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

The T^sub sk^ decreased below 30�C at each T^sub a^ in 60 minutes during moderate work. T^sub sk^ decreases easily to performance level, especially at the beginning of the exercise, because of redistribution of circulation and increase of vasoconstriction in response to cold. The degree of the decrement in T^sub sk^ was dependent on T^sub a^, rather than the metabolic rate of the marching. However, the duration of exposure to cold affects T^sub sk^. In the study by Cortili et al.10 T^sub sk^ was 26.8�C at T^sub a^ of 0�C and -20�C after 120 minutes and 56 minutes of exercise, respectively, at a moderate workload. A decrease of T^sub sk^ below 30�C means performance degradation, according to Lotens.17 However, this temperature might be too high for performance decrement, because a significant decrease in muscle performance was not detected until the T^sub sk^ was decreased below ~28�C.26 At a T^sub a^ of -25�C or lower (moderate work), T^sub sk^ decreased below 25�C (tolerance level), which could be tolerated but might increase risks of health disorders if the work period in the cold is prolonged. Because of its dependence on the ambient environment and on normal physiological responses to cold, the T^sub sk^ may be regarded as a secondary indicator of thermal physiological strain. However, it constitutes an important criterion for the appraisal of thermal comfort.

Cooling of the fingers and hands was typical at all T^sub a^ values. Handling of cold items (e.g., radio or other metal devices) accelerates cooling even at mild temperatures (0-5�C).23 At T^sub a^ below -15�C, the finger temperature decreased to 15�C or lower during moderate work, which might cause impairment of manual dexterity. The tolerance level (5�C) of the fingers was reached during moderate activity at T^sub a^ between -20�C and -28�C. Wearing mittens delayed or even prevented the decrement of finger temperature. It is recommended that thermally insulated mittens be used to cover the rubber gloves. In the present study, the cold discomfort experienced was mainly focused on the hands and fingers.

Because of the decrement of T^sub sk^, the rate of heat storage was negative, although T^sub re^ increased. The degree of the decrease in T^sub sk^ at the beginning of the marching and also the time of the start of the increase were dependent on T^sub a^. Storage of body heat in cold is affected by peripheral cooling and more reflects T^sub a^, like T^sub sk^. Estimation of body heat storage can also be dependent on weighing factors of T^sub re^ and T^sub sk^.27 Environment, acclimation, and type of clothing may affect the estimate of heat storage.27

In the present study, the applicability of strain indices (PSI and CSI)18,19 was tested in cold conditions when NBC protective clothing was used. CSI showed little or no cold strain even in the coldest conditions, unlike the other indicators (T^sub sk^ and storage of body heat). CSI seemed to describe adequately the cold strain during rest, but changes in core and surface temperatures attributable to the physical activity might have resulted in incorrect interpretation of cold strain.28