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(Database last updated on Sep 16, 2020)

ID Number 1216
Study Type Engineering & Physics
Model Modeling studies to characterize SARs in head of adults and children, RF tissue heating, temperature gradients, microwave hearing, and potential non-thermal bioeffects. A historical appendix by T.S. Ely presents the origin of the 6-min averaging time used in the microwave standard.

Modeling / calculations based upon a dimensionless form of the bioheat equation were used to predict tissue heating during RF exposures. Values were assigned for two constants: (t1) = heat convection by blood flow (~20-30 min for physiologically normal perfusion rates) and (t2) = heat conduction (varies as the square of distance over the heating area). Models included a tissue block with an insulated surface (simulating a large surface area of tissue exposed to RF) and a hemispherical region (simulating local exposure). Using these models, a thermal averaging time was developed and compared to similar temperature rises that would occur from infrared laser exposure within ANSI standard limits. Additional modeling studies look at whether absorbed radiofrequency energy can create a large enough temperature gradient in cells and molecules to affect a biological process. Authors' abstract: Foster et al. 1998 (IEEE #2305 and #1199): We consider the thermal response times for heating of tissue subject to nonionizing (microwave or infrared) radiation. The analysis is based on a dimensionless form of the bioheat equation. The thermal response is governed by two time constants: one (t1) pertains to heat convection by blood flow, and is of the order of 2030 min for physiologically normal perfusion rates; the second (t2) characterizes heat conduction and varies as the square of a distance that characterizes the spatial extent of the heating. Two idealized cases are examined. The first is a tissue block with an insulated surface, subject to irradiation with an exponentially decreasing specific absorption rate, which models a large surface area of tissue exposed to microwaves. The second is a hemispherical region of tissue exposed at a spatially uniform specific absorption rate, which models localized exposure. In both cases, the steady-state temperature increase can be written as the product of the incident power density and an effective time constant teff , which is defined for each geometry as an appropriate function of t1 and t2 . In appropriate limits of the ratio of these time constants, the local temperature rise is dominated by conductive or convective heat transport. Predictions of the block model agree well with recent data for the thresholds for perception of warmth or pain from exposure to microwave energy. Using these concepts, we developed a thermal averaging time that might be used in standards for human exposure to microwave radiation, to limit the temperature rise in tissue from radiation by pulsed sources. We compare the ANSI exposure standards for microwaves and infrared laser radiation with respect to the maximal increase in tissue temperature that would be allowed at the maximal permissible exposures. A historical appendix presents the origin of the 6-min averaging time used in the microwave standard. AUTHORS' ABSTRACT: Foster et al. 2016 (6568): 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 (30300 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 0.51 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 12 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.

Findings Not Applicable to Bioeffects
Status Completed With Publication
Principal Investigator Univ. Pennsylvania, USA
Funding Agency MMF
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  • Foster, KR et al. Bioelectromagnetics., (1999) Suppl 4:52-63
  • Foster, KR et al. Bioelectromagnetics, (1999) 20:112-116
  • Foster , KR et al. Bioelectromagnetics, (1998) 19:420-428
  • Ely, TS Bioelectromagnetics., (1998) 19:427-428
  • Foster, KR et al. IEEE Access., (2016) 4:5322-5326
  • Foster, KR et al. Health Physics., (2016) 111:528-541
  • Foster, K et al. Electronics Letters., (2017) 53(5):360-362
  • Foster, K Electronics Letters., (2017) 53:290-(1 page)
  • Foster, KR et al. IEEE Access., (2018) DOI: 10.1109/ACCESS.2018.2883175:11 pages-
  • Foster, K et al. Health Physics., (2018) 115:295-307
  • Foster, KR et al. Future Networks Tech Focus., (2019) 3:-(5 pages)
  • Foster, KR et al. IEEE Access., (2020) 8:10.1109/ACCESS.2020.3008322-(13 pages)
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