Wet military uniforms pose low risk of hypothermia while static in mild cold air

Wet clothing is less insulative than dry clothing and consequently increases heat loss in cold air. Tactical necessity can render removal of wet clothing impossible and/or require Warfighters to remain static to avoid detection, limiting heat production and posing a threat of hypothermia (core temperature <35 °C). This study aimed to characterize body temperatures and evaluate hypothermia risk while statically exposed to 5 °C air wearing three wet military uniforms. Further, low-speed loaded walking was evaluated as a strategy to raise end-static temperatures. Twelve adults (11 M, 1 F) randomly completed three wet-cold trials wearing either the Improved Hot Weather Combat Uniform (IHWCU), Army Combat Uniform (ACU), or ACU with silk-weight base layer (ACU+). Each trial involved 180 min of cold air (5.3 ± 0.3 °C, 0.8 m·s^-1) exposure after a clothed 2 min head-out immersion (34.0 ± 0.2 °C). Volunteers were static for 60 min followed by 120 min of walking with a rucksack. Rectal temperature (T_re) area under the curve did not differ among the three wet uniforms when static (p = 0.431) with T_re increasing, rather than decreasing, across the 60 min (IHWCU: +0.26 ± 0.19 °C, ACU: +0.37 ± 0.21 °C, ACU+: +0.36 ± 0.20 °C). Hypothermia risk with 60 min static wet-cold exposure therefore appears minimal, regardless of the military uniform worn, in an otherwise low stress cohort. End-static finger temperatures (IHWCU: 9.48 ± 2.30 °C, ACU: 9.99 ± 1.82 °C, ACU+: 9.27 ± 1.66 °C, p > 0.999) were reduced by ∼20-23 °C posing a considerable dexterity concern. Heat production of ∼210 W·m² appeared sufficient to begin to reverse negative cumulative heat storage and initiate slight elevations of rectal and peripheral temperatures, although finger temperatures increased <2 °C after 120 min. ClinicalTrials.gov ID: NCT05409937.