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

Assessment of incident power density in different shapes of averaging area for radio-frequency exposure above 6 GHz

Objective. The International Commission on Non-Ionizing Radiation Protection guidelines and IEEE C95.1-2019 standard for human protection from local electromagnetic field exposure above 6 GHz state that absorbed (or epithelial) power density (APD) and incident power density (IPD), averaged over a square area, are internal and external physical quantities, respectively, that set the exposure limit. Per exposure standards, the measurement procedure and evaluation of the IPD have been established in technical standards, where a circular averaging area is recommended only for non-planar surfaces in IEC/IEEE 63195-1 and -2. In this study, the effects of two averaging shapes on the APD and IPD are evaluated computationally to provide new insights from the viewpoint of exposure standards. Approach. The relation between the APD, IPD, and the steady-state temperature rise (heating factor) in rectangular and human models for exposure to a single dipole, dipole arrays, and the Gaussian beams is investigated computationally with finite-difference method. Main results. The maximum differences in the heating factor of the APD and IPD for square and circular averaging areas were 4.1% and 4.4% for the antenna–model distance >5 mm, respectively. These differences appear when the beam pattern on the model surface has an elliptical shape. For an antenna–model distance ≤5 mm and at frequencies ≤15 GHz, the heating factors for square averaging areas were not always conservative to those for circular ones (−7.8% for IPD), where only the antenna feed point are visible before beam formation. Significance. The heating factors of the APD and IPD for a circular averaging area are conservative for near-field exposure of canonical sources for frequencies up to 300 GHz, except for a beam with a significant major-to-minor axis ratio and an angle of 30°–60° to a square averaging area. This tendency would help bridge the gap between exposure and product standards.

Analysis of ICNIRP 2020 Basic Restrictions for Localized Radiofrequency Exposure in the Frequency Range above 6 GHz

ABSTRACT

ICNIRP 2020 guidelines have defined a practical temperature elevation threshold for human health effects, namely the operational adverse health effect threshold that forms the basis of the absorbed power and energy density basic restrictions. These basic restrictions for localized exposures at frequencies above 6 GHz were evaluated by comparing numerically computed temperature rise against the target temperature rise of 2.5 o C, which is the operational adverse health effect threshold divided by the occupational safety factor of 2. The numerical model employs the maximum absorbed power and energy density levels allowed by the occupational basic restriction for both pulsed and continuous wave exposures. These analyses were performed considering 3- and 4-tissue layer models and a variety of beam diameters, frequencies, and exposure durations. The smallest beam diameters were based on a study of theoretically achievable beam widths from half-wave resonant dipoles and show the impact of the averaging area on the computed temperature elevation. The results demonstrated that ICNIRP's assumed occupational safety factors in the frequency range above 6 GHz were not sufficiently maintained for all exposure scenarios and particularly for short pulse exposures at frequencies of 30 GHz or higher with small beam diameters. Worst-case tissue temperature elevations were estimated to be as much as 3.6 times higher than ICNIRP's target temperature increases. Consequently, the authors suggest a small modification in the application of the ICNIRP 2020 localized basic restrictions, thereby limiting the worst-case tissue temperature increases to 1.4 times the target value.

Assessment of SAR in Road-Users from 5G-V2X Vehicular Connectivity Based on Computational Simulations

(1) Background: Cooperative Intelligent Transportation Systems (C-ITS) will soon operate using 5G New-Radio (NR) wireless communication, overcoming the limitations of the current V2X (Vehicle-to-Everything) wireless communication technologies and increasing road-safety and driving efficiency. These innovations will also change the RF exposure levels of pedestrians and road-users in general. These people, in fact, will be exposed to additional RF sources coming from nearby cars and from the infrastructure. Therefore, an exposure assessment of people in the proximity of a connected car is necessary and urgent. (2) Methods: Two array antennas for 5G-V2X communication at 3.5 GHz were modelled and mounted on a realistic 3D car model for evaluating the exposure levels of a human model representing people on the road near the car. Computational simulations were conducted using the FDTD solver implemented in the Sim4Life platform; different positions and orientations between the car and the human model were assessed. The analyzed quantities were the Specific Absorption Rate on the whole body (SAR_wb), averaged over 10 g (SAR_10g) in specific tissues, as indicated in the ICNIRP guidelines. (3) Results: the data showed that the highest exposure levels were obtained mostly in the head area of the human model, with the highest peak obtained in the configuration where the main beam of the 5G-V2X antennas was more direct towards the human model. Moreover, in all configurations, the dose absorbed by a pedestrian was well below the ICNIRP guidelines to avoid harmful effects. (4) Conclusions: This work is the first study on human exposure assessment in a 5G-V2X scenario, and it expands the knowledge about the exposure levels for the forthcoming use of 5G in connected vehicles.

Assessment of Human Exposure Levels Due to Mobile Phone Antennas in 5G Networks

The recent deployment of 5G networks is bringing benefits to the population but it is also raising public concern about human RF-EMF exposure levels. This is particularly relevant considering the next 5G mobile devices, which are placed in close proximity to the subjects. Therefore, the aim of the following paper is focused on expanding the knowledge of the exposure levels in 5G exposure scenarios, specifically for mobile applications, using computational methods. The mobile antenna was designed considering the 5G technology innovations (i.e., mm-wave spectrum, beamforming capability, high gain and wide coverage), resulting in a phased-array antenna with eight elements at the working frequency of 27 GHz. To assess the exposure levels, different types of skin models with different grades of details and layers were considered. Furthermore, not only was the presence of a mobile phone user simulated, but also that of a person in their proximity, who could be hit by the main beam of the phased-array antenna. All the simulations were conducted in Sim4Life platform, where the exposure levels were assessed in terms of absorbed power density averaged over 4 cm² and 1 cm², following the ICNIRP guidelines. The results highlighted that the use of the homogeneous skin model led to the absorbed power density peaks being greatly underestimated, with respect to those obtained in multilayer skin models. Furthermore, interestingly, we found that the exposure levels obtained for the person passing nearby were slightly higher than those experienced by the mobile phone user himself. Finally, using the allowed input power for real mobile applications, all the values remained below the limits indicated by the ICNIRP guidelines.

Temperature Dynamics in Rat Brains Exposed to Near-Field Waveguide Outputs at 2.8 GHz

Biological effects in the microwave band of the radiofrequency (RF) spectrum are thermally mediated. For acute high-power microwave exposures, these effects will depend on transient time-temperature histories within the tissue. In this article, we summarize the transient temperature response of rats exposed to RF energy emanating from an open-ended rectangular waveguide. These exposures produced specific absorption rates of approximately 36 and 203 W/kg in the whole body and brain, respectively. We then use the experimentally measured thermal data to infer the baseline perfusion rate in the brain and modify a custom thermal modeling tool based upon these findings. Finally, we compare multi-physics simulations of rat brain temperature against empirical measurements in both live and euthanized subjects and find close agreement between model and experimentation. This research revealed that baseline brain perfusion rates in rat subjects could be larger than previously assumed in the RF thermal modeling literature, and plays a significant role in the transient thermal response to high-power microwave exposures. © 2021 Bioelectromagnetics Society.

Steady state temperature rise in multilayered tissue due to arbitrary periodic SAR using finite difference FFT and transfer function method

Steady state (SS) and transient temperature-rise in tissue from radiofrequency exposure forms the underlying basis for limits in international exposure guidelines. Periodically pulsed or intermittent exposures form a special case of having both peak and average levels, producing temperature-rise oscillations in the SS. Presented here is a method for determining tissue temperature-rise for periodic specific absorption rate (SAR) modulation having arbitrary waveform. It involves the finite difference solution of a form of the Pennes Bioheat Transfer equation (BHTE) and uses the concept of the transfer function and the Fast Fourier Transform (FFT). The time-dependent BHTE is converted to a SS harmonic version by assuming that the time-dependent SAR waveform and tissue temperature can both be represented by Fourier series. The transfer function is obtained from solutions of the harmonic BHTE for an assumed SAR waveform consisting of periodic impulses. The temperature versus time response for an arbitrary periodic SAR waveform is obtained from the inverse FFT of the product of the transfer function and the FFT of the actual SAR waveform. This method takes advantage of existing FFT algorithms on most computational platforms and the ability to store the transfer function for later re-use. The transfer function varies slowly with harmonic number, allowing interpolation and extrapolation to reduce the computational effort. The method is highly efficient for the case where repeated temperature-rise calculations for parameter variations in the SAR waveform are sought. Examples are given for a narrow, circularly symmetric beam incident on a planar skin/fat/muscle model with rectangular, triangular and cosine-pulsed SAR modulation waveforms. Calculations of temperature-rise crest factor as a function of rectangular pulse duty factor and pulse repetition frequency for the same exposure/tissue model are also presented as an example of the versatility of the method.

Human exposure to radiofrequency energy above 6 GHz: review of computational dosimetry studies

International guidelines/standards for human protection from electromagnetic fields have been revised recently, especially for frequencies above 6 GHz where new wireless communication systems have been deployed. Above this frequency a new physical quantity 'absorbed/epithelial power density' has been adopted as a dose metric. Then, the permissible level of external field strength/power density is derived for practical assessment. In addition, a new physical quantity, fluence or absorbed energy density, is introduced for protection from brief pulses (especially for shorter than 10 s). These limits were explicitly designed to avoid excessive increases in tissue temperature, based on electromagnetic and thermal modeling studies but supported by experimental data where available. This paper reviews the studies on the computational modeling/dosimetry which are related to the revision of the guidelines/standards. The comparisons with experimental data as well as an analytic solution are also been presented. Future research needs and additional comments on the revision will also be mentioned.

Stochastic Dosimetry Assessment of the Human RF-EMF Exposure to 3D Beamforming Antennas in indoor 5G Networks

The deployment of near future 5G networks will introduce modifications in the population’s exposure levels to radio-frequency electromagnetic fields (RF-EMFs). The present work aimed to face the challenge of studying the exposure variability in the presence of an access point (AP) at 3.7 GHz with 64 patch elements uniform planar array antenna and 3D beamforming capability. The novelty introduced in the methodology of the exposure’s evaluation was the combining of traditional computational methods with a new approach based on stochastic dosimetry, called polynomial chaos kriging method, in order to estimate the exposure levels for 1000 different antenna beamforming patterns with low computational efforts. The simulations were evaluated considering a child model and computing the specific absorption rate (SAR) in different tissues. The analysis of the results highlighted a high exposure variability scenario depending on the beamforming patterns of the array antenna and identified the ranges of elevation and azimuth angles of the main antenna beam that may cause the highest levels of exposure.

Guidelines for Limiting Exposure to Electromagnetic Fields (100 kHz to 300 GHz)

Radiofrequency electromagnetic fields (EMFs) are used to enable a number of modern devices, including mobile telecommunications infrastructure and phones, Wi-Fi, and Bluetooth. As radiofrequency EMFs at sufficiently high power levels can adversely affect health, ICNIRP published Guidelines in 1998 for human exposure to time-varying EMFs up to 300 GHz, which included the radiofrequency EMF spectrum. Since that time, there has been a considerable body of science further addressing the relation between radiofrequency EMFs and adverse health outcomes, as well as significant developments in the technologies that use radiofrequency EMFs. Accordingly, ICNIRP has updated the radiofrequency EMF part of the 1998 Guidelines. This document presents these revised Guidelines, which provide protection for humans from exposure to EMFs from 100 kHz to 300 GHz.

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

Both the current and newly proposed safety guidelines for local human exposure to millimeter-wave frequencies aim at restricting the maximum local temperature increase in the skin to prevent tissue damage. In this study, we show that the application of the current and proposed limits for pulsed fields can lead to a temperature increase of 10°C for short pulses and frequencies between 6 and 30 GHz. We also show that the proposed averaging area of 4 cm² , that is greatly reduced compared with the current limits, does not prevent high-temperature increases in the case of narrow beams. A realistic Gaussian beam profile with a 1 mm radius can result in a temperature increase about 10 times higher than the 0.4°C increase the same averaged power density would produce for a plane wave. In the case of pulsed narrow beams, the values for the time and spatial-averaged power density allowed by the proposed new guidelines could result in extreme temperature increases. Bioelectromagnetics. 2020;41:164-168. © 2019 Bioelectromagnetics Society.

Systematic Derivation of Safety Limits for Time-Varying 5G Radiofrequency Exposure Based on Analytical Models and Thermal Dose

Extreme broadband wireless devices operating above 10 GHz may transmit data in bursts of a few milliseconds to seconds. Even though the time- and area-averaged power density values remain within the acceptable safety limits for continuous exposure, these bursts may lead to short temperature spikes in the skin of exposed people. In this paper, a novel analytical approach to pulsed heating is developed and applied to assess the peak-to-average temperature ratio as a function of the pulse fraction α (relative to the averaging time [INCREMENT]T; it corresponds to the inverse of the peak-to-average ratio). This has been analyzed for two different perfusion-related thermal time constants (τ1 = 100 s and 500 s) corresponding to plane-wave and localized exposures. To allow for peak temperatures that considerably exceed the 1 K increase, the CEM43 tissue damage model, with an experimental-data-based damage threshold for human skin of 600 min, is used to allow large temperature oscillations that remain below the level at which tissue damage occurs. To stay consistent with the current safety guidelines, safety factors of 10 for occupational exposure and 50 for the general public were applied. The model assumptions and limitations (e.g., employed thermal and tissue damage models, homogeneous skin, consideration of localized exposure by a modified time constant) are discussed in detail. The results demonstrate that the maximum averaging time, based on the assumption of a thermal time constant of 100 s, is 240 s if the maximum local temperature increase for continuous-wave exposure is limited to 1 K and α ≥ 0.1. For a very low peak-to-average ratio of 100 (α ≥ 0.01), it decreases to only 30 s. The results also show that the peak-to-average ratio of 1,000 tolerated by the International Council on Non-Ionizing Radiation Protection guidelines may lead to permanent tissue damage after even short exposures, highlighting the importance of revisiting existing exposure guidelines.

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

In the international guidelines/standards for human protection against electromagnetic fields, the specific absorption rate (SAR) is used as a metric for radio-frequency field exposure. For radio-frequency near-field exposure, the peak value of the SAR averaged over 10 g of tissue is treated as a surrogate of the local temperature elevation for frequencies up to 3⁻10 GHz. The limit of 10-g SAR is derived by extrapolating the thermal damage in animal experiments. However, no reports discussed the difference between the time constant of temperature elevation in small animals and humans for local exposure. This study computationally estimated the thermal time constants of temperature elevation in human head and rat models exposed to dipole antennas at 3⁻10 GHz. The peak temperature elevation in the human brain was lower than that in the rat model, mainly because of difference in depth from the scalp. Consequently, the thermal time constant of the rat brain was smaller than that of the human brain. Additionally, the thermal time constant in human skin decreased with increasing frequency, which was mainly characterized by the effective SAR volume, whereas it was almost frequency-independent in the human brain. These findings should be helpful for extrapolating animal studies to humans.

Modeling Tissue Heating From Exposure to Radiofrequency Energy and Relevance of Tissue Heating to Exposure Limits: Heating Factor

This review/commentary addresses recent thermal and electromagnetic modeling studies that use image-based anthropomorphic human models to establish the local absorption of radiofrequency energy and the resulting increase in temperature in the body. The frequency range of present interest is from 100 MHz through the transition frequency (where the basic restrictions in exposure guidelines change from specific absorption rate to incident power density, which occurs at 3-10 GHz depending on the guideline). Several detailed thermal modeling studies are reviewed to compare a recently introduced dosimetric quantity, the heating factor, across different exposure conditions as related to the peak temperature rise in tissue that would be permitted by limits for local body exposure. The present review suggests that the heating factor is a robust quantity that is useful for normalizing exposures across different simulation models. Limitations include lack of information about the location in the body where peak absorption and peak temperature increases occur in each exposure scenario, which are needed for careful assessment of potential hazards. To the limited extent that comparisons are possible, the thermal model (which is based on Pennes' bioheat equation) agrees reasonably well with experimental data, notwithstanding the lack of theoretical rigor of the model and uncertainties in the model parameters. In particular, the blood flow parameter is both variable with physiological condition and largely determines the steady state temperature rise. We suggest an approach to define exposure limits above and below the transition frequency (the frequency at which the basic restriction changes from specific absorption rate to incident power density) to provide consistent levels of protection against thermal hazards. More research is needed to better validate the model and to improve thermal dosimetry in general. While modeling studies have considered the effects of variation in thickness of tissue layers, the effects of normal physiological variation in tissue blood flow have been relatively unexplored.

Temperature elevation in the human brain and skin with thermoregulation during exposure to RF energy

Background

Two international guidelines/standards for human protection from electromagnetic fields define the specific absorption rate (SAR) averaged over 10 g of tissue as a metric for protection against localized radio frequency field exposure due to portable devices operating below 3–10 GHz. Temperature elevation is suggested to be a dominant effect for exposure at frequencies higher than 100 kHz. No previous studies have evaluated temperature elevation in the human head for local exposure considering thermoregulation. This study aims to discuss the temperature elevation in a human head model considering vasodilation, to discuss the conservativeness of the current limit.

Methods

This study computes the temperature elevations in an anatomical human head model exposed to radiation from a dipole antenna and truncated plane waves at 300 MHz–10GHz. The SARs in the human model are first computed using a finite-difference time-domain method. The temperature elevation is calculated by solving the bioheat transfer equation by considering the thermoregulation that simulates the vasodilation.

Results

The maximum temperature elevation in the brain appeared around its periphery. At exposures with higher intensity, the temperature elevation became larger and reached around 40 °C at the peak SAR of 100 W/kg, and became lower at higher frequencies. The temperature elevation in the brain at the current limit of 10 W/kg is at most 0.93 °C. The effect of vasodilation became notable for tissue temperature elevations higher than 1–2 °C and for an SAR of 10 W/kg. The temperature at the periphery was below the basal brain temperature (37 °C).

Conclusions

The temperature elevation under the current guideline for occupational exposure is within the ranges of brain temperature variability for environmental changes in daily life. The effect of vasodilation is significant, especially at higher frequencies where skin temperature elevation is dominant.

Human exposure to pulsed fields in the frequency range from 6 to 100 GHz

Restrictions on human exposure to electromagnetic waves at frequencies higher than 3-10 GHz are defined in terms of the incident power density to prevent excessive temperature rise in superficial tissue. However, international standards and guidelines differ in their definitions of how the power density is interpreted for brief exposures. This study investigated how the temperature rise was affected by exposure duration at frequencies higher than 6 GHz. Far-field exposure of the human face to pulses shorter than 10 s at frequencies from 6 to 100 GHz was modelled using the finite-difference time-domain method. The bioheat transfer equation was used for thermal modelling. We investigated the effects of frequency, polarization, exposure duration, and depth below the skin surface on the temperature rise. The results indicated limitations in the current human exposure guidelines and showed that radiant exposure, i.e. energy absorption per unit area, can be used to limit temperature rise for pulsed exposure. The data are useful for the development of human exposure guidelines at frequencies higher than 6 GHz.