ABSTRACT
CONTENTS 5.1 Introduction ..................................................................................................................... 215 5.2 Maintenance of Heat Equilibrium ............................................................................... 216 5.3 Endogenous Heat Production (M) ............................................................................... 216 5.4 Thermoregulatory Control ............................................................................................ 217
5.4.1 Tolerance to Added Heat .................................................................................. 218 5.4.2 Heat Generated during Exercise ...................................................................... 218 5.4.3 Exposure to Radio Frequency........................................................................... 218 5.4.4 Fever...................................................................................................................... 218
5.5 Human Experimental Data ........................................................................................... 219 5.5.1 Overexposure....................................................................................................... 219
5.6 Magnetic Resonance Imaging ....................................................................................... 220 5.7 Thermal Perception and Nociception of Radio Frequency Energy ....................... 220 5.8 Behavioral Responses to Excessive Heat Load.......................................................... 221 5.9 Data from Animal Studies............................................................................................. 222
5.9.1 Metabolic Responses .......................................................................................... 222 5.9.2 Vasomotor Responses ........................................................................................ 223 5.9.3 Evaporative Adjustments during Radio Frequency Exposure ................... 223 5.9.4 Intense or Prolonged Exposure ........................................................................ 223 5.9.5 Thermal Hotspots ............................................................................................... 224
5.10 Summary .......................................................................................................................... 224 References ................................................................................................................................... 224
Much of the research on thermoregulation in the presence of absorbed radio frequency (RF) energy has used commonly available laboratory animals. These, like all mammals, are endotherms, with a dependence for body temperature on controlled-heat production by metabolism, but there are important limitations. These arise from their size and consequent surface-to-volume ratio and also differences in thermoregulatory mechanisms, such as sweating, which are species dependant. In general, the function of most mammalian tissues and organs depends on the maintenance of temperature within a
range of some 58C, although this will vary to some extent as a result of changes in activity, hormonal and psychological factors, as well as intake of food and physiologically active substances, such as pharmaceuticals and alcohol. Most tissues are tolerant to low temperatures, at least insofar as potential for recovery,
but since there is relatively low tolerance of overheating, organisms have evolved various sets of sophisticated control mechanisms to avoid this. The controls that are used in human physiological systems may be broadly categorized as behavioral and physiological. For behavioral control, the whole organism acts consciously or subconsciously to alter its environment with regard to radiated or convected heat energy or inhaled air. This provides gross control, which also serves to guard against entering or remaining in adverse thermal conditions, if they can be avoided. In humans this is connected to and mediated by much more complex behaviors, including anxiety, which attempts to predict an adverse environment, resulting in sophisticated avoidance behaviors. These mechanisms provide coarse control, and more primitive variants of such behaviors are also found in ectotherms such as reptiles. The added mechanism available to mammals, where body temperature is maintained to a set point, are the physiological automatic thermoregulatory systems, which are largely controlled by the autonomic nervous system, requiring a sophisticated network of sensors and a variety of control mechanisms that have to work in concert with other autonomic functions such as the metabolic demands of circulation.
