359 Chapter 20 The physiology of acute cold exposure, with particular reference to human performance in the cold Nigel A.S. Taylor Igor B. Mekjavic Michael J. Tipton CHAPTER CONTENTS 1 Introduction 359 2 Biophysics of heat transfer 360 2.1 Heat balance 360 2.2 The interaction of body composition and morphology 361 2.3 Predicting problems in heat balance 362 3 Neurophysiology of thermoreception 364 4 Autonomic regulation 365 4.1 Cardiovascular modulation 365 4.2 Shivering thermogenesis 367 5 Hormonal and body fluid responses 368 5.1 Hormonal changes 368 5.2 Body fluid regulation 368 6 Interactions of age, gender, physical fitness and exercise with the cold responses 369 6.1 Age and cold stress 369 6.2 Gender differences 370 6.3 Physical fitness 370 6.4 Exercise and work in the cold 371 7 Pathophysiogical considerations 372 7.1 Cold shock 373 7.2 Peripheral incapacitation 373 7.3 Freezing and non-freezing cold injuries 373 References 374 1 INTRODUCTION Modern man is of African descent (Ray et al. 2005), with northern migrations from the African and Asian continents (Piazza et al. 1981) right up to the Arctic Circle occurring about 40,000 years ago. These warm-climate hominids, who possessed behavioural and thermoregulatory characteris- tics that enabled survival in the cold, now found themselves in frequently inhospitable climates. Indeed, almost half of the landmass above 40° north latitude is above the freezing line (0°C). In the south, this region is restricted to the Antarctic Continent whilst in the north, the freezing line crosses through the US, Europe (Norway, Denmark, Germany, Italy, Romania, Turkey) and Asia (India, China, Japan; Bates and Bilello 1966). Air temperatures more than 20°C below skin tempera- ture are very stressful for unprotected humans. However, cold-water immersion at equivalent temperatures can rapidly become life threatening, even for people wearing clothing. This occurs for two reasons. First, water displaces air, which is a very good insulator. Indeed, most of the protective insulation provided by thermal protective cloth- ing is derived from the ability of the clothing to trap and retain a sufficiently large volume of air between the skin surface and the surrounding air. This trapped air is rapidly warmed, thereby providing a thermal buffer and reducing transcutaneous heat loss. Immersion removes this protec- tion. Second, the thermal conductivity of water (37°C) is 24 times greater than that of air (630.5 versus 26.2 mW· m -1 ·K -1 ). This means that, for the same thermal gradient, the potential for heat loss is elevated 24-fold during water immersion. In addition, the specific heat capacity of water is four times higher (4.179 versus 1.007 J·g -1 ·K -1 ), and its density is 827 times greater than air (0.9922 versus 0.0012 g·cm -3 ). The product of specific heat capacity and density yields a volume-specific heat capacity, which quan- tifies the amount of heat required to raise the temperature of a given volume of water by 1 K. For water, this value is 3431 times that of air at 37°C, and increasing by <0.01% at 15°C. Water not just has a greater capacity to accept thermal energy, but this energy transfer will proceed at a much