Radiofrequency/Microwave Safety Standards

R. C. Petersen

Introduction

Radiofrequency (RF)/microwave safety standards generally refer to standards, regulations, recommendations and guidelines that specify basic restrictions and exposure limits for the purpose of protecting human health. Contemporary standards are based on the results of critical evaluation and interpretation of the relevant scientific research – ideally, all laboratory and epidemiology research that relates any biological response, from short-term and long-term exposure, would be included. From this evaluation, a threshold is established for the most sensitive confirmed response that could be considered harmful to humans. To account for uncertainties in the data and to increase confidence that the limits are well below the levels at which an adverse effect could occur, the resulting threshold is lowered by a somewhat arbitrary safety factor. RF safety standards have evolved over several decades from a simple single value that is applicable over a broad band of frequencies e.g., 10 MHz to 100 GHz, to sophisticated frequency and time dependent limits that cover a much greater frequency range, e.g., 3 kHz to 300 GHz. The early single-value limits were usually expressed in terms of incident power density and were based on simple models predicting whole-body heating; contemporary standards address effects associated with electrostimulation at low frequencies, effects associated with whole-body heating, effects associated with surface heating and usually include limits on induced and contact, exposure to pulses of high peak but low average power, and localized exposure. The evolution of the development of the standards and guidelines developed by committees of the American National Standards Institute (ANSI), the International Commission on Non-Ionizing Radiation Protection (ICNIRP), the Institute of Electrical and Electrical Engineers (IEEE) and the National Council on Radiation Protection and Measurements (NCRP) is described below.

The early years

Although the interest in the potential effect on humans exposed to RF energy goes back almost a century, it was only toward the end of World War II that a concerted effort was made to try to understand the interaction of RF energy with biological systems and, from this understanding, establish criteria to protect against effects that could be considered harmful. The effort in the United States mainly stemmed from anecdotal reports of various effects by radar technicians and others who came in contact with various military radars, e.g., temporary male sterility from exposure to radar beams, the induction of opacities in the lens of the eye. Although by this time the heating effects of RF energy was well-understood and the technology had been applied in medicine for decades, e.g., RF diathermy, the anecdotal reports in conjunction with studies reporting lens opacities in the eyes of subject animals exposed to microwave energy (e.g., Richardson, et el., , Clark et al. , Daily, et al. ) and a report of cataracts in a radar technician (Hirsch and Parker ), resulted in a coordinated effort to understand the interaction of RF/microwave energy with biological systems and to establish safety limits. As clearly evidenced by the several orders of magnitude differences between the protection guides initially adopted by different organizations worldwide during the mid to late 1950’s, there was little agreement as to suitable protection criteria or an appropriate rationale for establishing these criteria.

Organized efforts to seek an understanding of the possible interaction mechanisms and the effects on human beings of exposure to electromagnetic energy at RF/microwave frequencies began in the United States with a number of meetings and symposia. Such meeting included the “Symposium on Physiologic and Pathologic Effects of Microwaves” held at the Mayo Clinic in 1955, the “First Annual Tri-Service Conference on Biological Hazards of Microwave Radiation” and the “Second Annual Tri-Service Conference on Biological Hazards of Microwave Radiation,” in 1957 and 1958, respectively . The purpose of these meetings was to bring together key researchers in the radiation hazards area in order to discuss ongoing research and identify needed research and, ultimately, establish science-based safety limits. As pointed out by Mumford , during the time period of these symposia, and even before, a number of widely different exposure limits were recommended and used by different organizations in the US. These recommendations, expressed in terms of incident power density, ranged from 100 W/cm² to 100 mW/cm². The upper level was based on an apparent threshold for opacities in the lens of the eye (cataracts), which was estimated by Hirsch and Parker to be of the order of 100 mW/cm². Others, e.g., Williams, et al. and Ely et al. , found higher thresholds but there was general agreement that 100 mW/cm² should be considered an approximate threshold for biological damage, i.e., levels above this value were considered hazardous and should be avoided. In 1953, one Department at Bell Telephone Laboratories added a 30 dB safety factor to the level considered hazardous and adopted 100 W/cm² as a safe level. In 1954 General Electric adopted 1 mW/cm² as a safe level, some organizations informally adopted 10 mW/cm² as a potentially hazardous level, and still others merely adopted 100 mW/cm² as a hazardous level without specifying a safe limit. Based on a number of animal studies and discussions at the symposia noted above, it became apparent by the late 1950’s that 100 W/cm² was too conservative, 100 mW/cm² was probably not conservative enough and most organizations adopted an exposure limit of 10 mW/cm² as recommended by Schwan and Li . This value was based on a simple thermal model that limited the rise in core temperature of an exposed individual to less than 1^ C, assuming that about half of the incident energy was absorbed. The frequency range was 10 MHz to 100 GHz.

USAS C95.1-1966 and ANSI C95.1-1974

In 1960, the first formal RF safety standards project was approved in the US when the American Standards Association (ASA)¹ approved the initiation of Radiation Hazards Standards Project C95 and the establishment of a committee (C95), which was charged with developing standards through an open consensus process. The scope of the committee was “Hazards to mankind, volatile materials, and explosive devices which are created by man-made sources of electromagnetic radiation. The frequency range of interest extends presently from 10 kHz to 100 GHz. It is not intended to include infrared, X-rays or other ionizing radiation.” The C95 Committee, co-sponsored by the Department of the Navy (Bureau of Ships), and the American Institute of Electrical Engineers,² was chaired by Schwan; there were six members on the committee, including the chairman. The committee deliberated for approximately six years and in 1966 the first C95.1 standard, USAS C95.1-1966, was published . The exposure limit was presented as a “Radiation Protection Guide” (RPG), defined as the radiation level which should not be exceeded without careful consideration of the reasons for doing so. The RPG for whole-body exposure was 10 mW/cm² across the frequency spectrum from 10 MHz to 100 GHz. Included were an averaging time of 6 minutes and a corresponding energy density limit of 1 mWh/cm². The 6 min averaging time appears to have come from the diathermy literature, although this is not stated in the standard. It is noted that the RPG is applicable in moderate thermal environments and that under conditions of moderate to sever heat stress the RPG should be reduced accordingly. The entire standard is less than one and one-half pages in length. Although some at the time considered the RPGs only applicable in the occupational environment, nowhere in the standard is this stated or implied.

A revision of USAS C95.1-1966 was published in 1974 by the American National Standards Institute (ANSI) as ANSI C95.1-1974 . The normative part of the standard was still less than two pages in length; the RPGs for continuous whole body exposure, expressed in terms of incident power density and energy density, remained at 10 mW/cm² and 1 mWh/cm², respectively. Because it was then recognized that important exposures could occur in the near field, particularly in the workplace, limits were also given separately in terms of mean squared electric and magnetic field strengths. Although the mean-squared electric and magnetic field strengths were each based on an equivalent power density of 10 mW/cm², it was considered important to assess each independently, at least at frequencies below 300 MHz. It is noted in the standard that the RPGs were based on the currently available literature, it was the consensus of the committee that effects associated with tissue heating remain dominant, and the RPGs should protect against such effects. It is also noted that at the time, sufficient information concerning modulation effects, peak power effects and frequency dependent effects was not adequate to substantiate adjustments to the RPGs to account for these effects . The frequency range over which the RPGs applied remained 10 MHz to 100 GHz. During the eight year development of the 1974 revision the working group (Subcommittee 4)³ had grown considerably in size, totaling almost 70 members.

ANSI C95.1-1982 Standard and the National Council on Radiation Protection and Measurements (NCRP) Recommendations (1986)

Each revision of the C95.1 standard was more scientifically sound, albeit more complex than its predecessor, with major changes appearing in ANSI C95.1-1982 (the revision of ANSI C95.1-1974). These changes were related to the significant advances that occurred in the 1970s in instrumentation and the techniques for measuring complex electromagnetic fields and, most important, in the field of RF dosimetry. Advances in dosimetry included the use of numerical techniques to study energy absorption patterns in simple spheroidal and block models of humans and animals and the use of thermography to study the absorption characteristics of complex realistic models of animals and anthropomorphic models of humans. These studies led to a clearer understanding of the frequency-dependent absorption by objects in an RF field, in particular the pronounced resonance over a narrow range of frequencies, the extent of which depended on the geometry and orientation of the object in the field. Under optimal exposure conditions, i.e., conditions yielding maximal absorption, it was found that the absorption cross section at resonance could be 2-3 times greater than the geometrical cross section (see Figure 1). From this understanding, it became apparent that realistic future protection guides should be frequency-dependent, something that was studied by Soviet scientists in the early 1960s, cf. Pressman . Thus, while the RPGs in the 1966 and 1974 C95.1 standards were independent of frequency, by the mid 1970’s it was recognized that the amount of RF energy absorbed by an object in the field would be frequency dependent—as should the RPGs.

RF dosimetry studies , i.e., the study of RF absorption in models of humans and animals, were carried out by Guy , , Guy et al. , Gandhi, et al. , , Durney , Durney et al. , Hagmann and Gandhi , and others, using thermographic techniques and numerical modeling. These studies led to an understanding of how the incident and internal electromagnetic fields are related as a function frequency, field polarization, and size, geometry, orientation and composition of the object in the field. They also led to the recognition of the need for a dosimetric quantity to relate the incident fields to the internal fields, a quantity that would be more directly related to a biological effect than the incident fields alone. This need became very apparent during the literature evaluation that led to the 1982 standard where the general criticism of the growing body of literature was a complete lack of consistency in reported results, particularly with respect to the field parameters necessary for determining the internal field distributions or energy absorbed from the field. In many cases only the incident power density was reported without mention of other parameters necessary for estimating these quantities and, hence, made the comparison of studies difficult at best. This lack of consistency and completeness also helped explain large differences in the incident power density reported for the same biological effect in different animal species, and in the same animal species under different exposure conditions. It was agreed that an appropriate quantity for establishing meaningful thresholds and allowing comparison across frequency and animal species should be analogous to “dose,” and “dose rate” used by the ionizing radiation community. This then would be the basic parameter that should be reported so that the results of studies at different frequencies, using different animal species and widely different exposure conditions e.g., plane wave, TEM cell, cylindrical cavity, could be compared. Once a threshold for an adverse effect is determined in terms of the “dose rate,” i.e., the rate at which energy is absorbed from the field, the growing understanding of RF dosimetry would provide the means for relating this threshold to the incident fields and, with a suitable safety factor, to realistic frequency-dependent RPGs.

Figure 1—Calculated whole-body average SAR versus frequency for simple models of the average man for three standard polarizations. The incident power density is 1 mW/cm². Curves E and H refer to exposure geometries where the major axis of the body is aligned with the electric field (E) and the magnetic field (H), respectively; K refers to the geometry where the direction of propagation is in the direction of the major axis of the body. (From Durney, et al. )

Various quantities and terms were proposed for an appropriate dosimetric quantity including “absorbed power density” expressed in units of W/cm³ or W/kg, and “dose, and “dose rate,” i.e., the energy imparted to a unit mass of biological material (dose) and the rate at which energy is imparted to unit mass (dose rate). After considerable discussion and debate within C95 Subcommittee 4 (SC4) during the 1970’s, there was consensus that dose and dose-rate were appropriate. To avoid confusion and the connotation associated with terms traditionally used in ionizing radiation protection, “dose” and “dose rate” were named “specific absorption” (SA), defined as the incremental energy absorbed by (dissipated in) an incremental mass, and “specific absorption rate” (SAR), defined as the time rate of incremental energy absorbed in (dissipated in) an incremental mass—specific meaning that it is unique to RF/microwave frequencies.. The units of SA and SAR are J/kg and W/kg respectively. Although SA and SAR first appear as the defining RF dosimetric quantities in the 1981 NCRP Scientific Committee 39 Report (No. 67) , it had already been accepted by C95 SC4 in the late 1970s during the development of C95.1-1982 and was used effectively to compare the results of studies in the database in order to determine an SAR threshold for effects considered adverse. From this threshold and the results of the increasing number of dosimetry studies, frequency-dependent limits expressed in terms of the incident fields were derived. These limits were called Radiofrequency Protection Guides (RFPG) in order to mitigate confusion with the term RPG used by the ionizing radiation community. Compliance with the RFPGs, ensures that the SAR remains below the threshold (with an adequate margin of safety) under various exposure conditions and for various size humans from infants to adults.

As indicated above, the 1966 and 1974 C95.1 standards were based on the assumption that effects to protect against are related to gross thermal effects associated with elevations in core temperature. By 1980, however, a number of studies reporting effects that occurred at levels where significant temperature increases were not observed or expected (i.e., “athermal effects”) began to appear in the scientific literature. These studies warranted careful examination and were included in the list of citations considered by SC4 during the development of the 1982 C95.1 standard. (It is pointed out in the 1982 standard that “classification and judgment of findings were made without prejudgment of mechanisms of effects,” i.e., the intent of the subcommittee was to protect exposed humans against harm from adverse effects associated with any interaction mechanism, including effects associated with an elevation in body temperature” ). During the literature selection process that led to ANSI C95.1-1982, several hundred experimental studies reporting effects associated with RF energy were reviewed and a select list of 32 studies was compiled in accordance with the following criteria: demonstrability (positive effects), relevance, reproducibility and dosimetric quantifiability (i.e., was the SAR reported or was there enough information in the report regarding the exposure setup to allow determination of the SAR).

Studies that demonstrated general evidence of morbidity or debilitation, chronic or acute, were emphasized . The bias toward positive findings added a degree of worst-case conservatism to the resulting exposure limits. When positive results were demonstrated for a specific biological endpoint by several laboratories, those studies that demonstrated the effect at the lowest SAR and longest exposure duration were selected. Biological endpoints were grouped in the 15 categories shown in Table 1 along with the number of studies that met the selection criteria in each category. Reports of specific effects induced by low frequency amplitude-modulated RF carriers, e.g., calcium efflux from chick brain tissue, were included but were not considered adverse for the following reasons: inability of the SC4 members to relate the effect to human health; the narrow range of effective modulation frequencies; the study author’s finding that the effect is reversible. The studies were reviewed by the biologists on SC4 and also by the physically trained scientists and engineers with emphasis on reliability, evidence of adverse effects, and whether the study had been independently replicated in another laboratory. The engineers also determined the SAR for each of the studies.

Following the critical evaluation of the selected studies, the subcommittee agreed that the most sensitive, reliable confirmed biological response that could be considered potentially harmful to humans is disruption of food-motivated learned behavior. Even though this effect is, modest, transient and represents an adaptive response, it serves to identify a threshold for potentially harmful effects . It was also assumed that while behavioral disruption was demonstrated to be transient and reversible after acute exposure, chronic exposure could lead to irreversible injury. The threshold for behavioral disruption was found to reliably occur within a narrow range of whole-body-averaged SARs between approximately 4 to 8 W/kg, across animal species from rodents to primates, frequencies from 600 MHz to 2450 MHz, and incident power densities that ranged from 10 to 50 mW/cm². Thus it was agreed by SC4 that the appropriate biological endpoint for acute exposures should be disruption of behavior, and the corresponding threshold, in terms of whole-body-average SAR, should be set at 4 W/kg. That is, SAR values above 4 W/kg could produce adverse effects while SARs below 4 W/kg were not shown to result in effects that could be considered hazardous.

Table 1—Category of exemplary reports selected from the experimental literature by SC4 for the development of ANSI C95.1-1982

	Environmental factors (effects of temperature on the specific endpoint – 3 studies
	Behavior and physiology – 6 studies
	Immunology – 4 studies
	Teratology – 1 study
	Central nervous system/blood-brain barrier – 4 studies
	Cataracts – no reliable studies were found reporting cataracts at levels  10 mW/cm2
	Genetics (no reliable studies were found reporting genetic effects at levels  10 mW/cm2
	Human studies – no reliable human studies were found
	Thermoregulation and metabolism – 5 studies
	Biorhythms – 1 study
	Endocrinology – 3 studies
	Development – 3 studies
	Evoked auditory response (RF hearing) – no studies
	Hematology – 2 studies
	Cardiovascular – 1 study

There was considerable deliberation during the development of ANSI C95.2-1982 as to an appropriate margin of safety and whether a single frequency-dependent RFPG should apply to exposures of the public and the worker. In order to ensure an adequate margin of safety, a safety factor of 10 was incorporated, which was considered adequate to protect members of the public and the worker because of the conservatism already built into the 4 W/kg threshold. Thus, a whole-body-averaged SAR value of 0.4 W/kg was adopted as the basis of the standard (basic restriction) from which the frequency-dependent RFPGs (also called derived limits, investigation levels, reference levels) would be derived.

By 1980 the field of RF dosimetry had advanced to the point where reliable techniques were available to determine the incident power density that would limit the whole-body-averaged SAR to a specific value. Theoretical analyses were carried out to determine the magnitude of the incident fields that would limit the whole-body-averaged SAR to 0.4 W/kg under worst-case exposure conditions, i.e., conditions that would maximize energy absorption. The results of these analyses demonstrated that under plane wave exposure conditions, energy absorption in models of humans, ellipsoids, animals, etc., is generally maximal when the major axis of the exposed object is aligned with E-field vector of the incident field and, under these exposure conditions, absorption increases with the square of frequency, reaches a maximum (resonance), then decreases linearly with increasing frequency over a limited range of frequencies, and then remains relatively constant. Work by Gandhi and others showed that under these conditions, maximum absorption (resonance) occurs when the length of the long axis of the exposed object is approximately 0.36 to 0.4 wavelengths. For example, the resonant frequency varies from about 79 MHz to 54 MHz, respectively, when the height of the body ranges from 1.52 m to 1.98 m. Moreover, it was found that when the object is in contact with a ground plane, the resonant frequency is about one-half the value found when it was not in contact. Data were complied from a number of dosimetry studies and a family of resonance curves and plotted as a function of whole-body-average SAR versus frequency for humans ranging in size from infants to tall adults, both in and not in conductive contact with a ground plane and with the long axis of the body parallel to the E-field vector of the incident field. The result of this exercise, normalized to an incident power density of 1 mW/cm² (which limits the maximum SAR at resonance to 0.4 W/kg) is shown in the Appendix of C95.1-1982, . The RFPGs were obtained by rearranging these data to determine the maximum incident power density that would limit the whole-body-averaged SAR to 0.4 /kg across the frequency range of interest. The results showed that the incident power density required to maintain an essentially constant SAR in humans of all sizes could be approximated by a broad resonance curve that decreased as 900/f² up to 30 MHz, above which it remained constant up to 300 MHz, then rose as f/300 to 1500 MHz, above which it remained relatively constant at 5 mW/cm². Although the limiting incident power density continues to increase with decreasing frequency for frequencies below 30 MHz, the RFPG was limited to 100 mW/cm² for frequencies below 3 MHz to prevent reactions at the body surface caused by the relatively high E-fields (> 600 V/m), e.g., perception and electric shock. The averaging time remained 6 min over the entire frequency range. There was some concern by members of the subcommittee about the 6 min averaging time as it applies to pulses of high peak power but low average power because time averaging single pulses of extremely short duration, e.g., a few microseconds, leads to unrealistically high exposure limits. No agreement was reached on how to treat this situation and, therefore, there are no explicit peak power limitations in the 1982 standard.

In addition to RFPGs for whole-body exposures, the 1982 standard contained the following exclusions: 1) The RFPG for whole body exposure at frequencies between 300 kHz and 100 GHz could be exceeded if it could be shown using laboratory techniques that the resulting SAR averaged over the whole body would not exceed 0.4 W/kg and the peak spatial average SAR could not exceed 8 W/kg as averaged over any one gram of tissue. The 8 W/kg was based on the peak to average SAR values reported in a number of animal studies where it was found that typically the peak to average SAR ratio was 20 to 1; 2) At frequencies between 300 kHz and 1 GHz, the RFPGs could be exceeded if the RF input power to the devices is 7 W or less, which is based on limiting the peak spatial-average SAR to 8 W/kg.

Finally, the standard contained the following caveat: “Because of the limitations of the biological effects database, these guides are offered as upper limits of exposure, particular for the population at large. Where exposure conditions are not precisely known or controlled, exposure reduction should be accomplished by reliable means to values as low as reasonably achievable [ALARA] .” This last sentence often has been quoted out of context by applying it to RF exposure in general.

NCRP Report No. 86

A number of important events occurred during the interval between approval of ANSI C95.1-1982 and IEEE C95.1-1991, including comprehensive reviews of the extant RF bioeffects literature by a scientific committee of the National Council on Radiation Protection and Measurements (NCRP).⁴ Although NCRP is concerned mostly with ionizing radiation, in 1973 Scientific Committee 53 (SC53 – now SC89-5) was convened to carry out a comprehensive review the scientific literature and make recommendations for limiting exposures to RF energy. SC53 consisted of 6 members, 5 advisory members and 5 consultants (NCRP )—8 of whom were at the time also members of SC4 of the ANSI C95 committee. Whereas SC4 adopted criteria for selecting studies specifically relevant to standard setting (e.g., demonstration of positive effects, relevance, reproducibility, dosimetric quantifiability), and consequently reviewed in detail a relatively small number of reports, SC53 carried out complete review of the literature and close to 1000 studies were included in the NCRP literature evaluation (the literature cutoff date was 1982 but a few 1983 references are included). With the quality of the selected reports ranging from excellent to poor, including some which appear to be nothing more than anecdotal reports, value judgments had to be made in interpreting and assessing the quality of each of the studies. Reports were divided roughly by biological endpoint into the categories shown in Table 2.

Table 2—Category of reports reviewed NCRP SC53 SC4 during the development of NCRP Report 86

As did SC4 of the C95 committee, the members of SC53 also concluded that the most sensitive and statistically significant biological endpoint was behavioral disruption. Although the carrier frequencies for behavioral disruption ranged from 225 to 5800 MHz, across animal species from laboratory rats to rhesus monkeys (see Table 3), the incident power densities ranged from 8 to 140 mW/cm² and the exposure conditions included near field, far field, planewave, multipath, CW and modulated RF, the SAR threshold for behavioral disruption narrowly ranged from 3 to 9 W/g, which is in fair agreement with the threshold reported in ANSI C95.1-1982.

Table 3—Comparison of power density and SAR thresholds for behavioral disruption in trained laboratory animals (from Osepchuk and Petersen )

Species and Conditions	225 MHz (CW)	1.3 GHz (Pulsed)	2.45 GHz (CW)	5.8 GHz (Pulsed)
Norwegian Rat Power Density SAR	— —	10 mW/cm2 2.5 W/kg	28 mW/cm2 5.0 W/kg	20 mW/cm2 4.9 W/kg
Squirrel Monkey Power Density SAR	— —	— —	45 mW/cm2 4.5 W/kg	40 mW/cm2 7.2 W/kg
Rhesus Monkey Power Density SAR	8 mW/cm2 3.2 W/kg	57 mW/cm2 4.5 W/kg	67 mW/cm2 4.7 W/kg	140 mW/cm2 8.4 W/kg

For the frequency range where surface effects predominate, SC53 went further than the C95 committee and recommended lowering the RFPG if there is a likelihood of coming into contact with grounded metallic objects. To prevent RF burns at the point of contact, the recommendation was to lower the RFPG such that the induced RF current does not exceed 200 mA. This is to be done on a case by case basis.

Recommendations, based on the low-frequency modulation-specific effects literature, e.g., calcium efflux studies, were also included. It was pointed out that it is not known whether these affects lead to a risk to human health, but the reliability of the studies and their independent confirmation in avian and mammalian species dictates the need for caution . The recommendation is as follows: “If a carrier frequency is modulated at a depth of 50% or greater at frequencies between 3 and 100 Hz, the exposure criteria for the general population shall also apply to occupational exposures.” The incorporation of this caveat, which was based on reported frequency and intensity “windows,” was extremely controversial and has not been accepted by other standard-setting bodies or incorporated into contemporary science-based standards and guidelines.

Although the RFPGs in the 1982 C95.1 standard and the NCRP Report are far more realistic and sophisticated than those used before 1982, both suffer serious shortcomings, including the following: 1) There are no limitations on peak power for pulses of high intensity but low average power—the 6 min averaging time allows exposure to short pulses in excess of those known to cause burns at frequencies where the energy is deposited superficially, i.e., frequencies above a few GHz; 2) There is no explicit guidance to limit induced current at the lower frequencies, i.e., frequencies below a few MHz, to prevent electric shock and RF burns; 3) The magnetic field strength limits, which correspond to the equivalent plane wave power density of the RFPG, is unrealistic at the low frequencies where the magnetic field is inefficiently coupled to the body; 4) No clear distinction is made between whole-body and partial-body exposure (Petersen ). Many of these issues were addressed in the next revision of the 1982 C95.1 standard, i.e., IEEE C95.1-1991.⁵ Table 4 is a comparison of the rationale between NCRP Report 86 and ANSI C95.1-1982.

Table 4—Comparison of rationale: ANSI/NCRP (from Petersen )

*General population

Institute of Electrical and Electronics Engineers (IEEE) C95.1-1991⁶

Almost immediately after ANSI C95.1-1982 was published, Subcommittee 4 of the ANSI C95 Committee began work on the next revision with emphasis on addressing some of the recognized shortcomings mentioned above. As with the earlier revisions, the literature was culled for relevant studies and a total of 321 papers were identified by the Literature Surveillance Working Group. (See Figure 2 for a graphical depiction of the literature evaluation process.) Although most of the selected reports were published before 1985, several reports, particularly those relating to shock, burns and peak-power effects, were published after 1985. Those peer-reviewed studies that reported effects at whole-body averaged SARs less than 10 W/kg, and which also met the criteria of the Engineering, Biological and Statistical Evaluation Working Groups, were sent to the Risk Assessment Working Group, whose charge was to determine the threshold SAR above which potentially deleterious effects are likely to occur in humans, even if the effects are reversible . As in the case of ANSI C95.1-1982 and the NCRP Report, the working group concluded that a threshold SAR of 4 W/kg is appropriate to protect against behavioral disruption, which was once again found to be the most sensitive and reliable biological endpoint. Without saying that behavioral disruption is a “thermal” effect, it was noted that behavioral disruption in laboratory animals was accompanied by a core temperature increase of approximately 1 C and the effect, regardless of the interaction mechanism, was reversible. Effects reported to be non-thermal, e.g., modulation specific effects such as changes in calcium efflux from chick brain tissue, were again considered but it was the consensus of the Risk Assessment Working Group that such effects were inconsistent, could not be related to human health, and, therefore, not useful for standard setting. It was also the consensus of the Risk Assessment Working Group that a safety factor of 10 to account for dosimetric, biological and other uncertainties would provide an adequate margin of safety, thereby yielding a basic restriction of 0.4 W/kg in terms of whole-body-averaged SAR.

Figure 2—Graphical depiction of the ANSI/IEEE literature evaluation process

Unlike C95.1-1982, however, which consisted of a single tier that was considered protective of all, the SC4 Societal Implications Working Group recommended following NCRP and including a separate lower tier for exposures that take place in uncontrolled environments. There recommendation was based on the following argument : “To some, it would appear attractive and logical to apply a larger, or different, safety factor to arrive at the guide for the general public. Supportive arguments claim subgroups of greater sensitivity (infants, the aged, the ill, and the disabled), potentially greater exposure durations (24 hours/day vs. 8 hours/day), adverse environmental conditions (excessive heat and/or humidity), voluntary versus involuntary exposure, and psychological/emotional factors that can range from anxiety to ignorance. Non-thermal effects, such as efflux of calcium ions from brain tissues, are also mentioned as potential health hazards .” However, this is followed by “The members of Subcommittee 4 believe the recommended exposure levels should be safe for all, and submit as support for this conclusion the observation that no reliable scientific data exist indicating that

1. Certain subgroups of the population are more at risk than others;

2. Exposure duration at ANSI C95.1-1982 levels is a significant risk;

3. Damage from exposure to electromagnetic fields is cumulative; or

4. Non-thermal (other than shock) or modulation-specific sequelae of exposure may be meaningfully related to human health.”

and “No verified reports exist of injury to human beings or of adverse effects on the health of human beings who have been exposed to electromagnetic fields within the limits of frequency and SAR specified by previous ANSI standards, including ANSI C95.1-1982.” Thus, any scientific justification for the lower tier is tenuous at best. However, the C95 standards are developed through an open consensus process and the majority of the voting members agreed that a lower tier is appropriate.

The lower tier was derived by reducing the upper tier value by a factor of 5, at least in the resonance region where SAR is important resulting in a whole-body-averaged SAR of 0.08 W/kg. However, unlike other standards and guidelines that set limits based on population groups, i.e., an upper tier for occupational exposure and a lower tier for exposure of the general population, the committee concluded that it would be more meaningful to address the exposure environment rather than the exposed population to help clarify the assignment of an appropriate set of limits to personnel, particularly in the workplace. Thus, the derived limits of the upper tier, now referred to as maximum permissible exposure values (MPE) to be consistent with the use of the term in other standards relating to non-ionizing radiation protection, e.g., the laser safety standards ANSI Z136.1 and IEC 60825, apply to exposures in controlled environments; the MPEs of the lower tier apply to exposures in uncontrolled environments. Controlled environments are considered locations where exposures may be incurred by individuals who are aware of and have control of their potential for exposure, e.g., as a concomitant of their employment, or by other cognizant persons; uncontrolled environments are locations where there is exposure of individuals who have no knowledge or control of their exposure (in living quarters, offices or in workplaces where there are no expectations that exposure levels may exceed the MPE recommended for lower tier). (See Figure 3 for graphical representation of the IEEE C95.1-1991 MPEs.)

F
igure 3—Graphical representation of the C95.1-1991 MPEs expressed in term of the E-field equivalent plane wave power density

When a draft is finally approved by the working group, following the same process mandated for sponsor balloting described below, (except that balloting is carried out by SC4—not the IEEE SA Balloting Center), the draft is submitted to the IEEE SA Balloting Center for sponsor ballot (i.e., by the parent committee – SCC28). Sponsor balloting begins when the IEEE Balloting Center notifies members of the balloting pool that a standard is ready for sponsor ballot and invites members to join the Ballot Group for that standard. The balloting pool consists of the parent committee members (SCC28 – now Technical Committee 95 of the IEEE International Committee on Electromagnetic Safety—ICES) ¹⁰ plus all interested parties that may have joined the balloting pool. The balloting pool is open to any IEEE Standards Association (IEEE SA) member, or any non-member who elects to pay a nominal fee to vote and receive drafts. Members of the parent committee who wish to vote, but are not members of the IEEE SA, first have to be approved by the SASB. Requests from the sponsor chair to the SASB secretary outlining why these individuals should be permitted to vote, what they bring to the committee, etc., are usually placed on the consent agenda of the next quarterly SASB meeting and, unless pulled off for discussion, are approved with the agenda. During this time the standard usually undergoes a mandatory editorial review by IEEE Standards Department project editors, a review by SCC10 (Terms and Definitions – to ensure that all terms and definitions are in accord with IEEE definitions where such definitions exist), SCC14 (Quantities, Units, and Letter Symbols – to ensure consistent use of units and letter symbols), and, in some cases, a legal review.

Approval at the sponsor level requires a 75% response from the members of the Ballot Group (including abstentions) and 75% affirmative votes (the ratio of positive to positive plus negative votes) after ballot resolution. Attempts must be made to resolve every negative ballot and every substantive comment that accompanied a ballot, and their resolution (by an ad hoc ballot resolution working group) must be circulated to the Ballot Group to allow each member to confirm, change his or her vote or comment (only on issues raised during the initial ballot or previous recirculation ballot). Once a consensus is achieved the standard, ballot results, copies of the PAR, copies of the recirculation ballots, ballot resolution, and other relevant material are submitted to the SASB Review Committee (RevCom), also a standing committee of the IEEE SASB. RevCom reviews the scope of the standard to ensure that it is in accord with the scope of the PAR, that the draft has gone though legal review (when necessary), editorial review, review by SCC10, and SCC14 and that the Policies and Procedures of the sponsor and those of the IEEE SASB have been meticulously followed to ensure that the process was open, transparent, and due process was afforded at every level. When these conditions are met, RevCom can deem the ballot valid and recommend approval by the IEEE SASB. (RevCom deals only with procedural issues—not technical issues.) Once approved, the draft standard becomes an IEEE standard and is forwarded to ANSI for public comment and recognition as an American National Standard. Because of the potential sensitivity of the C95 standards, ICES Policies and Procedures require formal balloting at the working group level adhering strictly to IEEE SASB procedures, i.e., all members of SC4 are invited to join the ballot group, all comments submitted with each ballot are addressed, each revised draft resulting from ballot resolution and all comments and their disposition are circulated to all members of the ballot group to allow them to reaffirm or change their original vote.

IEEE C95.1-2005

C95.1-2005 is far more detailed and inclusive than its predecessor C95.1-1991. The 2005 standard is divided into two major parts—normative and informative. The normative part contains the scope and purpose, the normative references, definitions, recommendations (basic restrictions and MPEs), rules for assessing compliance and the role of an RF safety program. The informative part contains 7 Annexes. The first three explain the revision process, summaries of the literature by biological endpoint, and the rationale for the revision. Examples of practical applications of the standard to typical exposure situations are also included as is a glossary of commonly used terms, the bibliography of seminal papers from the International EMF Project (IEEE/WHO) database that are cited in establishing the basic restrictions and thresholds, and a bibliography of other cited publications.

At the time C95.1-2005 was approved, SC4 had 132 members, 42% from outside the US representing 23 countries. Of the 132 members, 36 were from academia, 56 from laboratories and administrative branches of federal agencies and the Department of Defense, 22 were from industry, 26 were independent consultants, and 2 represented the general public. Of these, 73 participated in the balloting, 57 approved, 5 disapproved with comments and 11 abstained, resulting in 92% approval. During sponsor balloting, the Ballot Group had 59 members, 58 returned ballots, 51 approved, 2 disapproved with comments, 1 disapproved without comments and 4 abstained, resulting in 96% approval. The standard has grown in length from less than one and one-half pages (C95.1-1966) to more than 250 pages—the majority of which addresses the literature reviews and evaluations and the rationale, particularly as it applies to changes.

As with the earlier C95.1 standards, the revision of the 1991 standard began with the identification of relevant papers by the SC4 Literature Surveillance Working Group. The focus was on the identification of reliable studies reporting biological responses – from reversible effects and responses of adaptation to irreversible and biologically harmful effects. At the literature evaluation cutoff date, 31 December 2003, the Literature Surveillance Working Group identified over 2200 papers from a number of databases and inputs from federal agencies and other organizations that were regularly polled. Findings of studies published between 1950 and December 2003 were considered, including a number of studies reviewed for C95.1-1991. Although the literature cutoff date was December 2003, a few papers published in 2004 and 2005 were included. New insights gained from improved experimental and numerical methods and a better understanding of the effects of acute and chronic RF electromagnetic field exposures of animals and humans were considered during the evaluation process. Every attempt was made to include and to evaluate all of the relevant literature in the database, with emphasis on studies carried out under low level exposure conditions where increases in temperature could not be measured or were not expected. SC4 agreed that only peer-reviewed papers and technical reports of original research would constitute the primary database on which any risk analysis would be based. Abstracts and presentations at scientific meetings or technical conferences were expressly excluded. A list of all 1143 papers that were evaluated during the development of C95.1-2005 can be found in Annex E of the standard.¹¹

The literature evaluation was carried out by the Engineering, Epidemiology, In vivo, and In vitro Working Groups. In addition, a Mechanisms Working Group was established to evaluate the technical significance of particular interaction mechanisms with regard to standard-setting. The Engineering WG was tasked with assessing of the exposure systems, field characteristics and measurements, dosimetry, specific absorption rates, induced currents and fields, and temperature/humidity measurements and whether or not the information provided was sufficient to allow a full understanding of how the experiment was performed.

The Epidemiology WG was originally tasked with the evaluation of each paper for study design and population segments, quality of the methods and implementation, merit of data acquisition and analysis for specific endpoints, and presence or absence of positive statistical associations. Similarly, the In Vivo and In Vitro WGs examined the technological methodologies employed in each published paper, including the exposure conditions, specific organ systems and/or biological endpoints, the engineering and statistical methodologies employed, and the relevance of each study for standard-setting. The in vitro papers typically emphasized possible effects at the cellular level, including those on cell viability and proliferation, genotoxicity, cell transformation, molecular synthesis, and cell function; the in vivo papers typically examined possible effects of exposure on the whole organism or on specific organ systems, including effects on the embryo/fetus, reproductive ability, immunological system, functional alterations of the metabolic or thermoregulatory system, various histological endpoints, and behavioral changes.

Many of the evaluations went through a formal process beginning with the chair of each WG providing copies of each paper to two independent reviewers, together with specially designed and approved review forms. These forms were in a computer format that required numerical scoring by individual reviewers for entry into a computerized database. When a review was completed, the reviewer gave the paper an overall technical merit rating on a 5-point scale. The rating scale was: Very High = 5; Moderately High = 4; Acceptable = 3; Low = 2; and Very Low = 1. For ratings of 1 or 2, a request was made for justification for the low score in writing by the reviewer. Strong discordance between the two reviews of a given paper required a third independent review. Periodically, the chair of each WG would submit a summary of the completed evaluations to the Chair of the Risk Assessment WG (RAWG) whose charge was to evaluate the implied risk for human beings of exposure to RF electromagnetic fields.

After several years it became clear that the literature evaluation process would not be completed on time following the formal protocol described above. While the engineering WG evaluated nearly all of the papers in the database and the In Vivo WG evaluated more than 90% of their assigned papers, few epidemiology and in vitro papers were evaluated by members of their respective WGs because of a lack of qualified reviewers. Rather than try to evaluate every paper in the database following the protocol described above, certain individuals with considerable expertise in specific areas volunteered or were asked to prepare review papers and summarize their findings in specific topic areas. These included, for example, cancer induction or promotion, teratologic effects, ocular effects, epidemiology, thermoregulation, and animal behavior. In each topic area, one of the goals was to search for definable hazards. The texts and conclusions of the various review papers were made available to the RAWG; the summaries and conclusions from each review paper, which appear in Annex B of C95.1-2005, were further enhanced by 12 review papers published in Supplement 6, 2003 of Bioelectromagnetics . These included reviews of the epidemiology and in vitro literature. The evaluation process took advantage of all completed evaluations in the computerized database plus the review papers.

The overall results of the literature evaluation and review process were used to determine the thresholds of individual responses and dose response functions, i.e., the lowest level at which a potential harmful effect occurs and the function that relates dose rate, e.g., SAR, to response magnitude. The weight of evidence approach was used throughout to develop the thresholds and dose response functions, i.e., the same approach used to develop guidance for assessment of risk from chemical and other physical agents known to be hazardous.¹² SC4 agreed that the recommendations (basic restrictions and MPEs) should protect against “established adverse health effects in human beings associated with exposure to electric, magnetic and electromagnetic fields in the frequency range of 3 kHz to 300 GHz” . The term adverse health effect is defined as “A biological effect characterized by a harmful change in health.” Notes to the definition point out that 1) adverse effects do not include biological effects without a harmful health effect, changes in subjective feelings of well-being that are a result of anxiety about RF effects or impacts of RF infrastructure that are not physically related to RF emissions, or indirect effects caused by electromagnetic interference with electronic devices, and 2) sensations (perceptions by human sense organs) per se are not considered adverse effects. Thus a sensation of warmth at millimeter and other wavelengths and the microwave auditory effect under the underlying special conditions are not recognized as effects to be protected against by this standard. Painful or aversive electrostimulation resulting from exposure at frequencies below 0.1 MHz is treated as an adverse effect” . This definition, though somewhat narrower than the WHO definition of adverse effect, i.e., “A biological effect that has a detrimental effect on mental, physical and/or general well being of exposed people either in the short-term, or long term” (cf. ), was chosen to eliminate some of the ambiguity and subjectivity associated with the broader definition.

Once the hazard threshold was identified and enough supporting information was available, a safety factor was applied to the threshold to derive the basic restrictions and MPEs based on the best available scientific information using the conservative approach common in standard setting. The safety factor, which is influenced by the uncertainty in the knowledge of the degree of hazard associated with the hazard threshold, is selected to prevent exceeding the hazard threshold value with a sufficiently wide margin. The magnitude of a safety factors in the 2005 standard ranges from unity at low frequencies, where effects associated with electrostimulation occur, e.g., sensation, to significantly greater values at frequencies where heating effects occur, i.e., above 100 kHz. In all cases, however, the selection of the appropriate safety factor was based on informed expert opinion after considering the underlying biological and engineering uncertainties applicable to the exposed population for a broad range of exposure conditions.

The results of the literature evaluation and review process by the SC4 working groups did not provide evidence that would warrant a change in the scientific basis for the adverse effect level for frequencies between 100 kHz and 3 GHz. The threshold value for whole-body average SAR was again found to be 4 W/kg, and again the most reliable reproducible biological endpoint was found to be behavioral disruption of food-motivated behavior in laboratory animals, including non-human primates. Although this conclusion was based on the results of animal studies, it was agreed that whole-body-averaged SARs above 4 W/kg could be potentially harmful in humans. This is the same threshold SAR and endpoint found during the development of C95.1-1982, C95.1-1991, the 1986 NCRP report and the 1998 ICNIRP guidelines. The upper boundary of the frequency range over which whole-body-average SAR is considered the appropriate basic restriction metric was reduced from 6 GHz in the 1991 standard to 3 GHz based on RF penetration depth calculations. Also, peak spatial-average SAR values were changed from 1.6 W/kg and 8 W/kg for the lower and upper tiers to 2 W/kg and 10 W/kg, respectively, and the corresponding tissue averaging mass was changed from 1g to 10 g. This change is based partially on the biologically based rationale of ICNIRP related to exposure of the eyes, the extensive theoretical biophysical research quantifying RF energy penetration in biological tissue, and the desire to harmonize with the ICNIRP guidelines where scientifically justified.

The rationale to set exposure limits for effects associated with electrostimulation at the lower frequencies and temperature-related effects at higher frequencies is explained thoroughly in the standard. Improved numerical and measurement methods in RF dosimetry have increased knowledge about the SAR-temperature relationship following RF energy deposition in human tissue, which is essential when assessing potential biological and health effects of RF exposures. A number of special considerations have been reviewed and are explained in detail in the annexes of the standard.

The 2005 standard incorporates a reasonably large margin of safety and, unlike the earlier standards, an RF safety program is required to provide part of the margin of safety for those exposed above the lower tier, now called an “action level,” rather than exposures in uncontrolled environments. The choice of the term “action level” for the lower tier, rather than limits for the “general public” or “uncontrolled environment,” stems from the fact that the committee concluded that the weight of scientific evidence supports the conclusion that there is no measurable risk associated with RF exposures below the basic restrictions of the upper tier of this standard. The lower tier, with an additional safety factor, recognizes public concerns and also supports the process of harmonization with other recommendations and guidelines, e.g., the NCRP recommendations and the ICNIRP guidelines, and defines the level above which implementation of an RF safety program is recommended. The purpose of the action level is to initiate measures, i.e., implementation of an RF safety program as defined in IEEE C95.7-2005 , to prevent exposures above the upper tier. (The basic restrictions and MPEs of the lower tier can be used for the general population.) The standard is especially conservative, since the safety factors are applied against perception phenomena (electrostimulation and behavioral disruption), which are far less serious effects than any permanent pathology or even reversible tissue damage that could occur at much higher exposure levels than those for perception phenomena .

This revision of IEEE Std C95.1 maintains many of the characteristics of the previous standard but also contains a number of differences from earlier editions that address new dosimetry findings and that simplify the use and application of the standard. These similarities and differences are described in Annex C of C95.1-2005 and are summarized below.

Similarities:

• All relevant reported biological effects at either low (“non-thermal”) or high (“thermal”) levels were evaluated. Research on the effects of chronic exposure and speculations on the biological significance of low-level interactions have not changed the scientific basis of the adverse effect level.

• Whole-body-average and peak spatial-average SAR remain the basic restrictions over much of the RF spectrum and remain the same as in the earlier standards and guidelines, i.e., 0.4 and 0.08 W/kg.

• The MPE for exposures in controlled environments remain the same as in C95.1-1991.

• The averaging time remains 6 min for frequencies below 3 GHz for effects associated with tissue heating; but the averaging time for effects associated with electrostimulation is now 0.2 s for an rms measurement (not 1 s as in the 1999 Supplement to C95.1-1991 ).

Differences:

• Both C95.1-1991 and C95.1-2005 contain two tiers. While the weight of scientific evidence supports the conclusion that no measurable risk is associated with RF exposures below the limits of the upper tier, it is impossible to scientifically prove absolute safety and, hence, a lower tier has been set with an extra margin of safety. The lower tier recognizes public concerns, takes into account uncertainties in laboratory data and in exposure assessment, and supports the process of harmonization with other standards, e.g., the NCRP recommendations and the ICNIRP guidelines. While the basic restrictions and MPEs of the upper tier in both standards apply to exposures in controlled environments; the lower tier of the 2005 standard is now an action level, rather than specific limits for exposures in uncontrolled environments. This action level, above which an RF safety program shall be implemented to protect against exposures that exceed the upper tier, is tied to C95.7-2005 (RF safety programs) . For practical purposes, however, the lower tier may also be used for the general public.

• The upper frequency boundary over which the whole-body-averaged SAR is deemed to be the basic restriction (i.e., the “resonance” region) has been reduced from 6 GHz to 3 GHz.

• The lower tier MPEs for long-term exposure are different from those in C95.1-1991 and are in general more restrictive between 300 MHz and 300 GHz.

• The peak spatial-average SAR values have been changed from 1.6 W/kg and 8 W/kg for lower and upper tiers to 2 W/kg and 10 W/kg, respectively.

• The averaging mass for determining the peak spatial-average SAR has been changed from 1 g of tissue in the shape of a cube to 10 g of tissue in the shape of a cube.

• Although implicit in previous versions of the C95.1 standard, e.g., as an SAR exclusion, the present standard explicitly relies on “basic restrictions.”

• The C95.1-2005 requires the development and implementation of an RF safety program in controlled environments.

• A more realistic averaging time (based on the