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, Vo2max = 55.6 +/- 7.3 ml x kg(-1) x min(-1)) who walked 90 min at 35% Vo2max 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 (Tre), 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 Tre. Tre 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 Tre 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 Tre surpassed the "balance point," triggering compensatory responses.