Increasing relative humidity impacts low-intensity exercise in the heat

Area of Science:

• Environmental Physiology
• Human Thermoregulation
• Exercise Science

Background:

• Occupational and exercise demands often occur in hot-humid conditions.
• The specific impact of relative humidity (RH) on thermoregulation during low-intensity exercise is not fully understood.

Purpose of the Study:

• To investigate the isolated effects of varying RH on thermoregulation and perception during prolonged low-intensity exercise in a hot environment.

Main Methods:

• Thirteen healthy men performed a 90-minute low-intensity walking exercise (35% VO2max) in 35°C at four different RH levels (40%, 55%, 70%, 85%).
• Measurements included core and skin temperatures, heart rate, respiratory parameters, and perceptual responses.
• Partitional calorimetry variables were calculated using pre- and post-exercise body mass.

Main Results:

• Increasing RH progressively reduced evaporative heat loss and heat dissipation capacity.
• Higher RH levels led to significant increases in heat storage, heat production, and core body temperature (Tre).
• Tre significantly increased at 70% RH and 85% RH compared to 40% RH and 55% RH after 35-40 minutes.

Conclusions:

• Reduced sweat evaporation due to high RH was not compensated by nonevaporative or respiratory heat loss.
• A critical RH threshold for increased thermal strain was identified between 55% and 70%.
• Elevated core body temperature surpassing a "balance point" appeared to trigger compensatory physiological responses.