Comparison of thermoregulatory responses to occupational heat stress in warm humid and hot dry conditions

The comparison of thermoregulatory responses to occupational heat stress in warm humid and hot dry conditions is under-investigated. Therefore, we compared the thermoregulatory and perceptual responses during an occupational recommendation-approved work protocol performed in warm humid and hot dry environments. We hypothesized that these thermoregulatory and perceptual responses would be greater in hot dry compared to warm humid conditions due to higher ambient temperature and restricted sweat evaporation, despite following the National Institute for Occupational Safety and Health (NIOSH) recommendations. Thirteen healthy adults (4 females, 22 ± 3 y, 1.77 ± 0.11 m, 77.2 ± 17.4 kg, 24.7 ± 5.3 kg/m 2 body mass index, 1.93 ± 0.25 m2 body surface area, 45 ± 9 ml/kg/min maximal oxygen consumption) completed two matched 120-minute moderate-intensity intermittent walking (work-rest ratio: 2:1) in hot-dry (HD: 40.7 ± 0.2°C ambient temperature, 21 ± 6% relative humidity [RH], 29.3 ± 1.2°C wet-bulb globe temperature [WBGT]) and warm-humid (WH: 30.7 ± 0.1°C, 77 ± 2% RH, 28.4°C WBGT) conditions. Participants wore standard one layered coveralls (clo = 0.70) and drank 237 mL of water (~4ºC) every 20-minutes during trials. We elicited a targeted metabolic heat production (Hpro) of ~340 Watts via a validated equation using body mass and treadmill speed, then we used oxygen consumption and respiratory exchange ratio to verify Hpro during trials. Rectal temperature (Tre), heart rate (HR), rating of perceived exertion (RPE), and thermal sensation (TS) were measured at baseline and every 20-minutes during the trial. Body mass loss (BML) was calculated as the change in nude body mass from pre- to post-exercise, corrected for urine loss. Whole body sweat rate (WBSR) was calculated as body mass loss, corrected for urine loss and fluid consumption. Data were analyzed using linear mixed effects models with main effects of time, condition, and interaction or paired t-tests, as appropriate. Despite identical treadmill speed between trials, Hpro was higher in HD (403 ± 35) compared to WH (346 ± 27, p < 0.001). Participants in HD had a greater Tre than in WH, regardless of time point (p = 0.004). Tre also increased over time, regardless of condition (p < 0.001). Peak Tre in HD (37.94 ± 0.36°C) was significantly higher than in WH (37.68 ± 0.32°C, p = 0.006). In HD, 3/13 and in WH, 1/13 participants reached peak Tre >38°C. There was a main effect of time (p < 0.001) and interaction (p = 0.001), but not a main effect of condition (p = 0.053) for HR. No difference in peak HR was observed between HD (118 ± 30 bpm) and WH conditions (116 ± 27 bpm, p = 0.767). There was a main effect of time (p < 0.001), and condition (p = 0.012) for TS. There was a main effect of time (p < 0.001), interaction (p = 0.046), but not condition (p = 0.191) for RPE. BML (HD: -0.49% ± -0.62%, # of BML > 2%: 0/13; WH: -0.51% ± -0.94%, p = 0.955, # of BML > 2%: 0/13), and WBSR (0.55 ± 0.24 L/hr, WH: 0.44 ± 0.20 L/hr, p = 0.212) were not difference between conditions. When NIOSH recommendations were strictly adhered to during physical work in the heat, the majority of participants (85%) did not reach 38°C core temperature, and no one reached 2% BML. HD appears to induce greater thermoregulatory (Tre) and perceptual (TS) responses than the WH condition. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.