Time constants for temperature elevation in human models exposed to dipole antennas and beams in the frequency range from 1 to 30 GHz

Abstract

This study computes the time constants of the temperature elevations in human head and body models exposed to simulated radiation from dipole antennas, electromagnetic beams, and plane waves. The frequency range considered is from 1 to 30 GHz. The specific absorption rate distributions in the human models are first computed using the finite-difference time-domain method for the electromagnetics. The temperature elevation is then calculated by solving the bioheat transfer equation. The computational results show that the thermal time constants (defined as the time required to reach 63% of the steady state temperature elevation) decrease with the elevation in radiation frequency. For frequencies higher than 4 GHz, the computed thermal time constants are smaller than the averaging time prescribed in the ICNIRP guidelines, but larger than the averaging time in the IEEE standard. Significant differences between the different head models are observed at frequencies higher than 10 GHz, which is attributable to the heat diffusion from the power absorbed in the pinna. The time constants for beam exposures become large with the increase in beam diameter. The thermal time constant in the brain is larger than that in the superficial tissues at high frequencies, because the brain temperature elevation is caused by the heat conduction of energy absorbed in the superficial tissue. The thermal time constant is minimized with an ideal beam with a minimum investigated diameter of 10 mm; this minimal time constant is approximately 30 s and is almost independent of the radiation frequency, which is supported by analytic methods. In addition, the relation between the time constant, as defined in this paper, and 'averaging time' as it appears in the exposure limits is discussed, especially for short intense pulses. Similar to the laser guidelines, provisions should be included in the limits to limit the fluence for such pulses.

The Skin Tissue of the Human Head Subjected to Thermal Diffusion

“…An additional relaxation time accommodating the phase lag of the temperature gradient is analytically studied. Morimoto et al [21] studied the time constants of the temperature elevations in human head and body models exposed to simulated radiation from dipole antennas, electromagnetic beams, and plane waves within frequency range considered to be from 1 to 30 GHz. Tang et al discussed the thermal and vibrational characterization of human skin to find suitable and reproducible biomarkers for further clinical studies on intrinsic and extrinsic aging of dermis [22].…”

Comparison of Thermal Response for RF Exposure in Human and Rat Models

“…The thermal time constant has not been discussed until recently [23], except for a few studies on the exposure from handset antennas [20,21,22,23]. Instead, thermal damage in animal studies has been commented on in the international guidelines/standards.…”

“…Focusing on each condition, the thermal time constant can be expected to saturate as the effective SAR volume increases. The increment of the effective SAR volume results in thermal diffusion, causing the delay in temperature elevation [23]. This result suggests that the effective SAR volume can be estimated as a metric in superficial tissues.…”

“…Foster et al [12] derived an analytic formula to estimate the thermal time constant using a one-dimensional homogeneous model for intense brief exposures, especially at millimeter waves. Some studies using human anatomical models computed the thermal time constants for handset antennas without considering thermoregulation [20,21,22,23]. However, no reports have compared thermal time constants between humans and animals, making direct extrapolation of findings in animals to humans difficult.…”

Discussion on Spatial and Time Averaging Restrictions Within the Electromagnetic Exposure Safety Framework in the Frequency Range Above 6 GHz for Pulsed and Localized Exposures

“…In the above calculations, we used a value of 500 s [Foster et al, ], as suggested by the newly approved standard [IEEE, ]. However, for local exposure, this time constant may take values as small as 100 s [Morimoto et al, ], resulting in a roughly twofold increase of the maximum temperature increase of the same pulse width.…”