Strategies to Reduce the Heat Stress of Wearing New Biological and Chemical Protective Combat Uniforms in MOPP 1

Background The core temperature of the soldier in a MOPP 1 configuration has a substantial impact on how long they can continue to work once they transition to MOPP 4. The higher their temperature at the start of this transition the shorter will be their work time in MOPP 4. New hot weather biological and chemical (BC) protective ensembles have been designed as stand-alone combat uniforms that are intended to replace the concept of an overgarment that is donned only as the threat level increases above MOPP 1. Given that soldiers are also expected to wear fragmentation and assault vests, the heat strain in MOPP 1 can be considerable during work in hot environments. The present study has examined whether vents in the leg and the arm of the BC uniform assist in reducing the heat strain of the soldier under varied wind conditions. Methods Eight males (33.6 ± 7.7 y, 86.8 ± 10.8 kg, 48.9 ± 5.3 ml/kg/min VO2max) completed up to 3 hours of treadmill walking at 4 km/h in 40oC and 30% RH conditions while wearing a new hot weather BC stand-alone uniform, fragmentation and assault vests, and helmet, and carrying an artificial C7 rifle. The total additional weight of the clothing and equipment was 20 kg. Zippered vents on the lateral aspect of the upper and lower leg and the medial aspect of the upper arm were either opened or closed in low (1 m/s) or high (3.5 m/s) wind conditions. Rectal and skin temperatures, heart rate, ratings of perceived exertion and thermal comfort, metabolic rate, and vapour pressures measured at the skin surface and in the clothing layers were determined throughout the heat stress exposures. Results All subjects completed the 3 hours of exercise when the vents were opened regardless of the wind condition. However, when the vents were closed 3 of the subjects were unable to complete the exercise challenge in low wind and 1 in the high wind condition. The change in delta rectal temperature was elevated when the vents were closed (1.8 ± 0.6oC) compared with open (1.5 ± 0.5oC) and was lower in high (1.2 ± 0.3oC) compared with low (2.0 ± 0.5oC) wind. The vapour pressure within the clothing was significantly reduced when the vents were opened (4.7 ± 0.7 kPa) compared with closed (5.0 ± 0.7 kPa) with the greatest contributor to these differences being found on the leg (4.0 ± 0.6 and 4.3 ± 0.6 kPa for the open and closed condition, respectively) rather than on the torso which was covered by the fragmentation and tactical assault vests (5.6 ± 0.9 and 5.7 ± 0.9 kPa for the open and closed condition, respectively). Conclusion It was concluded that zippered vents on the leg and arm of the CB uniform could significantly reduce the heat strain of the soldier in MOPP 1 while working in hot environments. McLellan, T.M. (2005) Strategies to Reduce the Heat Stress of Wearing New Biological and Chemical Protective Combat Uniforms in MOPP 1. In Strategies to Maintain Combat Readiness during Extended Deployments – A Human Systems Approach (pp. 21-1 – 21-12). Meeting Proceedings RTO-MP-HFM-124, Paper 21. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp. Strategies to Reduce the Heat Stress of Wearing New Biological and Chemical Protective Combat Uniforms in MOPP 1 21 2 RTO-MP-HFM-124 1.0 INTRODUCTION Current military deployments involve exposure to hot environments together with the threat of exposure to biological and chemical (BC) agents. Different states of mission oriented protective posture (MOPP) demand various levels of protection from the lowest (MOPP 1) where the protective ensemble is carried to the highest (MOPP 4) where all of the clothing is worn and the respirator is used. Although the use of clothing ensembles protects the soldier from the hazards of exposure to BC agents, the clothing together with the hot temperatures creates a condition of uncompensable heat stress. Under these conditions physiologic tolerance is determined by three factors; the core temperature at the beginning of the exposure, the core temperature tolerated at exhaustion and the rate of increase of core temperature from the beginning to end of the heat stress [for review see 7]. Several nations have designed new BC clothing ensembles with a reduced thermal resistance and increased water vapour permeability in an attempt to lessen the thermal burden when the ensemble is worn in a MOPP 4 configuration [1, 11, 13, 15]. However, these new clothing designs also are worn as a stand-alone combat uniform, which may create greater thermal stress during MOPP 1 when compared to use of the conventional combat uniform. Thus, a true comparison of the effectiveness of the new BC ensembles for reducing thermal strain must include MOPP 1 as well as MOPP 4 comparisons. Any potential advantage of the new ensemble when worn in MOPP 4 may be offset by its disadvantage when worn in MOPP 1. It is well documented that elevations in core temperature at the beginning of heat stress exposure in MOPP 4 will significantly decrease exposure time [5, 6, 20]. An increased airflow over the skin surface promotes greater convective and evaporative heat transfer [17]. Several sporting companies incorporate zippered vents in their jackets and pants in an attempt to increase airflow over the skin during physical activity. The current study has considered the effectiveness of this same concept during MOPP 1 with the use of a new BC ensemble. In addition, this concept was examined under conditions of low and high wind since the importance of vents in the clothing may be more evident when airflow is low. It was hypothesized that thermal strain would be reduced with the use of vents in the clothing and that this effect would be greatest when airflow was low.