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

Military Medicine, Feb 2007 by Rissanen, Sirkka, Rintam�ki, Hannu

Eleven male subjects participated in the marching trials. Mean � SE values for age, body mass, height, and body mass index were 21.0 � 0.7 years, 180.5 � 2.0cm, 75.1 � 2.8kg, and 23.0 � 0.7 kg/m^sup 2^, respectively. To estimate the relative workload of the marching, maximal oxygen consumption (VO^sub 2peak^) and heart rate were determined for each subject on a treadmill, using open-circuit spirometry (MetaMax 3B; Cortex, Leipzig, Germany) and a heart rate monitor (Sport Tester; Polar Electro Oy, Kempele, Finland), respectively.

Garments

The subjects wore (1) underwear (T-shirt and long-sleeved/ legged underwear), (2) NBC middle wear, (3) NBC coverall (either butyl rubber suit [M85] or polyethene-coated suit [model 2001]), (4) respirator (M95), and (5) impermeable gloves and rubber boots. The NBC middle wear (model 1996) is made of impregnated activated carbon and includes jacket, trousers, separate hood, and socks. Standard combat clothing was worn under the NBC coverall once, when the T^sub a^ was -28�C.

Procedures and Measurements

The subjects performed marching exercises on two different types of terrain (snowy forest road and snowy forest terrain) at different T^sub a^ values (-33 to 0�C). The marching trial consisted of transportation (20 minutes) and marching first on a snowy forest road for 60 minutes and then on snowy forest terrain for 30 to 40 minutes. Each subject performed one to three trials at different T^sub a^ values.

T^sub re^ was measured with a thermistor (YSI 401; Yellow Springs Instruments, Dayton, Ohio) inserted 10 cm beyond the anal sphincter. Skin temperature was measured with 12 thermistors (YSI 427; Yellow Springs Instruments). T^sub re^ and skin temperatures were continuously recorded and saved into a data logger (Squirrel 1200 or 1000; Grant, Shepreth, United Kingdom; or SmartReaderPlus8; ACR Systems, Surrey, Canada) at 1-minute intervals. For heart rate measurements, the Polar heart rate monitor was used. Thermal sensation of the whole body, fingers, and toes was assessed by using a 7-point standardized scale (ISO 10551)15 and the rate of perceived exertion was assessed by using Borg's scale,16 at 15-minute intervals.

Calculations

Determination of heat and cold strain was based on performance criteria17 and on the indices introduced by Moran et al.,18,19 that is, physiological strain index (PSI)18 and cold strain index (CSI).19 Lotens17 categorized thermal strain as discomfort, performance degradation, and tolerance. Calculation of PSI is based on T^sub re^ and heart rate and that of CSI on T^sub re^ and mean skin temperature. PSI and CSI are scaled to range from 0 to 10 (no/little strain to very high strain).

The weighted mean skin temperature (T^sub sk^) was calculated according to the method described by Hardy and Dubois.20 The mean T^sub b^ was calculated as follows: T^sub b^ = 0.65-Tn. 0.35-Tsk. Storage of body heat (kilojoules per kilogram) was calculated from the T^sub b^ changes in comparison with a reference point temperature of T^sub b0^ = 36.3�C found for healthy, well-hydrated persons in a comfort zone, when T^sub re^ was 37.0 and T^sub sk^ was 33.5�C.21 The storage of body heat was calculated as: (T^sub bt^ - T^sub b0^)3.48-BW, where T^sub bt^ is T^sub b^ at time t, 3.48 is the specific heat of body tissue (kilojoules per kilogram per degree Celsius) and BW is body weight (kilograms).