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We examined whether fatigue during exertional heat stress occurred at a critical internal temperature independent of the initial temperature at the start of exercise. Microwaves (2.1 GHz; 100 mW/cm(2)) were used to rapidly (3-8 min) heat rats before treadmill exercise to exhaustion. In a repeated-measures design, food-restricted male Sprague-Dawley rats (n = 11) were preheated to three levels (low, medium, and high). In addition, two sham exposures, Sham 1 and Sham 2, were administered at the beginning and end of the study, respectively. At the initiation of exercise, hypothalamic (T(hyp)) and rectal (T(rec)) temperatures ranged from 39.0 degrees C to 42.8 degrees C (T(hyp)) and 42.1 degrees C (T(rec)). The treadmill speed was 17 m/min (8 degrees grade), and the ambient temperature during exercise was 35 degrees C. Each treatment was separated by 3 wk. Run time to exhaustion was significantly reduced after preheating. There was a significant negative correlation between run time and initial T(hyp) and T(rec) (r = 0.73 and 0.74, respectively). The temperatures at exhaustion were not significantly different across treatments, with a range of 41.9-42.2 degrees C (T(hyp)) and 42.2-42.5 degrees C (T(rec)). There were no significant differences in run time in the sham runs administered at the start and end of the investigation. No rats died as a result of exposure to any of the treatments, and body weight the day after each treatment was unaffected. These results support the concept that a critical temperature exists that limits exercise in the heat.

Whole body hyperthermia may produce vasodialation, nausea, and altered cognitive function. Animal research has identified brain regions that have important roles in thermoregulation. However, differences in both the cognitive and sweating abilities of humans and animals implicate the need for human research. Positron emission tomography (PET) was used to identify brain regions with altered activity during systemic hyperthermia. Human subjects were studied under cool (control) conditions and during steady-state hyperthermia induced by means of a liquid-conditioned suit perfused with hot water. PET images were obtained by injecting [(18)F]fluorodeoxyglucose, waiting 20 min for brain uptake, and then scanning for 10 min. Heating was associated with a 23% increase in resting metabolic rate. Significant increases in cerebral metabolic rate occurred in the hypothalamus, thalamus, corpus callosum, cingulate gyrus, and cerebellum. In contrast, significant decreases occurred in the caudate, putamen, insula, and posterior cingulum. These results are important for understanding the mechanisms responsible for altered cognitive and systemic responses during hyperthermia. Novel regions (e.g., lateral cerebellum) with possible thermoregulatory roles were identified.

We present critiques of epidemiologic studies and experimental investigations, published mostly in peer‐reviewed journals, on cancer and related effects from exposure to nonionizing electromagnetic fields in the nominal frequency range of 3 kHz to 300 GHz of interest to Subcommittee 4 (SC4) of the International Committee on Electromagnetic Safety (ICES). The major topics discussed are presented under the headings Epidemiologic and Other Findings on Human Exposure, Mammals Exposed In Vivo, Mammalian Live Tissues and Cell Preparations Exposed In Vitro, and Mutagenesis and Genotoxicity in Microorganisms and Fruit Flies. Under each major topic, we present minireviews of papers on various specific endpoints investigated. The section on Epidemiologic and Other Findings on Human Exposure is divided into two subsections, the first on possible carcinogenic effects of exposure from emitters not in physical contact with the populations studied, for example, transmitting antennas and other devices. Discussed in the second subsection are studies of postulated carcinogenic effects from use of mobile phones, with prominence given to brain tumors from use of cellular and cordless telephones in direct physical contact with an ear of each subject. In both subsections, some investigations yielded positive findings, others had negative findings, including papers directed toward experimentally verifying positive findings, and both were reported in a few instances. Further research on various important aspects may resolve such differences. Overall, however, the preponderance of published epidemiologic and experimental findings do not support the supposition that in vivo or in vitro exposures to such fields are carcinogenic. Bioelectromagnetics Supplement 6:S74–S100, 2003. Published 2003 Wiley‐Liss, Inc.

Digital anatomical models of man and animals are available for use in numerical calculations to predict electromagnetic field (EMF)-induced specific absorption rate (SAR) values. To use these models, permittivity values are assigned to the various tissues for the EMF frequencies of interest. There is, as yet, no consensus on what are the best permittivity data. This study analyzed the variability in published permittivity data and investigated the effects of permittivity values that are proportional on SAR calculations. Whole-sphere averaged and localized SAR values along the diameter of a 4-cm sphere are calculated for EMF exposures in the radio frequency range of 1 MHz to 1 GHz. When the dimensions of a sphere are small compared to the wavelength (i.e., wavelength inside the material is greater than ten times the dimensions of the object), the whole-sphere averaged SAR is inversely proportional to the permittivity of the material composing the sphere. However, the localized SAR values generally do not have the same relation and, as a matter of fact, vary greatly depending on the location within the sphere. These results indicate that care must be taken in choosing the permittivity values used in calculating SAR values and some estimate of the dependence of the calculated SAR values on variability in permittivity should be determined.