Heat Acclimatisation and Active Body Cooling Strategies to Mitigate Heat Stress for Operations Involving Bullet Proof Vests

Heat acclimatization (HA) and personal body-cooling are effective methods of heat stress mitigation. HA involves conditioning the body to function under hot conditions by exposing the individual to an incremental level of heat stress daily over 10 to 14 days. Although HA improves thermoregulation during heat exposure, effective physical heat exchange between the body and environment must still take place. Operating with body armour may limit the benefits of HA due to the increased physical workload and greater coverage of body surface. In terms of personal cooling, substantial amount of research has shown that a significant amount of body heat can be removed with the use of a personal body cooling system. Aim: The study aims to quantify the magnitude of heat strain when marching with bullet proof vests in hot and humid conditions, and to evaluate the effectiveness of heat acclimatisation and active body-cooling as heat stress mitigating strategies in these conditions. Methods: Thirtythree trained male volunteers were randomly assigned to the Bullet Proof Vest (BPV) (n= 15) or Bullet Proof Vest and Full Battle Order (BPV+FBO) (n=18) groups. Participants were involved in three heat stress tests each, before (preHA) and after (post-HA) heat acclimatisation and with active body cooling (A-cooling). These tests were conducted in an environmental chamber programmed to simulate 36 °C ambient heat, 65% relative humidity and 800 W/m 2 of simulated solar radiation. During the trials, participants walked at 4 km/h for two 60 min cycles. The exercise cycles were shortened if core temperature (Tc) reached 39.5 °C for 1 min before 60 min or volitional fatigue, whichever occurred earlier. Participants then rested for 30 min or until Tc decreased to 38 °C, whichever occurred first, at the end of each exercise cycle. Results: Marching with the BPV induced Tc to increase at a rate of 0.28°C/min and adding the FBO to the BPV increased the rate of rise of Tc to 0.034°C/min and 0.037°C/min, respectively. HA was not effective in suppressing the rate of rise in Tc, but was effective in increasing work duration by lowering the resting Tc and by increasing work tolerance. Active body cooling was the most effective strategy in mitigating heat strain and increasing work tolerance. Work tolerance was limited by symptoms of physical exhaustion and discomfort resulting from the BPV in 70% to 80% of the participants, whereas 20% to 30% of the participants interrupted their trials because of high Tc. Conclusion: Heat acclimatisation was effective in improving work duration and tolerance by inducing a significant decrease in resting core temperature and by increasing physical fitness in both the BPV and BPV+FBO conditions. Active body cooling effectively decreased the rate of rise of body temperature and provided an alternative avenue for heat dissipation through conduction, resulting in a significant increase in work tolerance. Conclusion: HA should be a part of the overall training strategy whilst active body cooling devices should be considered as part of the technology-driven solutions that will allow the BPV to provide the required ballistic protection with minimal burden on the physiology of the soldier.