Eleanor R. Adair

Heating and pain sensation produced in human skin by millimeter waves: comparison to a simple thermal model.

Cutaneous thresholds for thermal pain were measured in 10 human subjects during 3-s exposures at 94 GHz continuous wave microwave energy at intensities up to approximately 1.8 W cm(-2). During each exposure, the temperature increase at the skins surface was measured by infrared thermography. The mean (+/- s.e.m.) baseline temperature of the skin was 34.0+/-0.2 degrees C. The threshold for pricking pain was 43.9+/-0.7 degrees C, which corresponded to an increase in surface temperature of approximately 9.9 degrees C (from 34.0 degrees C to 43.9 degrees C). The measured increases in surface temperature were in good agreement with a simple thermal model that accounted for heat conduction and for the penetration depth of the microwave energy into tissue. Taken together, these results support the use of the model for predicting thresholds of thermal pain at other millimeter wave (length) frequencies.

Thresholds of microwave-evoked warmth sensations in human skin.

We measured thresholds for microwave-evoked skin sensations of warmth at frequencies of 2.45, 7.5, 10, 35, and 94 GHz. In the same subjects, thresholds of warmth evoked by infrared radiation (IR) were also measured for comparison. Detection thresholds were measured on the skin in the middle of the back in 15 adult male human subjects at all microwave (MW) frequencies and with IR. Long duration (10-s), large area (327-cm2) stimuli were used to minimize any differential effects of temporal or spatial summation. Sensitivity increased monotonically with frequency throughout the range of microwave frequencies tested. The threshold at 94 GHz (4.5 +/- 0.6 mW/cm2) was more than an order of magnitude less than at 2.45 GHz (63.1 +/- 6.7 mW/cm2), and it was comparable to the threshold for IR (5.34 +/- 1.07 mW/cm2).

Displacements of rectal temperature modify behavioral thermoregulation

Abstract During behavioral thermoregulation, the temperature of the body core rises (or falls) in response to cooling (or warming) the medial preoptic area of the hypothalamus. Brief thermal displacements of the deep viscera of squirrel monkeys, accomplished via a rectal thermode, stimulate behavioral alterations of skin temperature in similar fashion to comparable hypothalamic thermal displacements, but only 1 3 as effectively. Prolonged rectal warming (or cooling), superimposed upon hypothalamic cooling, modifies behavioral thermoregulation by lowering (or raising) the skin temperature displacement necessary for the monkeys to achieve thermal balance. The behavioral response thus depends upon whether the imposed rectal temperature opposes or supports the normal physiological response to hypothalamic cooling. On the other hand, hypothalamic warming appears to exert such strong control over mechanisms for lowering body temperature that superimposed rectal temperature displacements have no additional effect upon the behavioral response.

Radio frequency electromagnetic fields: mild hyperthermia and safety standards.

This chapter is a short review of literature that serves as the basis for current safe exposure recommendations by ICNIRP (International Commission on Non-Ionizing Radiation Protection, 1998). and the IEEE C95.1 (IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz, 2005) for exposure to radio frequency electromagnetic radiation (RF-EMF). Covered here are topics on dosimetry, thermoregulatory responses, behavioral responses, and how these have been used to derive safe exposure limits for humans to RF-EMF. Energy in this portion of the electromagnetic spectrum, 3 kHz-300 GHz, can be uniquely absorbed and is different from ionizing radiation both in dosimetry and effects. The deposition of thermalizing energy deep in the body by exposure to RF-EMF fields provides a unique exception to the energy flows normally encountered by humans. Behavioral effects of RF-EMF exposure range from detection to complete cessation of trained behaviors. RF-EMF is detectable and can in most cases, presumably by thermal mechanisms, support aversion and disruption or complete cessation (work stoppage) of behavior. Safety standards are based on behavioral responses by laboratory animals to RF-EMF, enhanced by careful studies of human thermoregulatory responses at four specific RF frequencies, thereby providing a conservative level of protection from RF-EMF for humans.

Behavioral thermoregulation in the squirrel monkey: Adaptation processes during prolonged microwave exposure.

During 10-min exposures to 2450-MHz microwaves at a power density of 6-8 m W/cm2, squirrel monkeys reliably select a cooler environment. Exposure duration, at power densities above and below this threshold, was the parameter investigated in these experiments. Monkeys were restrained in the far field of a horn antenna inside a 1.8 x 1.8 x 2.5 m anechoic chamber which was heated and cooled by forced convection. The animals learned to control the temperature of the circulating chamber air by selecting between cold (10-15 degrees C) and warm (50-55 degrees C) air sources. During the experiments, they were exposed to 12.4-cm (2450-MHz) continuous-wave microwaves for periods from 5 to 150 min. Microwave power densities explored were 4, 10, and 20 mW/cm2 which represent rates of whole-body energy absorption that range from approximately, .6 to 3.0 W/kg. No microwaves were present during 4-hr control experiments. The 4 mW/cm2 microwave exposure did not modify thermoregulatory behavior, no matter how long it lasted. The 10 and 20 mW/cm2 exposures stimulated the monkeys to select ambient temperatures 1.5 and 3.0 degrees C cooler than control levels, respectively. Except during the first microwave presentation of a series, or during the early minutes of a single long exposure, duration had no significant effect on selection of air temperature or on the body temperatures achieved thereby.

Control of thermoregulatory behavior by brief displacements of hypothalamic temperature

When local hypothalamic temperature is changed, squirrel monkeys behaviorally change their environmental temperature in the opposite direction by a proportional amount. Skin and deep body temperatures shift in the same direction the environment does. Several pairs of offered air temperatures, which the monkeys must alternate to select their environment, yield similar functional relations between the behavioral response and hypothalamic temperature.

Physiological and Perceptual Responses of Human Volunteers during Whole-Body RF Exposure at 450 MHz

The razionale behind all investigations of radio frequency (RF) bioeffects, whether in vivo or in vitro, ultimately concerns the effects of such exposure on human beings. Voluminous animal bioeffects data have been collected over the last 40–50 years. However, controlled laboratory investigations of human responses to such fields are meager and tend to be limited to sensory endpoints. We have initiated a series of studies to obtain accurate knowledge of human thermoregulatory efficiency in the RF environment; these data are designed to provide generalizable functional relationships that can serve as the basis for human safety guidelines. The functions will also be invaluable to the refinement of computer models designed to predict human thermoregulatory responses in the presence of RF energy and aid the extrapolation of animal data to the human condition. As adjuncts to the physiological data, we have also developed a set of criteria by which R.F-exposed humans may rate their perceptual impressions of their thermal environment and their physiological responses. The perceptual data are featured in this report.

Thermoregulation in the Squirrel Monkey

The squirrel monkey is an endotherm, an organism capable of maintaining a stable internal body temperature in the face of rather wide fluctuations in the temperature of the environment. Thermoregulation in endotherms is accomplished by fine adjustments in appropriate autonomic response systems, by which the body gains or loses heat, acting in concert with a wide range of behavioral maneuvers that provide a hospitable microclimate for the animal. Whenever possible, the behaviorally generated microclimate is thermally neutral, a situation that maximizes the economy of energy stores and water in the body, and minimizes the involvement of autonomic mechanisms. Thus, the description of thermoregulation in any endotherm involves detailed knowledge of thermoregulatory behavior, both instinctive and learned, and of individual autonomic processes of heat production and heat loss. As we shall see, the particular autonomic response that may be operative at a given time is dictated by the environmental temperature: i.e., squirrel monkeys shiver in the cold and sweat in the heat, but not the reverse, and they will avoid doing either one if an efficient behavioral maneuver is available to them.

Thermal Physiology of Radiofrequency Radiation (RFR) Interactions in Animals and Humans

In his 1988 review of the biological effects of radiofrequency (RF) fields, Erwin1 counseled his readers that “... Protecting humans from the real hazards and allaying groundless fears requires a self-consistent body of scientific data concerning the effects of the fields, levels of exposures which cause those effects, and which effects are deleterious (or beneficial or neutral). With that knowledge, appropriate guidelines for safety can be devised, while preserving the beneficial uses of radiofrequency radiation (RFR) energy for military or civilian purposes.” Erwin’s good counsel forms the framework for the present critical review of recent data on the thermal physiology of RFR interactions in animals and humans.

Thermoregulatory Responses of Febrile Monkeys During Microwave Exposure.

Abstract : We have examined experimentally the question of increased vulnerability to the thermalizing effects of MW exposure during febrile illness. In a controlled ambient temperature of 26 degrees C, autonomic mechanisms of heat production and heat loss were measured in febrile squirrel monkeys during 30-min exposures to 450 or 2450 MHz CW MW fields at different phases of the fever cycle (induction, plateau, defervescence). We have shown that MW energy absorbed during a febrile episode spares endogenous energy production, but may augment the fever if deposited deep in the body, as is the case during exposure at the resonant frequency. The fever may also be exacerbated if the MW exposure occurs late in the febrile episode, a condition that may put an organism at some risk, especially if the field strength exceeds safety guidelines.