Nicole E. Moyen1,2,3, Toby Mündel2, Andrea M. Du Bois1, Anthony B. Ciccone1, and Daniel A. Judelson1, FACSM; 1Exercise Physiology Laboratory, Department of Kinesiology, California State University, Fullerton; 2 School of Sport and Exercise, Massey University, Palmerston North, New Zealand; 3Human Performance Laboratory, Department of Health, Human Performance, and Recreation, University of Arkansas

Females increasingly occupy vocations that require working prolonged periods in hot-humid climates, however, research examining females under these conditions remains unclear. PURPOSE: To assess how increasing relative humidity (RH) influences females’ thermoregulation and perception during low-intensity exercise in the heat. METHODS: Ten healthy females (age=23±2 y, VO2max=46±6 ml×kg-1×min-1) walked 90 min at 35%VO2max in 35˚C at 55% RH (55RH), 70% RH (70RH), and 85% RH (85RH); all trials were completed during follicular phase. Investigators obtained 1) rectal temperature (Tre), mean-weighted skin temperature (Tsk), heart rate (HR), ambient temperature (Ta), and RH every five min, 2) pre and post nude body masses, and 3) respiratory measures every 30 min; partitional calorimetric variables were calculated from these measures. RESULTS: Significant RH*time interactions existed for all calorimetry variables (n=9; all p<0.0005) except heat production (p=0.139), convective respiratory heat loss (p=0.137), and required rate of evaporative cooling (Ereq; p=0.197). Each RH increase significantly decreased evaporative heat loss (E); all other calorimetry variables only significantly differed after 70RH. Combined, dry (convective+radiative) and respiratory heat losses only compensated for <30% of decreases in E. Tre, Tsk, and HR presented significant RH*time interactions (p<0.0005). 85RH Tre significantly exceeded 55RH and 70RH after 35 min. 70RH Tsk tended to exceed 55RH (p=0.057), and 85RH Tsk significantly exceeded 55RH and 70RH after 30 and 40 min, respectively. After 10 min, 85RH HR significantly exceeded 55RH and 70RH; after 50 min, 70RH HR significantly exceeded 55RH. 70RH sweat rate tended to exceed 55RH (p=0.063), and 85RH significantly exceeded both trials (55RH=0.6±0.2; 70RH=0.7±0.2; 85RH=0.9±0.3 L×h-1; p=0.001). CONCLUSION: The primary findings of this study were that females exercising at low intensities in 35°C only experienced significant thermoregulatory strain after RH exceeded 70%. As heat production and Ereq remained constant across trials, heat storage increased with each 15% rise in RH because dry and respiratory heat losses minimally offset decreased E. Hotter Tre,Tsk, and resultantly higher sweat rate, reflected heat storage increases as E decreased in each trial. Overall, at Ta ≤ 35 °C, we found females working/exercising for prolonged periods at low-intensities will remain at safe core temperatures up to 70% RH.

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