Cold and Heat Strain during Cold-Weather Field Training with Nuclear, Biological, and Chemical Protective Clothing

Military Medicine, Feb 2007 by Rissanen, Sirkka, Rintamäki, Hannu

The objective of this study was to quantify the thermal strain of soldiers wearing nuclear, biological, and chemical protective clothing during short-term field training in cold conditions. Eleven male subjects performed marching exercises at moderate and heavy activity levels for 60 minutes. Rectal temperature (T^sub re^), skin temperatures, and heart rate were monitored. Ambient temperature (TJ varied from -33 to 0°C. T^sub re^ was affected by changes in metabolism, rather than in T^sub a^. T^sub re^ increased above 38°C during heavy exercise even at -33°C. The mean skin temperature decreased to tolerance level (25°C) at T^sub a^ below -25°C with moderate exercise. Finger temperature decreased below 15°C (performance degradation) at T^sub a^ of - 15°C or cooler. The present results from the field confirm the previous results based on laboratory studies and show that risk of both heat and cold strain is evident, with cooling of extremities being most critical, while wearing nuclear, biological, and chemical protective clothing during cold-weather training.

Introduction

The design of nuclear, biological, and chemical (NBC) protective clothing is focused on protection against external threats and not on environmental weather conditions. However, weather conditions may have profound effects on performance. Decrements in performance are influenced by ambient temperature (T^sub a^), humidity, and metabolic rate.1 It is widely reported that physical work performance is decreased when NBC protective clothing is worn in a warm environment.1,2 Even when heat stress is not a significant factor, performance is degraded because of wearing NBC protective clothing. The cumbersome, restrictive, protective clothing and respirator pose substantial additional stress to the wearer, compared with normal clothing.2-4 The rate of increase in the rectal temperature (T^sub re^) was three to six times greater for various levels of NBC protective clothing than for standard combat clothing, decreasing the work tolerance time by 70 to 80% at different metabolic rates.1 Energy requirements and time to accomplish tasks increase as well.5 The energy cost of walking increases ~3% per 1 kg of clothing when multilayered clothing ensembles are worn.6 Also, hobbling effects of the clothing increase the energy cost.7 Moreover, performance degradation may be caused by impaired manual dexterity, by hampered vision and communication, and by respiratory and psychological stress.5,8 Because of the great impact of heat strain on physical work performance, attention has recently been paid to research and development in producing lightweight fabrics and personal cooling systems. Consequently, there has been success in reducing the risk of heat strain and increasing tolerance time.3

The material of NBC protective clothing may vary from totally impermeable fabric to more-permeable activated carbon fabric. NBC protective garment with impregnated activated carbon is used under normal combat clothing or, as in Finland, under a protective coverall with low water vapor permeability. The protective coverall is needed in situations such as cleaning of contaminated areas or vehicles with liquid detergents and when liquid chemicals are involved. The physiological strain is dependent on the vapor permeability of the NBC protective garment.

NBC weapons may also be used in cold-weather regions. In a cold environment, cold protection in addition to NBC protective clothing must be taken into account. Additional clothing layers increase the bulkiness of the protective clothing system.6,9 However, the cold protection of hands and fingers has remained of less concern. Cooling of the fingers is the main factor for discomfort and performance decreases in a cold environment.10-12 The risk of cold strain can be decreased by limiting the time of exposure to cold conditions, by increasing clothing layers, or by increasing the metabolic rate. Young et al.13 used simulation modeling to predict core and mean skin temperature changes during alternating bouts of exercise and different cold conditions. According to Young et al.,13 the previous experimental studies and the results of their simulation modeling suggested that there is no increased risk of hypothermia with wearing NBC protective clothing while working in the cold. Wearing NBC protective clothing during strenuous activity in cold weather may even increase the risk of heat strain.11,13,14

In the present study, the aim was to quantify the thermal strain of soldiers wearing NBC protective clothing in coldweather conditions during field training. It was hypothesized that heat strain would exist during heavy work down to a T^sub a^ of -25°C. Furthermore, whole-body cold strain would appear during light work at -20°C and moderate work at -25°C, and extremity cooling would appear during moderate work at -10°C.

Methods

Subjects

The study was approved by the ethics committee of the Hospital District of Helsinki and Uusimaa, Finland. All subjects were informed of all details of the experimental procedures. Each subject gave written informed consent before the first experiment.