Increasing Relative Humidity Impacts Low-Intensity Exercise in the Heat

Introduction: Many jobs require working or exercising at low intensities for extended periods in hot-humid climates; however, in these conditions the isolated effects of relative humidity (RH) remain unclear. Therefore, the purpose of this study was to examine how RH influenced thermoregulation and perception during low-intensity exercise in the heat. Methods: There were 13 healthy men (age = 23 +/- 2 yr. mass = 83.1 +/- 13.3 kg, height = 179.9 +/- 5.9 cm, (V) overdotO(2max) = 55.6 +/- 7.3 ml kg(-1).min(-1)) who walked 90 min at 35% (V) overdotO(2max) in 35 degrees C, completing trials at 40% RH (40RH), 55% RH (55RH), 70% RH (70RH), and 85% RH (85RH). Investigators obtained 1) rectal temperature (T-re), skin temperatures, heart rate, and perceptual measures every 5 min; 2) respiratory measures every 30 min; and 3) pre- and post-exercise nude body masses; these measures derived partitional calorimetry variables. Results: Maximal evaporative capacity and heat loss incrementally decreased as RH increased; nonevaporative and respiratory heat loss negligibly altered heat balance. Progressively raising RH significantly increased heat storage, heat production, and T-re. T-re in 40RH and 55RH matched; 70RH exceeded 40RH and 55RH after 35 min; and 85RH exceeded all trials after 40 min. Discussion: Nonevaporative and respiratory heat loss mechanisms failed to offset decreased sweat evaporation. Nonlinear increases in T-re appeared to catalyze responses in most other variables, which demonstrated similar but temporally delayed patterns. Under these circumstances, an RH threshold for increased thermal strain plausibly existed between 55-70RH; environmental characteristics indicated the threshold occurred not where heat stress became uncompensable, but instead where T-re surpassed the "balance point," triggering compensatory responses.