01OH012016-01A1). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.