Thermoregulatory Capacity And Metabolism During Moderate Physical Activity Are Unaffected By Humidity In Hot Conditions

The ability to thermoregulate in hot and humid environments is critical to preventing exertional heat injury in the workplace. It is unclear whether environmental relative humidity (RH) affects thermoregulatory capacity and metabolism in hot conditions. PURPOSE: The purpose of this study was to evaluate the difference in thermoregulatory capacity and metabolism, defined by Metabolic Equivalent of Activity (METS), core body temperature (CBT), water loss, and heart rate, during intermittent exercise in hot conditions with differing relative humidity. METHODS: Healthy participants (n = 37, age = 25.3 ± 7 yrs., Body Mass Index (BMI) = 25.3 ± 4.1 kg/m2, sex: F = 13 M = 24) completed intermittent industrial work-related tasks in a 32,500 L metabolic chamber with an ambient temperature of 30 °C for 60-180 minutes (mean: 91.1 ± 21.7 min). Tasks were completed under high (n = 23, 79 ± 1.9%) or low humidity (n = 14, 42.7 ± 4.2%) conditions. Heart rate, CBT, and oxygen consumption (VO2) were continuously monitored throughout the session. Water loss was calculated as (Body Weightpost-session -Body Weightpre-session) + (Water consumedduring-session -Urine excretedduring-session). Water loss is expressed as an absolute value (kg) and as a percentage of body weight taken at the beginning of the study with other anthropometric measures. METS were calculated as VO2/3.5/Body weight. Independent sample t-tests were used to compare the RH groups. Data are presented as Mean ± SD. RESULTS: Age (high: 24.1 ± 5.3 yrs vs. low: 27.2 ± 9.3 yrs, p = 0.25) and BMI (high: 25.1 ± 2.8 kg/m2 vs. low: 25.5 ± 5.6 kg/m2, p = 0.83) did not differ between groups. Metabolic activity was not different across RH conditions (METS: high: 3.81 ± 1.32 vs. low: 4.11 ± 0.49, p = 0.54). CBT (high: 37.3 ± 0.7 °C vs. low: 37.5 ± 0.4 °C, p = 0.20) and heart rate (high: 110.4 ± 21.7 bpm vs. low: 117.6 ± 19.5 bpm, p = 0.37) also did not differ. Water loss was not different when expressed as an absolute value (high: 0.7 ± 0.5 kg vs. low: 0.9 ± 0.3 kg p = 0.44) or as a percentage of body weight (high: 0.9 ± 0.6% vs. low: 1.2 ± 0.4% p = 0.14). CONCLUSION: These results demonstrate that when completing intermittent moderate-intensity exercise under hot conditions, thermoregulatory capacity and metabolism are unaffected by environmental relative humidity in young, healthy persons. This study was funded by Epicore Biosystems and the Massachusetts State Core Facility Voucher Program.