Heat strain in different hot environments hiking in wildland firefighting garments

Wildland firefighters can work at high intensity in hot environments for extended periods of time. The resulting heat strain may be modified by the environmental conditions (i.e., ambient temperature and humidity [RH]) even at equal wet-bulb globe temperatures (WBGTs). This investigation assessed if a hot and dry condition would create greater strain than moderate and high humidity at equivalent WBGT (28 °C). Twelve participants (age 24 ± 2 year) walked at 40%-50% maximum aerobic capacity for 90 and 40 min separated by a 20 min rest in dry (40 °C, 20% RH), moderate-humidity (34 °C, 50% RH), and high-humidity (29 °C, 90% RH) conditions wearing fire-resistant jacket, pants, gloves, and helmet with the neck and face exposed. Peak core temperature was higher in moderate-humidity (38.9 ± 0.2 °C, <i>p</i> = 0.01) and high-humidity (38.9 ± 0.6 °C, <i>p</i> < 0.01) than dry condition (38.5 ± 0.3 °C). Average net heat gain was less in dry (33 ± 22 W) compared to moderate-humidity (38 ± 23 W, <i>p</i> < 0.01) and high-humidity (39 ± 28 W, <i>p</i> < 0.01). Peak heart rate (174 ± 14 bpm, <i>p</i> = 0.94), physiological strain index (7.7 ± 1.4 score, <i>p</i> = 0.99), perceived exertion (8 ± 2 rating, <i>p</i> = 0.97), and perceptual strain index (7.3 ± 1.6 score, <i>p</i> = 0.99) were not different in high-humidity compared to the dry condition (167 ± 19 bpm, 6.9 ± 1.3 score, 6 ± 2 rating, 7.3 ± 1.7 score, respectively). Whole-body sweat rate (15 ± 6 mL/min, <i>p</i> = 0.58) and thermal sensation (7 ± 1 rating, <i>p</i> = 0.37) were not different. Hiking in a humid condition while wearing protective garments creates greater exertional heat strain compared to a dry condition of equivalent WBGT. Wildland firefighters should consider extra strategies to mitigate hyperthermia when humidity is high.