Patrick A. Mason

Effects of frequency, permittivity, and voxel size on predicted specific absorption rate values in biological tissue during electromagnetic-field exposure

Current electromagnetic-field (EMF) exposure limits have been based, in part, on the amount of energy absorbed by the whole body. However, it is known that energy is absorbed nonuniformly during EMF exposure. The development and widespread use of sophisticated three-dimensional anatomical models to calculate specific-absorption-rate (SAR) values in biological material has resulted in the need to understand how model parameters affect predicted SAR values. This paper demonstrate the effects of manipulating frequency, permittivity values, and voxel size on SAR values calculated by a finite-difference time-domain program in digital homogenous sphere models and heterogeneous models of rat and man. The predicted SAR values are compared to empirical data from infrared thermography and implanted temperature probes.

Skin heating effects of millimeter-wave irradiation-thermal modeling results

Millimeter microwaves (MMWs) are a subset of RF in the 30-300-GHz range, The proliferation of devices that operate in the MMW range has been accompanied by increased concern about their safety. As MMW irradiation has a very shallow penetration in tissue, the specific absorption rate is not a relevant parameter for dosimetry purposes. A thermal modeling program was used to investigate the tissue heating effects of MMW irradiation (100 GHz nominal) on the primate head. The objectives were to determine the extent to which the surface and subsurface tissue temperatures depend on applied energy density and the effects of blood flow and surface cooling on tissue temperatures. Two power ranges were considered: short-duration exposure to high power microwaves (HPMs), with power densities of 1.0, 1.5, 2.0, 2.5, or 3.0 W cm/sup -2/ for 3 s, and longer duration exposure to low-power microwaves (LPMs), with power densities of 0.1, 0.15, 0.2, 0.25, 0.3 W cm/sup -2/ for 30 s. The applied energies were comparable for both HPM and LPM exposures. The authors found both surface and subsurface temperatures increase as the energy level increases, with HPMs having a higher peak temperature than the LPMs for similar exposure energy densities. The surface temperature increase is linear with energy density for the HPMs, except under combined conditions of high blood flow (blood-flow rate of 8/spl times/10/sup -3/ g s/sup -1/ cm/sup -3/) and high-energy density (greater than 7.5 J cm/sup -2/), The LPM surface temperatures are not linear with incident energy. The peak surface temperature is affected by environmental conditions (convection coefficient, sweat rate). The magnitude of the temperature increase due to MMW exposure did not change with environmental conditions. The subsurface temperature increases are considerably damped, compared to the surface temperatures.

Radio frequency radiation of millimeter wave length : Potential occupational safety issues relating to surface heating

Currently, technology is being developed that makes use of the millimeter wave (MMW) range (30-300 GHz) of the radio frequency region of the electromagnetic spectrum. As more and more systems come on line and are used in everyday applications, the possibility of inadvertent exposure of personnel to MMWs increases. To date, there has been no published discussion regarding the health effects of MMWs; this review attempts to fill that void. Because of the shallow depth of penetration, the energy and, therefore, heat associated with MMWs will be deposited within the first 1-2 mm of human skin. MMWs have been used in states of the former Soviet Union to provide therapeutic benefit in a number of diverse disease states, including skin disorders, gastric ulcers, heart disease and cancer. Conversely, the possibility exists that hazards might be associated with accidental overexposure to MMWs. This review attempts to critically analyze the likelihood of such acute effects as burn and eye damage, as well as potential long-term effects, including cancer.

Radiofrequency-Radiation Exposure Does Not Induce Detectable Leakage of Albumin Across the Blood-Brain Barrier

Abstract McQuade, J. M., Merritt, J. H., Miller, S. A., Scholin, T., Cook, M. C., Salazar, A., Rahimi, O. B., Murphy, M. R. and Mason, P. A. Radiofrequency-Radiation Exposure Does Not Induce Detectable Leakage of Albumin Across the Blood-Brain Barrier. Radiat. Res. 171, 615–621 (2009). The blood-brain barrier (BBB) consists of tight junctions between the endothelial cells that line the capillaries in the central nervous system. This structure protects the brain, and neurological damage could occur if it is compromised. Several publications by researchers at Lund University have reported alterations in the BBB after exposure to low-power 915 MHz energy. These publications increased the level of concern regarding the safety of wireless communication devices such as mobile phones. We performed a confirmation study designed to determine whether the BBB is altered in rats exposed in a transverse electromagnetic (TEM) transmission line cell to 915 MHz energy at parameters similar to those in the Lund University studies. Unanesthetized rats were exposed for 30 min to either continuous-wave or modulated (16 or 217 Hz) 915 MHz energy at power levels resulting in whole-body specific absorption rates (SARs) of 0.0018–20 W/kg. Albumin immunohistochemistry was performed on perfused brain tissue sections to determine the integrity of the BBB. Chi-square analysis revealed no significant increase in albumin extravasation in any of the exposed animals compared to the sham-exposed or home cage control animals.

Oxidative stress precedes circulatory failure induced by 35-GHz microwave heating.

Sustained whole-body exposure of anesthetized rats to 35-GHz radio frequency radiation produces localized hyperthermia and hypotension, leading to circulatory failure and death. The physiological mechanism underlying the induction of circulatory failure by 35-GHz microwave (MW) heating is currently unknown. We hypothesized that oxidative stress may play a role in the pathophysiology of MW-induced circulatory failure and examined this question by probing organs for 3-nitrotyrosine (3-NT), a marker of oxidative stress. Animals exposed to low durations of MW that increased colonic temperature but were insufficient to produce hypotension showed a 5- to 12-fold increase in 3-NT accumulation in lung, liver, and plasma proteins relative to the levels observed in control rats that were not exposed to MW. 3-NT accumulation in rats exposed to MW of sufficient duration to induce circulatory shock returned to low, baseline levels. Leukocytes obtained from peripheral blood showed significant accumulation of 3-NT only at exposure levels associated with circulatory shock. 3-NT was also found in the villus tips and vasculature of intestine and within the distal tubule of the kidney but not in the irradiated skin of rats with MW-induced circulatory failure. The relationship between accumulation in liver, lung, and plasma proteins and exposure duration suggests either that nitro adducts are formed in the first 20 min of exposure and are then cleared or that synthesis of nitro adducts decreases after the first 20 min of exposure. Taken together, these findings suggest that oxidative stress occurs in many organs during MW heating. Because nitration occurs after microwave exposures that are not associated with circulatory collapse, systemic oxidative stress, as evidenced by tissue accumulation of 3-NT, is not correlated with circulatory failure in this model of shock.

Effects of Blood Flow on Skin Heating Induced by Millimeter Wave Irradiation in Humans

Abstract— We have previously reported species differences in the rate of skin heating in response to millimeter wavelength microwave exposure. We hypothesized that these differences were predominantly a function of species differences in the ability to increase skin blood flow during local heating. Mathematical modeling also suggested that, in humans, the rate of skin heating during prolonged millimeter wavelength exposure would be dependent on skin blood flow. In order to empirically test this hypothesis, we determined the role of baseline skin blood flow on the rate of cutaneous heating induced by 94-GHz microwave energy in humans (3 female, 3 male) using infrared thermography and laser Doppler imaging to measure skin temperature and relative skin blood flow, respectively. Millimeter wavelength exposure intensities used were high power (HP), 1 W·cm−2 for 4 s and low power, 175 mW cm−2 for 180 s. Skin blood flow was (a) normal, (b) eliminated using a blood pressure cuff to occlude forearm blood flow, or (c) elevated by heating the skin prior to irradiation. Results showed that for the HP exposures, these manipulations did not influence the rate of skin heating. For the low power exposures, occlusion of baseline skin blood flow had a small impact on the subsequent rate of heating. In contrast, a two-fold elevation in baseline skin blood flow had a profound impact on the subsequent rate of heating, resulting in a substantially lower rate of heating. Occlusion of an elevated skin blood flow reversed this lower rate of heating. The results of these studies demonstrate that relatively small changes in skin blood flow may produce substantial alterations in the rate of skin heating during prolonged 94-GHz exposure.

Gene Expression Changes in the Skin of Rats Induced by Prolonged 35 GHz Millimeter-Wave Exposure

Abstract Millenbaugh, N. J., Roth, C., Sypniewska, R., Chan, V., Eggers, J. S., Kiel, J. L., Blystone, R. V. and Mason, P. A. Gene Expression Changes in the Skin of Rats Induced by Prolonged 35 GHz Millimeter-Wave Exposure. Radiat. Res. 169, 288–300 (2008). To better understand the cellular and molecular responses to overexposure to millimeter waves, alterations in the gene expression profile and histology of skin after exposure to 35 GHz radiofrequency radiation were investigated. Rats were subjected to sham exposure, to 42°C environmental heat, or to 35 GHz millimeter waves at 75 mW/cm2. Skin samples were collected at 6 and 24 h after exposure for Affymetrix GeneChip analysis. The skin was harvested from a separate group of rats at 3–6 h or 24–48 h after exposure for histopathology analysis. Microscopic findings observed in the dermis of rats exposed to 35 GHz millimeter waves included aggregation of neutrophils in vessels, degeneration of stromal cells, and breakdown of collagen. Changes were detected in 56 genes at 6 h and 58 genes at 24 h in the millimeter-wave-exposed rats. Genes associated with regulation of transcription, protein folding, oxidative stress, immune response, and tissue matrix turnover were affected at both times. At 24 h, more genes related to extracellular matrix structure and chemokine activity were altered. Up-regulation of Hspa1a, Timp1, S100a9, Ccl2 and Angptl4 at 24 h by 35 GHz millimeter-wave exposure was confirmed by real-time RT-PCR. These results obtained from histopathology, microarrays and RT-PCR indicate that prolonged exposure to 35 GHz millimeter waves causes thermally related stress and injury in skin while triggering repair processes involving inflammation and tissue matrix recovery.

Regional distribution of Hsp70 in the CNS of young and old food-restricted rats following hyperthermia

We examined the effect of aging on the capacity of the brain to produce heat shock protein (Hsp70) in response to heat stress, using high-powered microwaves (HPM, 2.06 GHz, 2.2 W/cm(2)) to induce hyperthermia for periods so brief that thermoregulatory factors were functionally eliminated as confounding variables. Unanesthetized young (6 months) and old (25 months) male, food-restricted Sprague-Dawley rats were exposed to HPM to induce a mean peak tympanic temperature (T(ty)) of 42.2 degrees C within 30 s. T(ty) returned to <40.0 degrees C within 6 min post-exposure in both age groups. Rats were euthanized 6 or 24 h later for immunohistochemical determination of Hsp70 accumulation in 10 brain regions. HPM exposure induced significant increases in 7 of the 10 regions. There were no significant differences observed in the pattern or density of Hsp70 accumulation between the young and old rats at 6 h post-HPM exposure, with the exception of the medial vestibular nucleus, which demonstrated significantly greater Hsp70 accumulation in the old rats. There were significant differences between the age groups at 24 h post-exposure, however, there was no general pattern; i.e., depending on the brain region, aged rats displayed significantly greater, lesser, or similar increases in Hsp70 expression compared with young. Taken together, these results demonstrate that the brain of aged, food-restricted rats does not display a loss of capacity to accumulate Hsp70 in response to heat stress.

Inter-species extrapolation of skin heating resulting from millimeter wave irradiation: modeling and experimental results.

Abstract— This study reports measurements of the skin surface temperature elevations during localized irradiation (94 GHz) of three species: rat (irradiated on lower abdomen), rhesus monkey (posterior forelimb), and human (posterior forearm). Two exposure conditions were examined: prolonged, low power density microwaves (LPM) and short-term, high power density microwaves (HPM). Temperature histories were compared with calculations from a bio-heat transfer model. The mean peak surface temperature increase was approximately 7.0°C for the short-term HPM exposures for all three species/locations, and 8.5°C (monkey, human) to 10.5°C (rat) for the longer-duration LPM exposures. The HPM temperature histories are in close agreement with a one-dimensional conduction heat transfer model with negligible blood flow. The LPM temperature histories were compared with calculations from the bio-heat model, evaluated for various (constant) blood flow rates. Results suggest a variable blood flow model, reflecting a dynamic thermoregulatory response, may be more suited to describing skin surface temperature response under long-duration MMW irradiation.

Comparison Of Blood Pressure And Thermal Responses In Rats Exposed To Millimeter Wave Energy Or Environmental Heat

ABSTRACT Electromagnetic fields at millimeter wave lengths are being developed for commercial and military use at power levels that can cause temperature increases in the skin. Previous work suggests that sustained exposure to millimeter waves causes greater heating of skin, leading to faster induction of circulatory failure than exposure to environmental heat (EH). We tested this hypothesis in three separate experiments by comparing temperature changes in skin, subcutis, and colon, and the time to reach circulatory collapse (mean arterial blood pressure, 20 mmHg) in male Sprague-Dawley rats exposed to the following conditions that produced similar rates of body core heating within each experiment: (1) EH at 42°C, 35 GHz at 75 mW/cm2, or 94 GHz at 75 mW/cm2 under ketamine and xylazine anesthesia; (2) EH at 43°C, 35 GHz at 90 mW/cm2, or 94 GHz at 90 mW/cm2 under ketamine and xylazine anesthesia; and (3) EH at 42°C, 35 GHz at 90 mW/cm2, or 94 GHz at 75 mW/cm2 under isoflurane anesthesia. In all three experiments, the rate and amount of temperature increase at the subcutis and skin surface differed significantly in the rank order of 94 GHz more than 35 GHz more than EH. The time to reach circulatory collapse was significantly less only for rats exposed to 94 GHz at 90 mW/cm2, the group with the greatest rate of skin and subcutis heating of all groups in this study, compared with both the 35 GHz at 90 mW/cm2 and the EH at 43°C groups. These data indicate that body core heating is the major determinant of induction of hemodynamic collapse, and the influence of heating of the skin and subcutis becomes significant only when a certain threshold rate of heating of these tissues is exceeded.ABBREVIATIONS-MMW - millimeter wave, EH - environmental heat, MAP - mean arterial pressure, HR - heart rate, TC - colonic temperature, TSQ - subcutaneous temperature, Tsurf - skin surface temperature