Comment on Redmayne, M.; Maisch, D.R. ICNIRP Guidelines' Exposure Assessment Method for 5G Millimetre Wave Radiation May Trigger Adverse Effects. Int. J. Environ. Res. Public Health 2023, 20, 5267.

This article discusses the contention in the commented-upon paper that Brillouin precursors generated by 5G New Radio (5G NR) and other cellular systems are a possible cause of tissue damage at deeper layers of tissue than the power penetration depth of the carrier frequency. The original theory for Brillouin precursors from pulsed radiofrequency signals (RF-EMF) and speculation about their possible health effects dates back to the 1990's and was based on studies of the propagation of very short (nanosecond) ultrawide-bandwidth RF pulses through water. This assumption is not correct for cellular telephone signals due to their narrow bandwidth. The commented-on paper provides no alternative rationale as to why Brillouin effects should cause tissue damage from RF-EMF radiation from cellular and other communications systems. Other inaccuracies in this paper concerning thermal responses of tissue to RF-EMF are also noted.

Superlensing enables radio communication and imaging underwater.

Wireless radio communications provide a backbone to our technological civilization. However, radio communications are widely believed to be impossible in many situations where radios are surrounded by conductive media, such as underwater or underground, thus making ocean exploration difficult and creating well-known mine safety problems. In addition, since most imaging techniques rely on electromagnetic waves, the difficulty of electromagnetic wave propagation through biological tissues, which are mostly made of water, also severely limits bioimaging. Here we show that contrary to common beliefs, radio signals may be efficiently propagated through water over useful distances. Both radio communication and radio imaging through water may be enabled by superlensing of surface electromagnetic waves propagating along the water surface. We have demonstrated underwater radio communication over distances of several hundred skin depth in the MHz frequency range, which would require sensitivity below 10-100 W in a conventional radio communication channel. We also demonstrated subwavelength super-resolution radio imaging in the GHz range by using water surface as a superlens. Our results indicate new ways to perform bioimaging, as well as marine life safe techniques of wireless radio communication and imaging underwater, which are essential for ocean and seafloor exploration. We also anticipate that the developed techniques will provide invaluable means of studying the extraterrestrial water worlds, such as potentially inhabitable Jovian moons.

Mechanism of Capacitive Coupling of Proximal Electromagnetic Sources With Biological Bodies.

This study investigates the absorption of the induced E-field in homogeneous biological tissue exposed to highly localized field sources in proximity of the body, such as the charged tips of antennas, where E-field coupling dominates. These conditions are relevant for compliance testing of modern mobile phones where exposure is evaluated at small separation between radiators and the body. We derive an approximation that characterizes the decay of the induced E-field in the tissue as a function of distance. The absorption is quantified in terms of the local specific absorption rate (SAR) at the tissue surface as a function of the charge at the antenna tip. The approximation is based on the analytical evaluation of the E-fields of a charged disk under quasi-static conditions. We validate this approximation using full-wave simulations of dipoles. We demonstrate that the coupling mechanism of the E-field is dominated by the perpendicular field component and that wave propagation need not be considered for the characterization of the exposure. The surface SAR decreases approximately with the fourth power of the distance and with the square of the ratio of the permittivities of the tissue and free-space. The approximation predicts the induced maximum E-field with an accuracy of better than 1.5 dB. © 2022 Bioelectromagnetics Society.

Three Quarters of a Century of Research on RF Exposure Assessment and Dosimetry-What Have We Learned?

This commentary, by three authors with an aggregate experience of more than a century in technology and health and safety studies concerning radiofrequency (RF) energy, asks what has been learned over the past 75 years of research on radiofrequency and health, focusing on technologies for exposure assessment and dosimetry. Research programs on health and safety of RF exposure began in the 1950s, initially motivated by occupational health concerns for military personnel, and later to address public concerns about exposures to RF energy from environmental sources and near-field exposures from RF transmitting devices such as mobile phones that are used near the body. While this research largely focused on the biological effects of RF energy, it also led to important improvements in exposure assessment and dosimetry. This work in the aggregate has made RF energy one of the best studied potential technological hazards and represents a productive response by large numbers of scientists and engineers, working in many countries and supported by diverse funding agencies, to the ever rapidly evolving uses of the electromagnetic spectrum. This review comments on present needs of the field, which include raising the quality of dosimetry in many RF bioeffects studies and developing improved exposure/dosimetric techniques for the higher microwave frequencies to be used by forthcoming communications technologies. At present, however, the major uncertainties in dosimetric modeling/exposure assessment are likely to be related to the inherent variability in real-world exposures, rather than imprecision in measurement technologies.

Time-temperature Thresholds and Safety Factors for Thermal Hazards from Radiofrequency Energy above 6 GHz.

ABSTRACT: Two major sets of exposure limits for radiofrequency (RF) radiation, those of the International Commission on Nonionizing Radiation Protection (ICNIRP 2020) and the Institute of Electrical and Electronics Engineers (IEEE C95.1-2019), have recently been revised and updated with significant changes in limits above 6 GHz through the millimeter wave (mm-wave) band (30-300 GHz). This review compares available data on thermal damage and pain from exposure to RF energy above 6 GHz with corresponding data from infrared energy and other heat sources and estimates safety factors that are incorporated in the IEEE and ICNIRP RF exposure limits. The benchmarks for damage are the same as used in ICNIRP IR limits: minimal epithelial damage to cornea and first-degree burn (erythema in skin observable within 48 h after exposure). The data suggest that limiting thermal hazard to skin is cutaneous pain for exposure durations less than ≈20 min and thermal damage for longer exposures. Limitations on available data and thermal models are noted. However, data on RF and IR thermal damage and pain thresholds show that exposures far above current ICNIRP and IEEE limits would be required to produce thermally hazardous effects. This review focuses exclusively on thermal hazards from RF exposures above 6 GHz to skin and the cornea, which are the most exposed tissues in the considered frequency range.

Effects of 171 MHz Low-Intensity Electromagnetic Field on Glucocorticoid and Mineral Corticoid Activity of the Adrenal Glands of Rats.

A sub-acute electromagnetic field (EMF) biological effect study was carried out on rats exposed in the Transverse ElectroMagnetic exposure chamber at 171 MHz Continuous Wave (CW). The experiments involved three exposure levels (15, 25, and 35 V/m) for 15 days with triplicate parallel sham-exposed controls in each series. All exposure conditions were simulated for the evaluation of the electromagnetic energy distribution and specific absorption rate (SAR) in the rat phantoms. Studies have shown a biphasic biological response depending on time and absorbed electromagnetic energy. Under low SAR, approximately 0.006 W/kg, EMF exposure leads to the stimulation of adrenal gland activity. This process is accompanied by an initial increase of daily excretion of corticosterone and Na+ , which is seen as a higher Na+ /K+ ratio, followed by a decrease of these parameters over time. It is possible that EMF exposure causes a stress response in animals, which is seen as an increased adrenal activity. Bioelectromagnetics. 2019;40:578-587. © 2019 Bioelectromagnetics Society.

EXPOSURE ASSESSMENT OF PORTABLE WIRELESS DEVICES ABOVE 6 GHz.

The emerging 5 G wireless devices working at frequencies above 6 GHz are expected to have antenna arrays formed by dipoles, slots, patches or their combination. At lower frequencies, the accepted criteria for exposure compliance is stated in terms of specific absorption rate. IEEE and ICNIRP are adopting epithelial or transmitted power density (PD through body surface) as the dosimetric reference for frequencies above 6 GHz, which entails the measurement of free space PD. Theoretical and numerical results presented in this article show that it is possible to perform meaningful free space PD assessments at half wave (λ/2) distance from arrays and, with the proper instrumentation, as close as λ/(2π). However, if a dissipative body is placed very close (<λ/2π) to the arrays, its reflection and absorption of RF energy can change the electric currents and charges over the antenna. The relevance of such an effect should be further investigated, for instance by means of experimental analysis including measurements of tissue heating when in the presence of a strong reactive near field.

Theoretical and numerical assessment of maximally allowable power-density averaging area for conservative electromagnetic exposure assessment above 6 GHz.

The objective of this paper is to determine a maximum averaging area for power density (PD) that limits the maximum temperature increase to a given threshold for frequencies above 6 GHz. This maximum area should be conservative for any transmitter at any distance >2 mm from the primary transmitting antennas or secondary field-generating sources. To derive a generically valid maximum averaging area, an analytical approximation for the peak temperature increase caused by localized exposure was derived. The results for a threshold value of 1 K temperature rise were validated against simulations of a series of sources composed of electrical and magnetic elements (dipoles, slots, patches, and arrays) that represented the spectrum of relevant transmitters. The validation was successful for frequencies in which the power deposition occurred superficially (i.e., >10 GHz). In conclusion, the averaging area for a PD limit of 10 W/m2 that conservatively limits the temperature increase in the skin to less than 1 K at any distance >2 mm from the transmitters is frequency dependent, increases with distance, and ranges from 3 cm2 at <10 GHz to 1.9 cm2 at 100 GHz. In the far-field, the area depends additionally on distance and the antenna array aperture. The correlation was found to be worse at lower frequencies (<10 GHz) and very close to the source, the systematic evaluation of which is part of another study to investigate the effect of different coupling mechanisms in the reactive near-field on the ratio of temperature increase to incident power density. The presented model can be directly applied to any other PD and temperature thresholds. Bioelectromagnetics. 39:617-630, 2018. © 2018 Wiley Periodicals, Inc.

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.

Tissue models for RF exposure evaluation at frequencies above 6 GHz.

Exposures to radiofrequency (RF) energy above 6 GHz are characterized by shallow energy penetration, typically limited to the skin, but the subsequent increase in skin temperature is largely determined by heat transport in subcutaneous layers. A detailed analysis of the energy reflection, absorption, and power density distribution requires a knowledge of the properties of the skin layers and their variations. We consider an anatomically detailed model consisting of 3 or 4 layers (stratum corneum, viable epidermis plus dermis, subcutaneous fat, and muscle). The distribution of absorbed power in the different tissue layers is estimated based on electrical properties of the tissue layers inferred from measurements of reflected millimeter wavelength energy from skin, and literature data for the electrical properties of fat and muscle. In addition, the thermal response of the model is obtained using Pennes bioheat equation as well as a modified version incorporating blood flow rate-dependent thermal conductivity that provides a good fit to experimentally-found temperature elevations. A greatly simplified 3-layer model (Dermis, Fat, and Muscle) that assumes surface heating in only the skin layer clarifies the contribution of different tissue layers to the increase in surface skin temperature. The model shows that the increase in surface temperature is, under many circumstances, determined by the thermal resistance of subcutaneous tissues even though the RF energy may be deposited almost entirely in the skin layer. The limits of validity of the models and their relevance to setting safety standards are briefly discussed. Bioelectromagnetics. 39:173-189, 2018. © 2018 Wiley Periodicals, Inc.

Thermal Modeling for the Next Generation of Radiofrequency Exposure Limits: Commentary.

This commentary evaluates two sets of guidelines for human exposure to radiofrequency (RF) energy, focusing on the frequency range above the "transition" frequency at 3-10 GHz where the guidelines change their basic restrictions from specific absorption rate to incident power density, through the end of the RF band at 300 GHz. The analysis is based on a simple thermal model based on Pennes' bioheat equation (BHTE) (Pennes 1948) assuming purely surface heating; an Appendix provides more details about the model and its range of applicability. This analysis suggests that present limits are highly conservative relative to their stated goals of limiting temperature increase in tissue. As applied to transmitting devices used against the body, they are much more conservative than product safety standards for touch temperature for personal electronics equipment that are used in contact with the body. Provisions in the current guidelines for "averaging time" and "averaging area" are not consistent with scaling characteristics of the bioheat equation and should be refined. The authors suggest the need for additional limits on fluence for protection against brief, high intensity pulses at millimeter wave frequencies. This commentary considers only thermal hazards, which form the basis of the current guidelines, and excludes considerations of reported "non-thermal" effects of exposure that would have to be evaluated in the process of updating the guidelines.

Thermal Response of Human Skin to Microwave Energy: A Critical Review.

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 (30-300 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 0.5-1 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 1-2 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.

Near-field radiofrequency electromagnetic exposure assessment.

Personal wireless telecommunication devices, such as radiofrequency (RF) electromagnetic field (EMF) sources operated in vicinity of human body, have possible adverse health effects. Therefore, the correct EMF assessment is necessary in their near field. According to international near-field measurement criteria, the specific absorption rate (SAR) is used for absorbed energy distribution assessment in tissue simulating liquid phantoms. The aim of this investigation is to validate the relationship between the H-field of incident EMF and absorbed energy in phantoms. Three typical wireless telecommunication system frequencies are considered (900, 1800 and 2450 MHz). The EMF source at each frequency is an appropriate half-wave dipole antenna and the absorbing medium is a flat phantom filled with the suitable tissue simulating liquid. Two methods for SAR estimation have been used: standard procedure based on E-field measured in tissue simulating medium and a proposed evaluation by measuring the incident H-field. Compared SAR estimations were performed for various distances between sources and phantom. Also, these research data were compared with simulation results, obtained by using finite-difference time-domain method. The acquired data help to determine the source near-field space characterized by the smallest deviation between SAR estimation methods. So, this region near the RF source is suitable for correct RF energy absorption assessment using the magnetic component of the RF fields.

In vitro exposure: Linear and non-linear thermodynamic events in Petri dishes.

We conducted an electromagnetic-thermal analysis of Petri dishes filled with different medium volumes under different radio frequency exposure conditions with the aim of identifying linear and non-linear parameters that might explain contradictory results of many in vitro bioelectromagnetic experiments. We found that power loss density and temperature depend on shape, size, and orientation of the exposed sample with respect to direction of incident energy, showing that the liquid medium acts as a receiving antenna. In addition, we investigated the possibility of convection from thermodynamic principles within the liquid medium. For a 35 mm diameter Petri dish, a 2 or 4 ml medium volume is too small to support vertical convection. Conversely, horizontal convective motion is possible for H-polarization exposures at 1.8 GHz.

Absence of nonlinear responses in cells and tissues exposed to RF energy at mobile phone frequencies using a doubly resonant cavity.

A doubly resonant cavity was used to search for nonlinear radiofrequency (RF) energy conversion in a range of biological preparations, thereby testing the hypothesis that living tissue can demodulate RF carriers and generate baseband signals. The samples comprised high-density cell suspensions (human lymphocytes and mouse bone marrow cells); adherent cells (IMR-32 human neuroblastoma, G361 human melanoma, HF-19 human fibroblasts, N2a murine neuroblastoma (differentiated and non-differentiated) and Chinese hamster ovary (CHO) cells) and thin sections or slices of mouse tissues (brain, kidney, muscle, liver, spleen, testis, heart and diaphragm). Viable and non-viable (heat killed or metabolically impaired) samples were tested. Over 500 cell and tissue samples were placed within the cavity, exposed to continuous wave (CW) fields at the resonant frequency (f) of the loaded cavity (near 883 MHz) using input powers of 0.1 or 1 mW, and monitored for second harmonic generation by inspection of the output at 2f. Unwanted signals were minimised using low pass filters (≤ 1 GHz) at the input to, and high pass filters (≥ 1 GHz) at the output from, the cavity. A tuned low noise amplifier allowed detection of second harmonic signals above a noise floor as low as -169 dBm. No consistent second harmonic of the incident CW signals was detected. Therefore, these results do not support the hypothesis that living cells can demodulate RF energy, since second harmonic generation is the necessary and sufficient condition for demodulation.