Radiofrequency Exposure Limits Research Articles

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

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.

Thermal Analysis of Averaging Times in Radio-Frequency Exposure Limits Above 1 GHz

This paper considers the problem of choosing an appropriate “averaging time” in radiofrequency (RF) exposure limits to protect against thermal hazards, focusing on the RF frequency range above 3–10 GHz. Analysis is based on examination of the dynamic properties of thermal models for tissue using Pennes’ bioheat equation. Three models are considered: a baseline model consisting of a uniform half space with dielectric and thermal properties similar to those of human skin with adiabatic boundary conditions; a layered 1D model with dielectric and thermal properties similar to those of skin, fat, and underlying muscle, with convective boundary conditions appropriate for room environments; and exposures to the head of an anatomically detailed image-based model (“Taro”). RF exposure consisted of plane wave radiation incident on the two planar models, and radiation from resonant dipoles located 1.5 cm from the head model, at frequencies ranging from 1 to 300 GHz. The dynamic properties of the models were explored by analytic solution of the baseline model, and from numerical solutions of the thermal responses of the layered and head models. From the step responses of the models (increases in surface temperature to a suddenly imposed exposure), the impulse and frequency responses of the models were obtained. In the frequency domain, the thermal models exhibit extreme lowpass characteristics with cutoff (−3 dB response) frequencies below 1 mHz. The impulse response to millimeter wave radiation (30–300 GHz) shows a sharp peak at zero time, due to short term accumulation of heat near the surface, which dissipates quickly as heat is conducted into deeper layers of tissue. Simple analytical results of a further simplified model assuming purely surface heating agree well with results of a more detailed assessment for millimeter waves. Response of the model to pulse trains and to single maximum fluence “big bang” pulses in which all allowable energy over a 6-min averaging time is delivered in one short pulse raises the possibility of excessive transient temperature increases at the tissue surface from exposure to short high-fluence pulses at mm-wave frequencies. Such exposures are not produced by current technologies apart from certain military weapons systems but may occur from future high-power mm-wave technology. By contrast, simulations of exposure from a communications waveform at 1.9 GHz show extremely tiny transient temperature fluctuations. The results generally confirm the present choice of 6 min for an averaging time in the current generation of RF exposure limits but suggest the need for additional limits on fluence for brief high-fluence pulses at mm-wave frequencies. This paper addresses thermal hazards only, and a larger range of evidence would need to be evaluated as well in revising exposure limits.

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.

Occupational exposures to radiofrequency fields: results of an Israeli national survey

Relatively high exposures to radiofrequency (RF) fields can occur in the broadcast, medical, and communications industries, as well in occupations that use RF emitting equipment (e.g. law enforcement). Information on exposure to workers employed in these industries and occupations is limited. We present results of an Israeli National Survey of occupational RF field levels at frequencies between ~100 kHz and 40 GHz, representing Industrial Heating, Communications, Radar, Research, and Medicine. Almost 4300 measurements from 900 sources across 25 occupations were recorded and categorised as ‘routine’, ‘incidental’, or ‘unintended’. The occupation-specific geometric means (GMs) of the percentage of the American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit values (TLVs) for each of the three exposure scenarios are presented together with the geometric standard deviation (GSD). Additionally, we present estimates of occupation-specific annual personal exposures and collective exposures. The vast majority of the GM of routine exposures ranged from a fraction to less than 1% of ACGIH TLVs, except for Walkie-Talkie (GM 94% of ACGIH), Induction Heating (17%), Plastic Welding (11%), Industrial Heating (6%) and Diathermy (6%). The GM of incidental and unintended exposures exceeded the TLV for one and 14 occupations, respectively. In many cases, the within-occupation GSD was very large, and though the medians remained below TLV, variable fractions of these occupations were projected to exceed the TLV. In rank order, Walkie-Talkie, Plastic Welding, and Induction Heating workers had the highest annual cumulative personal exposure. For cumulative collective exposures within an occupation, Walkie-Talkie dominated with 96.3% of the total, reflecting both large population and high personal exposure. A brief exceedance of the TLV does not automatically translate to hazard as RF exposure limits (issued by various bodies, including ACGIH) include a 10-fold safety factor relative to thermal thresholds and are based on a 6 min averaging period.

IEEE Committee on Man and Radiation--COMAR technical information statement radiofrequency safety and utility Smart Meters.

This Technical Information Statement describes Smart Meter technology as used with modern electric power metering systems and focuses on the radio frequency (RF) emissions associated with their operation relative to human RF exposure limits. Smart Meters typically employ low power (-1 W or less) transmitters that wirelessly send electric energy usage data to the utility company several times per day in the form of brief, pulsed emissions in the unlicensed frequency bands of 902-928 MHz and 2.4-2.48 GHz or on other nearby frequencies. Most Smart Meters operate as wireless mesh networks where each Smart Meter can communicate with other neighboring meters to relay data to a data collection point in the region. This communication process includes RF emissions from Smart Meters representing energy usage as well as the relaying of data from other meters and emissions associated with maintaining the meter's hierarchy within the wireless network. As a consequence, most Smart Meters emit RF pulses throughout the day, more at certain times and less at others. However, the duty cycle associated with all of these emissions is very small, typically less than 1%, and most of the time far less than 1%, meaning that most Smart Meters actually transmit RF fields for only a few minutes per day at most. The low peak power of Smart Meters and the very low duty cycles lead to the fact that accessible RF fields near Smart Meters are far below both U.S. and international RF safety limits whether judged on the basis of instantaneous peak power densities or time-averaged exposures. This conclusion holds for Smart Meters alone or installed in large banks of meters.

Risk management policies and practices regarding radio frequency electromagnetic fields: results from a WHO survey.

This study aims to describe current risk management practices and policies across the world in relation to personal exposures from devices emitting radiofrequency fields, environmental exposures from fixed installations and exposures in the work environment. Data from 86 countries representing all WHO regions were collected through a survey. The majority of countries (76.8 %) had set exposure limits for mobile devices, almost all (90.7 %) had set public exposure limits for fixed installations and 76.5 % had specified exposure limits for personnel in occupational settings. A number of other policies had been implemented at the national level, ranging from information provisions on how to reduce personal exposures and restrictions of usage for certain populations, such as children or pregnant women to prevention of access around base stations. This study suggests that countries with higher mobile subscriptions tend to have set radiofrequency exposure limits for mobile devices and to have provisions on exposure measurements about fixed installations.

COMAR finding on radiofrequency safety and utility smart meters

This Technical Information Statement describes Smart Meter technology as used with modern electric power metering systems and focuses on the radio frequency (RF) emissions associatedwith their operation relative to human RF exposure limits. Smart Meters typically employ lowpower (~1 W or less) transmitters that wirelessly send electric energy usage data to the utilitycompany several times per day in the form of brief, pulsed emissions in the unlicensed frequencybands of 902-928 MHz and 2.4-2.48 GHz or on other nearby frequencies. Most Smart Metersoperate as wireless mesh networks where each Smart Meter can communicate with otherneighboring meters to relay data to a data collection point in the region. This communicationprocess includes RF emissions from Smart Meters representing energy usage as well as therelaying of data from other meters and emissions associated with maintaining the meter'shierarchy within the wireless network. As a consequence, most Smart Meters emit RF pulsesthroughout the day, more at certain times and less at others. However, the duty cycle associatedwith all of these emissions is very small, typically less than 1%, and most of the time far lessthan 1%, meaning that most Smart Meters actually transmit RF fields for only a few minutes perday at most. The low peak power of Smart Meters and the very low duty cycles lead to the factthat accessible RF fields near Smart Meters are far below both U.S. and international RF safe-tylimits whether judged on the basis of instantaneous peak power densities or time-averaged exposures. This conclusion holds for Smart Meters alone or installed in large banks of meters.

Telemetry Systems

Telemetry Systems

Radiofrequency fields associated with the Itron smart meter

This study examined radiofrequency (RF) emissions from smart electric power meters deployed in two service territories in California for the purpose of evaluating potential human exposure. These meters included transmitters operating in a local area mesh network (RF LAN, ∼250 mW); a cell relay, which uses a wireless wide area network (WWAN, ∼1 W); and a transmitter serving a home area network (HAN, ∼70 mW). In all instances, RF fields were found to comply by a wide margin with the RF exposure limits established by the US Federal Communications Commission. The study included specialised measurement techniques and reported the spatial distribution of the fields near the meters and their duty cycles (typically <1 %) whose value is crucial to assessing time-averaged exposure levels. This study is the first to characterise smart meters as deployed. However, the results are restricted to a single manufacturer's emitters.

Temperature changes associated with radiofrequency exposure near authentic metallic implants in the head phantom—a near field simulation study with 900, 1800 and 2450 MHz dipole

Along with increased use of wireless communication devices operating in the radiofrequency (RF) range, concern has been raised about the related possible health risks. Among other concerns, the interaction of medical implants and RF devices has been studied in order to assure the safety of implant carriers under various exposure conditions. In the RF range, the main established quantitative effect of electromagnetic (EM) fields on biological tissues is heating due to vibrational movements of water molecules. The temperature changes induced in tissues also constitute the basis for the setting of RF exposure limits and recommendations. In this study, temperature changes induced by electromagnetic field enhancements near passive metallic implants have been simulated in the head region. Furthermore, the effect of the implant material on the induced temperature change was evaluated using clinically used metals with the highest and the lowest thermal conductivities. In some cases, remarkable increases in maximum temperatures of tissues (as much as 8 °C) were seen in the near field with 1 W power level whereas at lower power levels significant temperature increases were not observed.

Thermal stress and radiation protection principles

Exposure to radiofrequency (RF) fields can occur in residential, occupational and medical settings. Since many technologies use RF fields, it is important to fully investigate their effects on the human body. Since the demonstrated effect of RF exposure is heating, it is important to critically evaluate studies of elevated temperature effects on the human body, from the cellular and tissue level to the whole body level, including potential effects on the susceptible groups such as the very young and the very old. WHO convened a Workshop in the Spring of 2002 on the subject of Adverse Temperature Levels in the Human. The goal of the workshop was to evaluate most recent data useful for the development of science-based RF exposure limits. This paper outlines radiation protection principles that underline such an evaluation. It discusses the quality of literature needed for sound scientific reviews, provides the hierarchy of scientific evidence used to establish effects, distinguish between biological effects and adverse health consequences and indicates how evidence is evaluated. In addition, criteria for determining the most sensitive effects, the value of an effect that has a dose-response and methods of extrapolation are also described. Finally, the need to account for scientific uncertainty in the formulation of guidance on exposure is discussed.

Assessment of guidelines for limiting exposures to emf using methods of probabilistic risk analysis.

Allowable limits of human exposure to radiofrequency fields commonly include a "factor of safety," typically between 10 to 50, which is somewhat arbitrary. The broad objective in our work is to assess radiofrequency exposure limits, hazard thresholds, and safety factors using methods of probabilistic risk analysis. We focus our analysis on the variables affecting peak radiofrequency specific energy absorption rate (SAR) values in the brain from digital mobile telephones operating at approximately 900 MHz. As SAR is defined as a product of positive random variables, it is not unreasonable to assume that SAR has a lognormal distribution. Our analysis of component SAR variables such as conductivity and permittivity of grey brain matter and radiated field strengths using experimental and numerical modeling data strongly supports our hypothesis that SAR values are distributed lognormally. It then follows that the probability that the SAR exceeds a certain threshold can be derived directly and is shown to be very low for handset SARs relative to presently allowable standard limits.

Electromagnetic Fields Around Induction Heating Stoves

Strengths of the magnetic and electric fields around two induction heating stoves were measured at the frequencies of operation of the devices, i.e., in the tens of kilohertz range and at the power line frequency and its harmonics. The measurements were performed for various cast iron and stainless steel cookware.Exposure levels of users of the stoves were compared with existing recommendations for human radiofrequency exposure limits and exposures produced by other home appliances.

Microwave and radio-frequency exposure limits

The paper summarises the exposure limits that have been promulgated by national organisations of different countries, and the rationales that have been advanced for establishing these limits. The limits differ by orders of magnitude, largely because of different philosophical approaches to standard making. The limits were developed, in the first instance, because of concern about high-power radar equipment, but have gradually been extended to cover very much lower frequencies. New standards are being developed in the USA which are much more restrictive as regards RF exposure. The justification for these is examined.