Thermal Response of Human Skin to Microwave Energy: A Critical Review

This is a review/modeling study of heating of tissue by microwave energy in the frequency range from 3 GHz through the millimeter frequency range (30-300 GHz). The literature was reviewed to identify studies that reported RF-induced increases in skin temperature. A simple thermal model, based on a simplified form of Pennes' bioheat equation (BHTE), was developed, using parameter values taken from the literature with no further adjustment. The predictions of the model were in excellent agreement with available data. A parametric analysis of the model shows that there are two heating regimes with different dominant mechanisms of heat transfer. For small irradiated areas (less than about 0.5-1 cm in radius) the temperature increase at the skin surface is chiefly limited by conduction of heat into deeper tissue layers, while for larger irradiated areas, the steady-state temperature increase is limited by convective cooling by blood perfusion. The results support the use of this simple thermal model to aid in the development and evaluation of RF safety limits at frequencies above 3 GHz and for millimeter waves, particularly when the irradiated area of skin is small. However, very limited thermal response data are available, particularly for exposures lasting more than a few minutes to areas of skin larger than 1-2 cm in diameter. The paper concludes with comments about possible uses and limitations of thermal modeling for setting exposure limits in the considered frequency range.

Abnormal Motor Activity and Thermoregulation in a Schizophrenia Rat Model for Translational Science

Background

Schizophrenia is accompanied by altered motor activity and abnormal thermoregulation; therefore, the presence of these symptoms can enhance the face validity of a schizophrenia animal model. The goal was to characterize these parameters in freely moving condition of a new substrain of rats showing several schizophrenia-related alterations.

Methods

Male Wistar rats were used: the new substrain housed individually (for four weeks) and treated subchronically with ketamine, and naive animals without any manipulations. Adult animals were implanted with E-Mitter transponders intraabdominally to record body temperature and locomotor activity continuously. The circadian rhythm of these parameters and the acute effects of changes in light conditions were analyzed under undisturbed circumstances, and the effects of different interventions (handling, bed changing or intraperitoneal vehicle injection) were also determined.

Results

Decreased motor activity with fragmented pattern was observed in the new substrain. However, these animals had higher body temperature during the active phase, and they showed wider range of its alterations, too. The changes in light conditions and different interventions produced blunted hyperactivity and altered body temperature responses in the new substrain. Poincaré plot analysis of body temperature revealed enhanced short- and long-term variabilities during the active phase compared to the inactive phase in both groups. Furthermore, the new substrain showed increased short- and long-term variabilities with lower degree of asymmetry suggesting autonomic dysregulation.

Conclusions

In summary, the new substrain with schizophrenia-related phenomena showed disturbed motor activity and thermoregulation suggesting that these objectively determined parameters can be biomarkers in translational research.

Index of thermal stress for cows (ITSC) under high solar radiation in tropical environments

This paper presents a new thermal stress index for dairy cows in inter-tropical regions, with special mention to the semi-arid ones. Holstein cows were measured for rectal temperature (T R), respiratory rate (F R) and rates of heat exchange by convection (C), radiation (R), skin surface evaporation (E S) and respiratory evaporation (E R) in the north eastern region of Brazil, after exposure to sun for several hours. Average environmental measurements during the observations were air temperature (T A) 32.4 °C (24.4-38.9°), wind speed (U) 1.8 m.s(-1) (0.01-11.0), relative humidity 63.6 % (36.8-81.5) and short-wave solar radiation 701.3 W m(-2) (116-1,295). The effective radiant heat load (ERHL) was 838.5 ± 4.9 W m(-2). Values for the atmospheric transmittance (τ) were also determined for tropical regions, in order to permit adequate estimates of the solar radiation. The average value was τ = 0.611 ± 0.004 for clear days with some small moving clouds, with a range of 0.32 to 0.91 in the day period from 1000 to 1300 hours. Observed τ values were higher (0.62-0.66) for locations near the seacoast and in those regions well-provided with green fields. Effects of month, location and time of the day were all statistically significant (P < 0.01). A total of 1,092 data were obtained for cows exposed for 1 to 8 h to sun during the day; in 7 months (February, March, April, July, August, September and November), 4 days per month on the average. A principal component analysis summarised the T R, F R, C, R, E S and E R measurements into just one synthetic variable (y 1); several indexes were then obtained by multiple regression of y 1 on the four environmental variables and its combinations, by using Origin 8.1 software (OriginLab Corp.). The chosen equation was the index of thermal stress for cows, ITSC = 77.1747 + 4.8327 T A - 34.8189 U + 1.111 U (2) + 118.6981 P V - 14.7956 P V (2) - 0.1059 ERHL with r (2) = 0.812. The correlations of ITSC with T R, F R, C, E S, R and E R were 0.275, 0.255, -0.493, -0.647, -0.818 and 0.075, respectively. Correlations of the index with the physiological variables are presented, and ITSC is compared to three other indexes.

Modeling the effect of adverse environmental conditions and clothing on temperature rise in a human body exposed to radio frequency electromagnetic fields

This study considers the computationally determined thermal profile of a fully clothed, finely discretized, heterogeneous human body model, subject to the maximum allowable reference level for a 1-GHz radio frequency electromagnetic field for a worker, and also subject to adverse environmental conditions, including high humidity and high ambient temperature. An initial observation is that while electromagnetic fields at the occupational safety limit will contribute an additional thermal load to the tissues, and subsequently, cause an elevated temperature, the magnitude of this effect is far outweighed by that due to the conditions including the ambient temperature, relative humidity, and the type of clothing worn. It is envisaged that the computational modeling approach outlined in this paper will be suitably modified in future studies to evaluate the thermal response of a body at elevated metabolic rates, and for different body shapes and sizes including children and pregnant women.

[The influence of general infrared sauna on the antioxidant systems in the blood of volunteers]

This comprehensive clinical and laboratory study was designed to elucidate the antioxidant status of the volunteers before and after a course of general infrared irradiation. It was shown that the effect of high temperatures promotes the development of oxidative stress that is followed by the formation of adaptive reactions in the form of activation of antioxidant protection, enhancement of non-specific responses of the cells, increase of stability and restoration of structural homeostasis of erythrocyte membranes. The molecular mechanism of endothelium-independent vasodilation develops by the end of the treatment period; it may serve as the compensatory-adaptive reaction needed to maintain the adequate tone of the vascular wall and thereby to support the functioning of mechanisms supporting physical thermoregulation. The results of the study give evidence of the stimulatory influence of the described method on the adaptive and protective potential of the organism. It is concluded that the proposed scheme of physiotherapeutic treatment may be used for prophylactic purposes.

Effects of chronic exposure to radiofrequency electromagnetic fields on energy balance in developing rats

The effects of radiofrequency electromagnetic fields (RF-EMF) on the control of body energy balance in developing organisms have not been studied, despite the involvement of energy status in vital physiological functions. We examined the effects of chronic RF-EMF exposure (900 MHz, 1 V m(-1)) on the main functions involved in body energy homeostasis (feeding behaviour, sleep and thermoregulatory processes). Thirteen juvenile male Wistar rats were exposed to continuous RF-EMF for 5 weeks at 24 °C of air temperature (T a) and compared with 11 non-exposed animals. Hence, at the beginning of the 6th week of exposure, the functions were recorded at T a of 24 °C and then at 31 °C. We showed that the frequency of rapid eye movement sleep episodes was greater in the RF-EMF-exposed group, independently of T a (+42.1 % at 24 °C and +31.6 % at 31 °C). The other effects of RF-EMF exposure on several sleep parameters were dependent on T a. At 31 °C, RF-EMF-exposed animals had a significantly lower subcutaneous tail temperature (-1.21 °C) than controls at all sleep stages; this suggested peripheral vasoconstriction, which was confirmed in an experiment with the vasodilatator prazosin. Exposure to RF-EMF also increased daytime food intake (+0.22 g h(-1)). Most of the observed effects of RF-EMF exposure were dependent on T a. Exposure to RF-EMF appears to modify the functioning of vasomotor tone by acting peripherally through α-adrenoceptors. The elicited vasoconstriction may restrict body cooling, whereas energy intake increases. Our results show that RF-EMF exposure can induce energy-saving processes without strongly disturbing the overall sleep pattern.

[The influence of ultrahigh-frequency electromagnetic radiation and low-intensity laser radiation on the body core temperature and basal metabolism in rats with systemic inflammation]

The effects of ultrahigh-frequency electromagnetic radiation (UHF EMR) and low-intensity laser irradiation (LILI) on the body and skin temperature, oxygen consumption, production of carbon dioxide and heat release were investigated in the experiments on intact rats and during LPS-induced polyphasic fever. It was found that UHF EMR with the wavelength of 4,9 mm, 5,6 mm or 7,1 mm and LILI with the wavelength of 0.47 microm, 0.67 microm and 0.87 microm caused modulation of basal metabolism and thermal response to systemically administered lipopolysaccharide (LPS). These findings suggest that the most pronounced antipyretic and hypometabolic effects were observed after the treatment with UHF EMR at 7,1 mm and LILI at 470 microm.

Effects of shade on physiological changes, oxidative stress, and total antioxidant power in Thai Brahman cattle

The purpose of this study was to assess the effects of artificial shade, tree shade, and no shade on physiological changes, oxidative stress, and total antioxidant power in Thai Brahman cattle. Twenty-one cattle were divided into three groups: cattle maintained under artificial shade, under tree shade, and without shade. On days 1, 7, 14, 21, and 28 of the experimental period, after the cattle were set in individual stalls for 2 h, physiological changes, thiobarbituric acid reactive substances (TBARS), and total antioxidant power were investigated. The results revealed that the respiratory rate, heart rate, sweat rate and the neutrophil/lymphocyte ratio of the no-shade cattle were significantly higher than those of cattle maintained under artificial shade and tree shade (P < 0.05). During the early period of heat exposure, the total antioxidant power of the no-shade cattle was lower than those of cattle maintained under artificial shade and tree shade, but the total antioxidant power of cattle maintained under artificial shade and tree shade were not different (P > 0.05). However, rectal temperature and packed cell volume of the cattle in all groups did not differ (P > 0.05). These results showed that artificial shade and tree shade can protect cattle from sunlight compared to no shade, and that the effectiveness of tree shade for sunlight protection is at an intermediate level.

Electromagnetic analysis of an RF rectangular resonant cavity applicator for hyperthermic treatment using whole-body voxel human model of Japanese adult male

The numerical whole-body voxel human model (numerical model) developed by National Institute of Information and Communications Technology (NICT) was assumed and hyperthemic treatment using radio-frequency wave was investigated. We assumed 51 different human tissues and organs with 2-mm spatial resolution in the numerical model, inserted it into the resonant cavity applicator, and Maxwell's equations were solved by FDTD method with variable mesh. We obtained the realistic energy patterns for a deep-seated tumor as compared to those obtained in our previous studies.

Thermal radiation absorbed by dairy cows in pasture

The goal of the present paper was to assess a method for estimating the thermal radiation absorbed by dairy cows (0.875 Holstein-0.125 Guzerath) on pasture. A field test was conducted with 472 crossbred dairy cows in three locations of a tropical region. The following environmental data were collected: air temperature, partial vapour pressure, wind speed, black globe temperature, ground surface temperature and solar radiation. Average total radiation absorbed by animals was calculated as R(abs) = 640.0 +/- 3.1 W .m(-2). Absorbed short-wave radiation (solar direct, diffuse and reflected) averaged 297.9 +/- 2.7 W m(-2); long wave (from the sky and from terrestrial surfaces) averaged 342.1 +/- 1.5 W m(-2). It was suggested that a new environmental measurement, the effective radiant heat load (ERHL), could be used to assess the effective mean radiant temperature (T*(mr)). Average T*(mr) was 101.4 +/- 1.2 degrees C, in contrast to the usual mean radiant temperature, T(mr) = 65.1 +/- 0.5 degrees C. Estimates of T*(mr) were considered as more reliable than those of T (mr) in evaluating the thermal environment in the open field, because T (mr) is almost totally associated only with long wave radiation.

Radio frequency heating at 9.4T (400.2 MHz): in vivo thermoregulatory temperature response in swine

In vivo thermoregulatory temperature response to radio frequency (RF) heating at 9.4T was studied by measuring temperatures in nine anesthetized swine. Temperatures were measured in the scalp, brain, and rectum. The RF energy was deposited using a four-loop head coil tuned to 400.2 MHz. Sham RF was delivered to three swine to understand the thermal effects of anesthesia (animal weight = 54.16 kg, SD = 3.08 kg). Continuous wave (CW) RF energy was delivered to the other six animals for 2.5-3.4 h (animal weight = 74.01 +/- 26.0 kg, heating duration = 3.05 +/- 0.29 h). The whole-head specific absorption rate (SAR) varied between 2.71 W/kg and 3.20 W/kg (SAR = 2.93 +/- 0.18 W/kg). Anesthesia caused the brain and rectal temperatures to drop linearly. Altered thermoregulatory response was detected by comparing the difference in the temperature slopes before and after the RF delivery from zero. RF heating statistically significantly altered the rate of cooling down of the animal. The temperature slope changes correlated well with the RF energy per unit head weight and heating duration, and the maximum rectal temperature change during heating in heated animals. The temperature slope changes did not correlate well to the whole-head average SARs.

Application of heat- and steam-generating sheets to the lumbar or abdominal region affects autonomic nerve activity

Effects of applying a heat- and steam-generating (HSG) sheet on peripheral hemodynamics and autonomic nerve activity were examined. An HSG sheet was applied to the lumbar or abdominal region. Measurements included skin temperature at the lumbar and abdominal regions and the fingertip, total hemoglobin, tissue oxygen saturation ratio (StO2), pupillary light reflex, changes in ECG R-R interval blood pressure and percutaneous electrogastrography (EGG). A heat-generating sheet without steam was used as the control. Based on the present findings, application of the HSG sheet to the lumbar or abdominal region may improve peripheral hemodynamics and inhibit sympathetic nerve activity, resulting in parasympathetic nerve activity dominance.

Thermophysiological responses of human volunteers to whole body RF exposure at 220 MHz

Since 1994, our research has demonstrated how thermophysiological responses are mobilized in human volunteers exposed to three radio frequencies, 100, 450, and 2450 MHz. A significant gap in this frequency range is now filled by the present study, conducted at 220 MHz. Thermoregulatory responses of heat loss and heat production were measured in six adult volunteers (five males, one female, aged 24-63 years) during 45 min whole body dorsal exposures to 220 MHz radio frequency (RF) energy. Three power densities (PD = 9, 12, and 15 mW/cm(2) [1 mW/cm(2) = 10 W/m(2)], whole body average normalized specific absorption rate [SAR] = 0.045 [W/kg]/[mW/cm(2)] = 0.0045 [W/kg]/[W/m(2)]) were tested at each of three ambient temperatures (T(a) = 24, 28, and 31 degrees C) plus T(a) controls (no RF). Measured responses included esophageal (T(esoph)) and seven skin temperatures (T(sk)), metabolic rate (M), local sweat rate, and local skin blood flow (SkBF). Derived measures included heart rate (HR), respiration rate, and total evaporative water loss (EWL). Finite difference-time domain (FDTD) modeling of a seated 70 kg human exposed to 220 MHz predicted six localized "hot spots" at which local temperatures were also measured. No changes in M occurred under any test condition, while T(esoph) showed small changes (< or =0.35 degrees C) but never exceeded 37.3 degrees C. As with similar exposures at 100 MHz, local T(sk) changed little and modest increases in SkBF were recorded. At 220 MHz, vigorous sweating occurred at PD = 12 and 15 mW/cm(2), with sweating levels higher than those observed for equivalent PD at 100 MHz. Predicted "hot spots" were confirmed by local temperature measurements. The FDTD model showed the local SAR in deep neural tissues that harbor temperature-sensitive neurons (e.g., brainstem, spinal cord) to be greater at 220 than at 100 MHz. Human exposure at both 220 and 100 MHz results in far less skin heating than occurs during exposure at 450 MHz. However, the exposed subjects thermoregulate efficiently because of increased heat loss responses, particularly sweating. It is clear that these responses are controlled by neural signals from thermosensors deep in the brainstem and spinal cord, rather than those in the skin.

A physio-anthropological approach in evaluation of human adaptability to living environment: in the case of artificial light environment

Attention has been focused on human adaptability to human-adopted artificially modern and comfortable environments which may not always match the adaptability by the physiological state of humans. This discrepancy was found to induce a slight tension in the human body on evaluation with a physio-anthropological approach. Although a standard methodological approach has yet to be established, the present study attempted to evaluate human adaptability to artificial light environment, based on evaluations from three major possible perspectives: a) central nervous system, b) autonomic nervous system and c) biological rhythm. In order to detect the slight tension, human volunteers were exposed to an artificial light environment, and the appropriate physiological parameters were then examined from photic signal pathways in a brain. The findings revealed that a higher color temperature of fluorescent lamps induced a slight tension, although many other factors remained to be elucidated.

Modification of internal temperature regulation for cutaneous vasodilation and sweating by bright light exposure at night

Bright light (BL) exposure at night leads to suppressed secretion of melatonin and an attenuated fall in internal temperature at rest from the night to the early morning. However, it is unknown at the present whether typical diurnal variations in reflex responses to thermal challenges are similarly affected by BL exposure at night. We investigated the control of cutaneous vasodilator and sweating responses to hyperthermia in the early morning after artificial BL exposure at night, compare with dim light (DL) exposure. Six subjects stayed awake in a semi-supine position under DL (120 lx) or BL (2800 lx) conditions between 21.00 and 04.30 h. Urine samples were collected at 04.30 h. Beginning at 05.30 h, the lower legs were immersed for 50 min in 42 degrees C water. The subjects remained awake for 21 h until the end of hot water immersion. Urinary 6-sulphatoxymelatonin levels following BL were significantly lower than after DL. Oesophageal temperature (T es) before heating was significantly higher following BL [36.41+/-0.10 (DL) vs. 36.55+/-0.09 (BL) degrees C]. The T es thresholds for the onset of cutaneous vasodilation and sweating were significantly higher with BL than with DL conditions (approximately 0.15 degrees C, respectively). We found that the internal temperature threshold for thermoregulatory control of cutaneous vasodilation and sweating responses to passive heating in the early morning can be modified by the level of light exposure the prior night. Thus both basal internal temperature and the regulation of internal temperature are modified by BL exposure at night.

Short photoperiod enhances thermogenic capacity in Brandt's voles

Environmental cues play important roles in the regulation of an animal's physiology and behavior. In the present study, we examined the effects of short photoperiod (SD) on body weight as well as on several physiological, hormonal, and biochemical measures indicative of thermogenic capacity to test our hypothesis that short photoperiod stimulates increases in thermogenesis without cold stress in Brandt's voles. SD voles showed increases in basal metabolic rate (BMR) and nonshivering thermogenesis (NST) during the 4-week photoperiod acclimation. At the end, these voles (SD) had lower body weights, higher levels of cytochrome C oxidase (COX) activity and mitochondrial uncoupling protein-1 (UCP1) contents in brown adipose tissues (BAT), and higher concentrations of serum tri-iodothyronine (T3) and thyroxine (T4) compared to LD voles. No differences were found between male and female voles in any of the above-mentioned measurements. Together, these data indicate that SD experience enhances thermogenic capacity similarly in males and females of Brandt's voles.

High sensitivity of human melatonin, alertness, thermoregulation, and heart rate to short wavelength light

Light can elicit acute physiological and alerting responses in humans, the magnitude of which depends on the timing, intensity, and duration of light exposure. Here, we report that the alerting response of light as well as its effects on thermoregulation and heart rate are also wavelength dependent. Exposure to 2 h of monochromatic light at 460 nm in the late evening induced a significantly greater melatonin suppression than occurred with 550-nm monochromatic light, concomitant with a significantly greater alerting response and increased core body temperature and heart rate ( approximately 2.8 x 10(13) photons/cm(2)/sec for each light treatment). Light diminished the distal-proximal skin temperature gradient, a measure of the degree of vasoconstriction, independent of wavelength. Nonclassical ocular photoreceptors with peak sensitivity around 460 nm have been found to regulate circadian rhythm function as measured by melatonin suppression and phase shifting. Our findings-that the sensitivity of the human alerting response to light and its thermoregulatory sequelae are blue-shifted relative to the three-cone visual photopic system-indicate an additional role for these novel photoreceptors in modifying human alertness, thermophysiology, and heart rate.

Temperature rises in the crystalline lens from focal irradiation

Many types of ophthalmic instruments produce a concentrated focal irradiance in the lens. Instruments that illuminate large areas of the retina-known as "Maxwellian-view," are but one example, and there are concerns about the potential hazards associated with this optical system. The transfer of the heat generated in the human eye in Maxwellian-view illumination or similar focal-beam situations was simulated using a mathematical model to determine the temperature elevations induced in the human eye. The maximum temperature rise in the lens region was examined to quantitatively assess the potential thermal hazard to the lens. It was shown that Maxwellian-view illumination or similar focal-beam situations can cause thermal injury to the lens under certain conditions, and that this hazard is greater for incident wavelengths of about 320-420 nm than for longer wavelengths. The risk of thermal injury increases as exposure duration increases, and the risk tends to increase as the beam waist diameter or Maxwellian-view angle decreases.

Numerical modelling of thermal effects in rats due to high-field magnetic resonance imaging (0.5–1 GHz)

A finite-difference time-domain (FDTD) thermal model has been developed to compute the temperature elevation in the Sprague Dawley rat due to electromagnetic energy deposition in high-field magnetic resonance imaging (MRI). The field strengths examined ranged from 11.75-23.5 T (corresponding to 1H resonances of 0.5-1 GHz) and an N-stub birdcage resonator was used to both transmit radio-frequency energy and receive the MRI signals. With an in-plane resolution of 1.95 mm, the inhomogeneous rat phantom forms a segmented model of 12 different tissue types, each having its electrical and thermal parameters assigned. The steady-state temperature distribution was calculated using a Pennes 'bioheat' approach. The numerical algorithm used to calculate the induced temperature distribution has been successfully validated against analytical solutions in the form of simplified spherical models with electrical and thermal properties of rat muscle. As well as assisting with the design of MRI experiments and apparatus, the numerical procedures developed in this study could help in future research and design of tumour-treating hyperthermia applicators to be used on rats in vivo.

Long-term, irradiation-induced effects on thermoregulation in the skin after thermal stimulation

PURPOSE

The morphological effect of radiation on the skin has been adequately analyzed, but the functional effect has received little attention. The purpose of this study was to examine the long-term effects of radiation on the skin from the viewpoint of function.

MATERIALS AND METHODS

Physiological changes in the irradiated skin of patients who had undergone breast-conserving therapy for the treatment of breast cancer were examined throughout the follow-up period. Thermal stimulation was applied to both breasts, and changes in skin temperature and sweating reactivity of irradiated and non-irradiated skin were measured.

RESULTS

From three weeks to the end of radiotherapy, the resting skin temperature of the irradiated region was significantly elevated, while the rate of sweating was lower. More than two years after radiotherapy, the elevated resting skin temperature of the irradiated region had returned to within the range observed for non-irradiated skin, although an abnormally high increase in skin temperature after thermal stimulation continued to be observed for more than two years after radiotherapy. At the same time, sweating after thermal stimulation continued to be suppressed.

CONCLUSION

Present observations suggest that functional effects, such as the skin temperature and sweating ability of irradiated skin, persist longer than readily visible morphological changes.

Efficacy of a single sequence of intermittent bright light pulses for delaying circadian phase in humans

It has been shown in animal studies that exposure to brief pulses of bright light can phase shift the circadian pacemaker and that the resetting action of light is most efficient during the first minutes of light exposure. In humans, multiple consecutive days of exposure to brief bright light pulses have been shown to phase shift the circadian pacemaker. The aim of the present study was to determine whether a single sequence of brief bright light pulses administered during the early biological night would phase delay the human circadian pacemaker. Twenty-one healthy young subjects underwent a 6.5-h light exposure session in one of three randomly assigned conditions: 1) continuous bright light of approximately 9,500 lux, 2) intermittent bright light (six 15-min bright light pulses of approximately 9,500 lux separated by 60 min of very dim light of <1 lux), and 3) continuous very dim light of <1 lux. Twenty subjects were included in the analysis. Core body temperature (CBT) and melatonin were used as phase markers of the circadian pacemaker. Phase delays of CBT and melatonin rhythms in response to intermittent bright light pulses were comparable to those measured after continuous bright light exposure, even though the total exposure to the intermittent bright light represented only 23% of the 6.5-h continuous exposure. These results demonstrate that a single sequence of intermittent bright light pulses can phase delay the human circadian pacemaker and show that intermittent pulses have a greater resetting efficacy on a per minute basis than does continuous exposure.

Modeling thermal responses in human subjects following extended exposure to radiofrequency energy

BACKGROUND

This study examines the use of a simple thermoregulatory model for the human body exposed to extended (45 minute) exposures to radiofrequency/microwave (RF/MW) energy at different frequencies (100, 450, 2450 MHz) and under different environmental conditions. The exposure levels were comparable to or above present limits for human exposure to RF energy.

METHODS

We adapted a compartmental model for the human thermoregulatory system developed by Hardy and Stolwijk, adding power to the torso skin, fat, and muscle compartments to simulate exposure to RF energy. The model uses values for parameters for "standard man" that were originally determined by Hardy and Stolwijk, with no additional adjustment. The model predicts changes in core and skin temperatures, sweat rate, and changes in skin blood flow as a result of RF energy exposure.

RESULTS

The model yielded remarkably good quantitative agreement between predicted and measured changes in skin and core temperatures, and qualitative agreement between predicted and measured changes in skin blood flow. The model considerably underpredicted the measured sweat rates.

CONCLUSIONS

The model, with previously determined parameter values, was successful in predicting major aspects of human thermoregulatory response to RF energy exposure over a wide frequency range, and at different environmental temperatures. The model was most successful in predicting changes in skin temperature, and it provides insights into the mechanisms by which the heat added to body by RF energy is dissipated to the environment. Several factors are discussed that may have contributed to the failure to account properly for sweat rate. Some features of the data, in particular heating of the legs and ankles during exposure at 100 MHz, would require a more complex model than that considered here.

Rewarming index of the lower leg assessed by infrared thermography in adolescents with type 1 diabetes mellitus

The aim of this study was to determine whether infrared thermography before and after challenge of the lower leg in cold water may be a useful tool to detect abnormalities in skin blood flow in adolescent asymptomatic patients with type 1 diabetes mellitus (DM1) and to assess the optimal setting of skin temperature measurements. Twenty-five adolescents (10 female, 15 male, mean age 21.2 +/- 6.2 years, body mass index [BMI] 23.0 +/- 2.1 kg/m2) with a duration of DMI of 13.8 +/- 5.4 years and mean HbA1c levels 8.5 +/- 1.3% were compared to age- and sex-matched controls (BMI 22.9 +/- 2.2 kg/m2). Seven defined sites of the lower leg were assessed by infrared thermography before and for 10 min after exposure of the leg to 14 degrees C cold water. As skin temperature before exposure to cold water differs from individual to individual and basal temperature was significantly warmer in patients at the tip of the first (p < 0.05) and fifth (p < 0.05) toe, the rewarming index was calculated in order to compare data. Rewarming indexes of skin temperature during the whole measurement procedure (0-10 min) were significantly lower at the tip of the first (p < 0.05) and fifth (p < 0.01) toes and from minute 2-10 also at the inner ankle (p < 0.05) in patients compared to healthy controls. Rewarming indexes of the other four sites were not significantly different between patients and controls. Infrared thermography of the lower leg after cold water exposure is an easily applicable method and a useful tool to detect abnormalities of skin blood flow in adolescents with DM1 especially at the tips of the first and fifth toes and the inner ankle.

Behavioral and cognitive effects of microwave exposure

This paper presents an overview of the recent behavioral literature concerning microwave exposure and discusses behavioral effects that have supported past exposure standards. Other effects, which are based on lower levels of exposure, are discussed as well, relative to setting exposure standards. The paper begins with a brief discussion of the ways in which behavioral end points are investigated in the laboratory, together with some of the methodological considerations pertinent to such studies when radio frequency (RF) exposure is involved. It has been pointed out by several sources that exposure to RF radiation can lead to changes in the behavior of humans and laboratory animals that can range from the perceptions of warmth and sound to lethal body temperatures. Behavior of laboratory animals can be perturbed and, under certain other conditions, animals will escape and subsequently avoid RF fields; but they will also work to obtain a burst of RF energy when they are cold. Reports of change of cognitive function (memory and learning) in humans and laboratory animals are in the scientific literature. Mostly, these are thermally mediated effects, but other low level effects are not so easily explained by thermal mechanisms. The phenomenon of behavioral disruption by microwave exposure, an operationally defined rate decrease (or rate increase), has served as the basis for human exposure guidelines since the early 1980s and still appears to be a very sensitive RF bioeffect. Nearly all evidence relates this phenomenon to the generation of heat in the tissues and reinforces the conclusion that behavioral changes observed in RF exposed animals are thermally mediated. Such behavioral alteration has been demonstrated in a variety of animal species and under several different conditions of RF exposure. Thermally based effects can clearly be hazardous to the organism and continue to be the best predictor of hazard for homosapiens. Nevertheless, similar research with man has not been conducted. Although some studies on human perception of RF exist, these should be expanded to include a variety of RF parameters.

Thermoregulatory responses to RF energy absorption

This white paper combines a tutorial on the fundamentals of thermoregulation with a review of the current literature concerned with physiological thermoregulatory responses of humans and laboratory animals in the presence of radio frequency (RF) and microwave fields. The ultimate goal of research involving whole body RF exposure of intact organisms is the prediction of effects of such exposure on human beings. Most of the published research on physiological thermoregulation has been conducted on laboratory animals, with a heavy emphasis on laboratory rodents. Because their physiological heat loss mechanisms are limited, these small animals are very poor models for human beings. Basic information about the thermoregulatory capabilities of animal models relative to human capability is essential for the appropriate evaluation and extrapolation of animal data to humans. In general, reliance on data collected on humans and nonhuman primates, however fragmentary, yields a more accurate understanding of how RF fields interact with humans. Such data are featured in this review, including data from both clinic and laboratory. Featured topics include thermal sensation, human RF overexposures, exposures attending magnetic resonance imaging (MRI), predictions based on simulation models, and laboratory studies of human volunteers. Supporting data from animal studies include the thermoregulatory profile, response thresholds, physiological responses of heat production and heat loss, intense or prolonged exposure, RF effects on early development, circadian variation, and additive drug-microwave interactions. The conclusion is inescapable that humans demonstrate far superior thermoregulatory ability over other tested organisms during RF exposure at, or even above current human exposure guidelines.

[Effects of the microwave exposure at elevated ambient temperature on the thermo-compensatory responses of small laboratory animals]

Thermogenic effectiveness of electromagnetic irradiation (EMI) of UHF range (7 GHz) in the dependence on intensity (10-50 mW/cm2) and environmental temperature (22 degrees and 30 degrees C) was studied in experiments with mice and rats. Negative influence of high ambient temperature on thermoregulate responses of animals at microwave exposure was showed. It is concluded that this interaction should been taken into account for hygienic standardization of non-ionizing EMI.

Local heating of human skin by millimeter waves: A kinetics study

Heating rates of human skin exposed locally to 42.25 GHz mm waves, coming from a waveguide (WG) opening or a YAV device designed for therapeutic application, were studied in vivo using infrared (IR) thermography. For both radiators, the power density distribution was described by a circularly symmetrical Gaussian type function on the exposed skin surface. Insertion of a small thermocouple (d = 0.1 mm) in the exposed area did not produce any significant artifact, either in the power density distribution or kinetics measurement, providing it was perpendicular to the E vector. The heating kinetics in the skin exposed with either the WG opening or the YAV device were well fitted to solutions of the 2-D bio-heat transfer equation for homogeneous tissue. Changes in irradiating beam size (1-8 mm) had no detectable effect on the initial (0.3-3.0 s) phase of the heating kinetics. However, the amplitude of the kinetics decreased substantially with decreasing the beam size. As the temperature rise in the time interval necessary for reliable measurement of the initial temperature rise rate was very small, an accurate experimental determination of specific absorption rate (SAR) becomes practically impossible at the low intensities normally used in our experiments. The correct SAR values may be found from fitting of the model to the heating kinetics. Bioelectromagnetics 24:571-581, 2003.

Model analysis of tissue responses to transient and chronic heating

Thermal models are used to analyze responses of muscle and lung tissue to transient (30-45 min) and chronic (4-7 week) heating in vivo. The general bioheat model, which describes one-dimensional temperature dynamics, incorporates heat conductance and perfusion. In general, perfusion changes with time and distance from a heated surface. One of the main objectives of this study was to analyze long-term perfusion change, which reflects tissue adaptation associated with angiogenesis. The database for these models was obtained using heated disks implanted in calves for up to seven weeks. Tissue temperature distributions were obtained repeatedly from thermistors protruding 1 to 10 mm from the heated disk surface. The perfusion parameter was estimated from the transient experiments at least several times each week by nonlinear, least-squares fitting of the model predicted temperature to the measured temperature response. Chronic heating at a heat flux 0.08 W/cm2 caused perfusion of muscle tissue to increase with postimplant day (PID). Under the same conditions, lung tissue perfusion increased with chronic heating from early to late PID, but less than that for muscle tissue. During chronic heating above 42 degrees C and below 50 degrees C, a decrease in tissue temperature is associated with higher perfusion that develops with time. Over seven weeks, perfusion of muscle tissue near the heated disk surface increased by about 70% at 0.08 W/cm2 and 40% at 0.06 W/cm2. Furthermore, the model can be used to predict tissue and perfusion changes continuously over weeks for heat fluxes around 0.08 W/cm2.

Skin temperature changes induced by strong static magnetic field exposure

High intensity static magnetic fields, when applied to the whole body of the anesthetized rat, have previously been reported to decrease skin temperature. The hypothesis of the present study was that in diamagnetic water, molecules in the air play significant roles in the mechanism of skin temperature decrease. We used a horizontal cylindrical superconducting magnet. The magnet produced 8 T at its center. A thermistor probe was inserted in a subcutaneous pocket of the anesthetized rats to measure skin temperature. Animals (n=10) were placed in an open plastic holder in which the ambient air was free to move in any direction (group I). Animals (n=10) were placed in a closed holder in which the air circulation toward the direction of weak magnetic field was restricted (group II). Each holder was connected to a hydrometer to measure humidity around the animal in the holder. The data acquisition phase consisted of a 5 min baseline interval, followed by inserting the animal together with the holder into the center of the magnet bore for a 5 min exposure and a 5 min postexposure period outside the bore. In group I, skin temperature and humidity around the animal significantly decreased during exposure, followed by recovery after exposure. In group II, skin temperature and humidity did not decrease during the measurement. The skin temperature decrease was closely related to the decrease in humidity around the body of the animal in the holder, and the changes were completely blocked by restricting the air circulation in the direction of the bore entrance. Possible mechanisms responsible for the decrease in skin temperature may be associated with magnetically induced movement of water vapor at the skin surface, leading to skin temperature decrease.

Thermal heating of human tissue induced by electromagnetic fields of magnetic resonance imaging

The paper presents a simulation of the transient temperature distribution in the human body caused by induced eddy currents during magnetic resonance imaging (MRI). In a first simulation the validity of the used heat conduction equation was proven using a simple example of a cool-down-process of a sphere. Thereafter the heating of a phantom model with an implanted electrode placed in a MRI-System (active body coil) was examined. The resulting increase in temperature was compared with existing measurements. Finally the implications of the heating of the tissue are discussed based on the observed experimental and numerical results.

Specific absorption rate and temperature elevation in a subject exposed in the far-field of radio-frequency sources operating in the 10-900-MHz range

The exposure of a subject in the far field of radiofrequency sources operating in the 10-900-MHz range has been studied. The electromagnetic field inside an anatomical heterogeneous model of the human body has been computed by using the finite-difference time-domain method; the corresponding temperature increase has been evaluated through an explicit finite-difference formulation of the bio-heat equation. The thermal model used, which takes into account the thermoregulatory system of the human body, has been validated through a comparison with experimental data. The results show that the peak specific absorption rate (SAR) as averaged over 10 g has about a 25-fold increase in the trunk and a 50-fold increase in the limbs with respect to the whole body averaged SAR (SARWB). The peak SAR as averaged over 1 g, instead, has a 30- to 60-fold increase in the trunk, and up to 135-fold increase in the ankles, with respect to SARWB. With reference to temperature increases, at the body resonance frequency of 40 MHz, for the ICNIRP incident power density maximum permissible value, a temperature increase of about 0.7 degrees C is obtained in the ankles muscle. The presence of the thermoregulatory system strongly limits temperature elevations, particularly in the body core.

[Effects of microwave radiation on conditioned behavior of rats]

Research has been carried out to investigate the effects of microwave exposure (7 GHz, surface energy density 10-50 mW/cm2, SAR 2.1-10.5 W/kg) on learned behaviors of rats in the paradigm of conditioned avoidance reflex. It was shown that transitory reductions in conditioned behavior after acute microwave exposure occurred at an SAR equal to the intensity of rat basal metabolism. It was found cumulative effects for intermittent exposures of rats at a power density of 10 mW/cm2.

[Thermocompensatory reactions of rabbits in response to microwave irradiation at different environmental temperatures]

Thermogenic effectiveness of electromagnetic irradiation (EMI) of UHF range (7 GHz) in the dependence on intensity (10-100 mW/cm2) and environmental temperature was studied in experiments with rabbits. Synergistic interaction of EMI and high ambient temperature was established. The existence of optimal EMI and high ambient temperature was established. The existence of optimal EMI intensity at which the synergy was maximal was shown. It is concluded that this interaction should be taken into account for hygienic standardization of nonionizing EMI.

Human exposure to 2450 MHz CW energy at levels outside the IEEE C95.1 standard does not increase core temperature

Permission was received from the Brooks AFB Institutional Review Board and the AF Surgeon General's Office to exceed the peak power density (PD = 35 mW/cm(2)) we had previously studied during partial body exposure of human volunteers at 2450 MHz. Two additional peak PD were tested (50 and 70 mW/cm(2)). The higher of these PD (normalized peak local SAR = 15.4 W/kg) is well outside the IEEE C95.1 guidelines for partial body exposure, as is the estimated whole body SAR approximately 1.0 W/kg. Seven volunteers (four males, three females) were tested at each PD in three ambient temperatures (T(a) = 24, 28, and 31 degrees C) under our standard protocol (30 min baseline, 45 min RF exposure, 10 min baseline). The thermophysiological data (esophageal and six skin temperatures, metabolic heat production, local sweat rate, and local skin blood flow) were combined with comparable data at PD = 0, 27, and 35 mW/cm(2) from our 1999 study to generate response functions across PD. No change in esophageal temperature or metabolic heat production was recorded at any PD in any T(a). At PD = 70 mW/cm(2), skin temperature on the upper back (irradiated directly) increased 4.0 degrees C in T(a) = 24 degrees C, 2.6 degrees C in T(a) = 28 degrees C, and 1.8 degrees C in T(a) = 31 degrees C. These differences were primarily due to the increase in local sweat rate, which was greatest in T(a) = 31 degrees C. Also at PD = 70 mW/cm(2), local skin blood flow on the back increased 65% over baseline levels in T(a) = 31 degrees C, but only 40% in T(a) = 24 degrees C. Although T(a) becomes an important variable when RF exposure exceeds the C95.1 partial body exposure limits, vigorous heat loss responses of blood flow and sweating maintain thermal homeostasis efficiently. It is also clear that strong sensations of heat and thermal discomfort will motivate a timely retreat from a strong RF field, long before these physiological responses are exhausted. Published 2001 Wiley-Liss, Inc.

Partial-body exposure of human volunteers to 2450 MHz pulsed or CW fields provokes similar thermoregulatory responses

Many reports describe data showing that continuous wave (CW) and pulsed (PW) radiofrequency (RF) fields, at the same frequency and average power density (PD), yield similar response changes in the exposed organism. During whole-body exposure of squirrel monkeys at 2450 MHz CW and PW fields, heat production and heat loss responses were nearly identical. To explore this question in humans, we exposed two different groups of volunteers to 2450 MHz CW (two females, five males) and PW (65 micros pulse width, 10(4) pps; three females, three males) RF fields. We measured thermophysiological responses of heat production and heat loss (esophageal and six skin temperatures, metabolic heat production, local skin blood flow, and local sweat rate) under a standardized protocol (30 min baseline, 45 min RF or sham exposure, 10 min baseline), conducted in three ambient temperatures (T(a) = 24, 28, and 31 degrees C). At each T(a), average PDs studied were 0, 27, and 35 mW/cm2 (Specific absorption rate (SAR) = 0, 5.94, and 7.7 W/kg). Mean data for each group showed minimal changes in core temperature and metabolic heat production for all test conditions and no reliable differences between CW and PW exposure. Local skin temperatures showed similar trends for CW and PW exposure that were PD-dependent; only the skin temperature of the upper back (facing the antenna) showed a reliably greater increase (P =.005) during PW exposure than during CW exposure. Local sweat rate and skin blood flow were both T(a)- and PD-dependent and showed greater variability than other measures between CW and PW exposures; this variability was attributable primarily to the characteristics of the two subject groups. With one noted exception, no clear evidence for a differential response to CW and PW fields was found.

Effects of solar radiation and feeding time on behavior, immune response and production of lactating ewes under high ambient temperature

A 6-wk trial was performed with 40 late-lactation Comisana ewes, which were either exposed to or protected from solar radiation and fed either in the morning (EXPM, PROM) or afternoon (EXPA, PROA) during summer in a Mediterranean climate. Behavioral traits of ewes were recorded once per week from 0800 to 2000 h. Rectal temperature (RT) and respiration rate (RR) were measured twice weekly at 1430 h. The phytohemagglutinin (PHA) skin test was performed to induce nonspecific delayed-type hypersensitivity at d 10, 20, and 32 of the experiment. Jugular blood samples were taken from ewes at the beginning and at d 21 and 42 of the experiment. Ewe milk yield was recorded daily. Individual milk samples were analyzed weekly for milk composition, coagulating properties, somatic cell count (SCC) and polymorphonuclear neutrophil leukocyte counts (PMNLC) and every 2 wk for bacteriological characteristics. Solar radiation and the interaction of solar radiation x time of feeding had significant effects on rectal temperatures. EXPM ewes had higher rectal temperatures than EXPA ewes, which in turn exhibited higher RT compared with PROM and PROA ewes. EXP groups also had significantly higher respiration rates than PRO groups. Immune response was lower in EXPM ewes at d 10 and in EXPM, EXPA, and PROM animals at d 20 compared with PROA ewes. Exposure to solar radiation resulted in decreased plasma concentrations of alanine amino-transferase, alkaline phosphatase, potassium, and magnesium, as well as in increased levels of nonesterified fatty acids and aspartate amino-transferase. Milk yield and composition were not changed by exposure to solar radiation and time of feeding, but the EXPM treatment resulted in lower yields of casein and fat and reduced clot firmness compared with the three other treatments. Milk SCC was similar across treatments, but PMNLC was higher in EXPM than in PROM and PROA milk. EXPM animals also had the greatest amounts of total and fecal coliforms and of Pseudomonadaceae as well as the highest number of mastitis related pathogens in their milk. Results suggest that provision of shaded areas can play a major role in helping lactating ewes to minimize the adverse effects of high ambient temperatures on thermal balance and energy and mineral metabolism. Changing the time of feeding to late afternoon may be beneficial to exposed ewes in lowering their heat loads during the warmest hours of the day, thereby reducing the detrimental impact of thermal stress on immune function and udder health.

[Experimental estimation of thermogenic levels of acute microwave exposure for different animal species]

In experiments on laboratory animals it was found the different patterns of reactions in response to acute thermogenic 7 GHz CW microwave exposures. The effects were related specifically to the intensity of the field. SAR equal 0.5-0.7 power of basal metabolism of different species did not result in increase of body temperature. At SARs over intensity of basal metabolism (up to 1.5-2.0 basal metabolism for the mice and for the rats and up to 1.5 for the rabbits) the "stepped" pattern of body heating with periods of rectal temperature stabilization was observed. This results may have implications for prognostic estimation of dose-temporal limits of endurable intensities of microwaves.

Body heating induced by sub-resonant (350 MHz) microwave irradiation: cardiovascular and respiratory responses in anesthetized rats

These experiments were designed to investigate the effects of sub-resonant microwave (MW) exposure (350 MHz, E orientation, average power density 38 mW/cm2, average whole-body specific absorption rate 13.2 W/kg) on selected physiological parameters. The increase in peripheral body temperature during 350 MHz exposure was greater than that in earlier experiments performed at 700 MHz (resonance). Heart rate and mean arterial blood pressure were significantly elevated during a 1 degree C increase in colonic temperature due to 350 MHz exposure; respiratory rate showed no significant change. The results are consistent with other investigators' reports comparing sub-resonance exposures with those at resonance and above.

A thermal model for human thresholds of microwave-evoked warmth sensations

Human thresholds for skin sensations of warmth were measured at frequencies from 2.45 to 94 GHz. By solving the one-dimensional bioheat equation, we calculated the temperature increase at the skin surface or at a depth of 175 microm at incident power levels corresponding to the observed thresholds. The thermal analysis suggests that the thresholds correspond to a localized temperature increase of about 0.07 degrees C at and near the surface of the skin. We also found that, even at the highest frequency of irradiation, the depth at which the temperature receptors are located is not a relevant parameter, as long as it is within 0.3 mm of the surface. Over the time range of the simulation, the results of the thermal model are insensitive to blood flow, but sensitive to thermal conduction; and this sensitivity increases strongly with frequency. We conclude with an analysis of the effect of thermal conduction on surface temperature rise, which becomes a dominant factor at microwave frequencies over 10 GHz.

Regional brain heating during microwave exposure (2.06 GHz), warm-water immersion, environmental heating and exercise

Nonuniform heating may result from microwave (MW) irradiation of tissues and is therefore important to investigate in terms of health and safety issues. Hypothalamic (Thyp), cortical (Tctx), tympanic (Tty), and rectal (Tre) temperatures were measured in rats exposed in the far field, k-polarization (i.e., head pointed toward the transmitter horn and E-field in vertical direction) to two power densities of 2.06 GHz irradiation. The high-power density (HPM) was 1700 mW/cm2 [specific absorption rate (SAR): hypothalamus 1224 W/kg; cortex 493 W/kg]; the low-power density (LPM) was 170 mW/cm2 (SAR: hypothalamus 122.4 W/kg; cortex 49.3 W/kg). The increase (rate-of-rise, in degrees C/s) in Thyp was significantly greater than those in Tctx or Tre when rats were exposed to HPM. LPM produced more homogeneous heating. Quantitatively similar results were observed whether rats were implanted with probes in two brain sites or a single probe in one or the other of the two sites. The qualitative difference between regional brain heating was maintained during unrestrained exposure to HPM in the h-polarization (i.e., body parallel to magnetic field). To compare the temperature changes during MW irradiation with those produced by other modalities of heating, rats were immersed in warm water (44 degrees C, WWI); exposed to a warm ambient environment (50 degrees C, WSED); or exercised on a treadmill (17 m/min 8% grade) in a warm ambient environment (35 degrees C, WEX). WWI produced uniform heating in the regions measured. Similar rates-of-rise occurred among regions following WSED or WEX, thus maintaining the pre-existing gradient between Thyp and Tctx These data indicate that HPM produced a 2-2.5-fold difference in the rate-of-heating within brain regions that were separated by only a few millimeters. In contrast, more homogeneous heating was recorded during LPM or nonmicrowave modalities of heating.

Light-dark differences in the effects of ambient temperature on gaseous metabolism in newborn rats

Body temperature (T(b)) of rat pups (7-9 days old) raised under a 12:12-h light-dark (L-D) regimen (L: 0700-1900, D: 1900-0700) was consistently higher in D than in L by approximately 1.1 degrees C. We tested the hypothesis that the L-D differences in T(b) were accompanied by differences in the set point of thermoregulation. Measurements were performed on rat pups at 7-9 days after birth. O(2) consumption (VO(2)) and CO(2) production (VCO(2)) were measured with an open-flow method during air breathing, as ambient temperature (T(a)) was decreased from 40 to 15 degrees C at the constant rate of 0.5 degrees C/min. At T(a) >/=33 degrees C, VO(2) was not significantly different between L and D, whereas VCO(2) was higher in L, suggesting a greater ventilation. Over the 33 to 15 degrees C range the VO(2) values in D exceeded those in L by approximately 30%. Specifically, the difference was contributed by differences in thermogenesis at T(a) = 30 to 20 degrees C. As T(a) was decreased, the critical temperature at which VO(2) began to rise was lower in L. We conclude that the higher T(b) of rat pups in D is accompanied by a higher set point for thermoregulation and a greater thermogenesis. These results are consistent with the idea that, in newborns, endogenous changes in the set point of thermoregulation contribute to the circadian oscillations of T(b).

Human exposure at two radio frequencies (450 and 2450 MHz): similarities and differences in physiological response

Thermoregulatory responses of heat production and heat loss were measured in two different groups of seven adult volunteers (males and females) during 45-min dorsal exposures of the whole body to 450 or 2450 MHz continuous-wave radio frequency (RF) fields. At each frequency, two power densities (PD) were tested at each of three ambient temperatures (T(a) = 24, 28, and 31 degrees C) plus T(a) controls (no RF). The normalized peak surface specific absorption rate (SAR), measured at the location of the subject's center back, was the same for comparable PD at both frequencies, i.e., peak surface SAR = 6.0 and 7.7 W/kg. No change in metabolic heat production occurred under any exposure conditions at either frequency. The magnitude of increase in those skin temperatures under direct irradiation was directly related to frequency, but local sweating rates on back and chest were related more to T(a) and SAR. Both efficient sweating and increased local skin blood flow contributed to the regulation of the deep body (esophageal) temperature to within 0.1 degrees C of the baseline level. At both frequencies, normalized peak SARs in excess of ANSI/IEEE C95.1 guidelines were easily counteracted by normal thermophysiological mechanisms. The observed frequency-related response differences agree with classical data concerning the control of heat loss mechanisms in human beings. However, more practical dosimetry than is currently available will be necessary to evaluate realistic human exposures to RF energy in the natural environment.

Light masking of circadian rhythms of heat production, heat loss, and body temperature in squirrel monkeys

Whole body heat production (HP) and heat loss (HL) were examined to determine their relative contributions to light masking of the circadian rhythm in body temperature (Tb). Squirrel monkey metabolism (n = 6) was monitored by both indirect and direct calorimetry, with telemetered measurement of body temperature and activity. Feeding was also measured. Responses to an entraining light-dark (LD) cycle (LD 12:12) and a masking LD cycle (LD 2:2) were compared. HP and HL contributed to both the daily rhythm and the masking changes in Tb. All variables showed phase-dependent masking responses. Masking transients at L or D transitions were generally greater during subjective day; however, L masking resulted in sustained elevation of Tb, HP, and HL during subjective night. Parallel, apparently compensatory, changes of HL and HP suggest action by both the circadian timing system and light masking on Tb set point. Furthermore, transient HL increases during subjective night suggest that gain change may supplement set point regulation of Tb.

Effects of different light intensities during the daytime on circadian rhythm of core temperature in humans

The present study was to determine the effects of bright light exposure during the daytime on core temperature rhythm. Eight female students participated as subjects. They were exposed to bright light of 5000 lux or dim light of 200 lux for 13 hours (06:30-19:30) for two consecutive days. Except the sleep period (22:30-06:30) and during the bright light exposure, light intensity was controlled at 200 lux. Rectal temperature data were collected every 10 min throughout the whole experimental period. The mean level of rectal temperature was not significantly different between the bright and the dim light conditions. However, the evening fall and the morning rise of rectal temperature were significantly greater in the bright light conditions on Day 2 compared to the dim. Furthermore, cosinor analysis showed that the acrophase of rectal temperature rhythm was earlier on Day 2 in the bright light conditions than the dim, and was significantly delayed on Day 2 compared to Day 1 in the dim light conditions. These results suggest that low intensity during the daytime for two consecutive days may induce a phase delay of core temperature rhythm rather than the bright light exposure at least in normally entrained female subjects.

Effects of solar radiation, dietary energy, and time of feeding on thermoregulatory responses and energy balance in cattle in a hot environment

Ten growing heifers were either exposed to or protected from solar radiation, offered a diet of either high (H) or low (L) ME, and fed either in the morning or afternoon during a hot summer. Heifers that consumed the H diet had a greater water intake, DMI, metabolizable energy intake, energy expenditure, and retained energy than heifers that consumed the L diet. Solar radiation did not have an effect on any of these variables. Furthermore, dietary energy and time of measurement had an effect on rectal temperature (Tr), respiration rate (RR), heart rate (HR), and rate of oxygen uptake (VO2); solar radiation had an effect on Tr and RR but not on HR and VO2; and time of feeding had an effect only on VO2. Heifers coped with greater heat loads by increasing RR and the difference in Tr between morning and afternoon. It seems that a lowered body temperature in the morning is a physiological mechanism used by animals to prepare for the heat load that develops during the day. Heat production (HP) and HR throughout the day were affected mainly by the time of feeding and not by the environmental heat load. Feeding in the afternoon increased HP in the cooler hours of the day when heat losses from the animal through conduction and radiation were more efficient. With a pending high heat load situation, reducing feed quality and(or) changing the time of feeding to the late afternoon could be beneficial to the animals in reducing their heat loads.

[Dynamics of the changes in various integral indices of the functional state of the body of white rats during the prolonged action of x-radiation and external heat]

Effects of low doses of X-radiation (0.02 Gr. a session, for 25 days), external heat (daily 4-h exposure in a thermochamber at 37.0 degrees C) and combined (consecutive) action of these factors on mature Wistar male rats were studied. Experimental animals were somewhat behind in body mass increment during exposure to the physical factors as compared to control rats. Radiation promoted a rise in rectal temperature, external heating to its small decrease, while combined exposure to the factors levelled the effects. There were certain changes in the CNS, as indicated by a small increase in the summation-threshold index. The heart rate did not significantly change. The changes normalized within 1-5 weeks after cessation of the exposures.

Do metabolic responses to solar radiation scale directly with intensity of irradiance?

Endotherms exposed to air temperatures below thermal neutrality reduce their metabolic heat production when exposed to sunlight. The physiological effects of this additional source of heat gain from the environment usually are assumed to be proportional to the intensity of irradiance if other factors are held constant. We test this assumption by measuring changes in metabolic heat production produced by exposing a small mammal, the Siberian hamster (Phodopus sungorus) to four intensities of simulated solar radiation (0 W m-2, 317 W m-2, 634 W m-2 and 950 W m-2). In the absence of solar radiation, metabolic heat production is inversely correlated with air temperature over the measured range of 3-27 degrees C. The respiratory quotient varies significantly with ambient temperature, indicating that the catabolic substrate and the thermal equivalent of oxygen consumed or carbon dioxide produced also vary with temperature. The depression of metabolic heat production resulting from exposure to simulated solar radiation is not simply a multiple of the intensity of irradiance. Rather, metabolic responses to higher levels of irradiance are blunted by 14-29% compared with those expected on the basis of the response to less intense irradiance. Because changes in irradiance levels do not have simple linear effects upon the animal's metabolic heat production, even in a simplified situation, significant errors may accumulate in biophysical analyses in which an animal's responses to a restricted set of radiative conditions are measured and the results are extrapolated to a wider range observed in nature.

Age does not affect thermal and cardiorespiratory responses to microwave heating in calorically restricted rats

This study sought to determine whether age influences the thermal distribution and cardiorespiratory responses to 35 GHz microwave (MW) heating. Male Sprague-Dawley rats (n = 8/group) 3 to 4 mo old (young), 15 to 16 mo old (middle-aged), and 24 to 25 mo old (old) were used. All animals were restricted to 63% of ad libitum feed. Rats were anesthetized (ketamine) and a catheter was placed into a carotid artery for measurement of mean arterial blood pressure (MAP). Anesthetized rats were then exposed to MWs until death, while measuring MAP, heart rate and temperatures at five sites (colonic, left and right tympanic, subcutaneous, and tail). Before MW exposure, there were no significant (p < .05) differences among age groups in measured parameters. MW produced increases in temperature at each of the measured sites; temperatures at death did not differ between groups. Heart rate increased throughout MW exposure, in a statistically identical manner in all age groups. MAP was initially well maintained and then, in the latter phases of heating, precipitously declined until death, with no discernible age-related difference. Respiration rate was not altered by MW exposure in any group. Finally, there were no group differences in the MW exposure time required to induce death. Thus, age does not alter thermal and cardiorespiratory responses to 35 GHz MW heating in food-restricted rats.