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Health and safety issues for microwave power transmission
Pergamon
HEALTH
0038-092X(95)00083-6
Solar Energy Vol. 56, No. 1, pp. 53-60, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-092X/96 $15.00 + 0.00
AND SAFETY ISSUES FOR MICROWAVE TRANSMISSION
POWER
J O H N M. O S E P C H U K Full Spectrum Consulting, 248 Deacon Hayes Road, Concord, MA 01742, U.S.A. Abstract--A general public perception that microwaves are hazardous has been a key obstacle for acceptance of microwave power transmission (MPT). This perception will eventually dissipate and then attention will focus on a real technical problem, that of interference (RFI). This can range from perceptible through annoying to hazardous. A program of actions is proposed to accelerate the goal of public acceptance of MPT. In this paper, a historical review shows that the solar power satellite (SPS) was reviewed a number of times relative to potential microwave exposure hazards. In all cases, no "show-stopper" was found but often the shibboleth "more research is needed" was aired. It is shown that standards for safe exposure to microwaves are the most important asset in convincing an audience that microwave exposure associated with M P T or SPS is safe. Standard-setting, world-wide, is shown to converge towards rational limits that are supportive of the M P T / S P S concepts. In recent times there has been the proposed substitute of "risk communication" ("prudent avoidance"). This is an unwise substitute for standards. Other aspects of microwave exposure standards are the new interface with R F I - - h e n c e the need for a rational division of responsibility between the radiators and the victim devices, like medical electronics-using both radiation limits and susceptibility limits. Beneficial applications of microwave exposure are being developed. Several studies are recommended which could put into perspective the likelihood of improbable events that represent "catastrophe"--e.g. the inadvertent focusing of a great amount of energy into inhabited areas.
1. INTRODUCTION
1980) of microwave bioeffects as they related to the SPS. The conclusions in the official review (Gutman, 1980) were that there were no serious anticipated problems (show-stoppers), but it would be wise to continue research on longterm chronic exposures to confirm the safety of environmental levels around an SPS rectenna site. Student perceptions (cited in Gutman, 1980), however, remained typically cynical and displayed an antipathy to "expert" opinions, presumed to be controlled by the establishment. Some student perceptions (MIT, 1973), however, were supportive of the SPS. During the 1980s, as considerable research on long-term chronic exposure to microwaves continued, the perception problem was intensified into a controversy (Steneck, 1985) and a "misperception" (Osepchuk, 1982) problem with extra-scientific dimensions. In 1986, this author (Osepchuk, 1986) reviewed the environmental issues for the SPS and concluded that the "perception" problem remained a number one problem even if no scientific basis for it existed. Other environmental issues were deemed controllable although it was predicted (Osepchuk, 1986) that interference (RFI) would eventually become the most serious technical issue to be faced in the deployment of the SPS. It was stressed that increasing beneficial uses of microwaves, the attendant familiarity with the entity
The idea of microwave power transmission in recent decades was brought forth by W. C. Brown (Brown, 1984) and Dunn (Dunn et al., 1966). Ever since Peter Glaser (Glaser, 1968) proposed the idea of the solar power satellite (SPS), there has been a lingering distrust of M P T in the general public because of vague fears of the effects of microwave exposure. There sometimes is voiced the fear that somehow a malfunction might cause an M P T system to accidentally "focus" its energy onto an inhabited area. This is, of course, an extremely unlikely and maybe impossible event but it forms the basis for the fear of a "catastrophe". The more common worry is that somehow or other longterm chronic exposure to low levels of microwaves might be harmful and even cause cancer. These fears are largely based on misinformation (Osepchuk, 1979) that appears in the writings (e.g. Brodeur, 1978) of some misguided advocates for the public. These fears have risen and fallen but in general they have evolved into a generalized electrophobia (Adair, 1990). This has been intensified in recent years because of the fear of power line fields (Nair et al., 1989). In recognition of these fears, the U.S. Department of Energy (DOE), during its extensive program review of the SPS concept in the 1970s supported a substantial assessment (Justesen, 53
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J.M. Osepchuk
and the benign nature of casual exposures would ease the problem of public acceptance. In the late eighties, the perception problem was complicated and even heightened by the emergence of a fear (Nair et al., 1989) that 60 Hz magnetic fields could cause cancer. As popularized by Brodeur (Brodeur, 1989), the danger is restricted to fields produced by power lines, distribution lines and substations under the control of the utilities, and not appliances and other common exposure sources. His stated viewpoint was that exposures to appliances are transient but exposure to power lines are chronic. Brodeur and the media (e.g. Microwave News, 1990) in this way have intensified the fear of "long-term" (time) exposure without relation to levels (intensities) and totally divorced from scientifically-supported absorbed dose (energy) considerations (Osepchuk, 1992). In 1992, even after thorough exposition (Osepchuk, 1992b) of the facts about electromagnetic energy (bioeffects and RFI), students (ISU, 1992) still voiced an apprehension of the long-term exposure problem while not appreciating the reality of the RFI problem. Opposition to electromagnetic energy has surfaced in a series of well-publicized media events in the U.S.; these have targeted (in rough historical order) radar, microwave ovens, high-voltage power lines, microwave-relay towers, videodisplay terminals, broadcast towers, distribution lines, electric blankets, police radar, hand-held cellular phones and cellular towers. Not targeted have been the children's walkie-talkie which emits more power than most police radars; most household appliances; electric trains (commuting); mobile radio (police, fire, and taxis); amateur radio; medical procedures; as well as infrared/optical sources like lasers, halogen lamps, etc. In a 1994 review (Osepchuk, 1994), the author states that the reasons for these distinctions remain extra-scientific. In fact, today because of the emergence of epidemiological studies, with their weak and inconsistent evidence, as the only credible reason for fearing low-level electromagnetic fields, there is a cry to do away with threshold-based standards. Instead, policies of prudent avoidance (Morgan, 1992) or risk communication (Florig, 1992) are proposed in which no level is assumed safe and probabilistic assessments are generated. These mimic procedures common to the assessment of chemical and ionizing-radiation hazards but without any scientific basis for applying the
non-threshold hypothesis to nonionizing electromagnetic energy. The author concluded (Osepchuk, 1994) that the best response by responsible technologists is the support of standards-setting, public education and a small amount of directed hard-science bioeffect/hazards research. The fact that many other emerging technologies are threatened (Osepchuk and Petersen, 1993) has drawn together their disparate interests into organizations (Electromagnetic Energy Association, 1994) which are devoted to working toward rational standards for safe use of electromagnetic energy, and public education on the benefits as well as the hazards of electromagnetic energy. 2. REVIEW OF B I O E F F E C T R E S E A R C H
Since the beginning of organized research programs on microwave bioeffects in the 1950s (Michaelson, 1971), there have literally been thousands of papers published over the world. By and large, robust effects occur only well above exposure limits existing anywhere in the world. There has been a history of many claims of anomalous low-level, non-thermal effects. However, most of these are found difficult to replicate or exhibit unsatisfying ambiguities--so much so that the term "Cheshire Cat" has been applied (Carstensen, 1987) to describe the elusive character of much of the research on electromagnetic bioeffects. Some, (Foster and Pickard, 1987) have questioned how long one must tolerate such ambiguous research before calling a halt. Even the author (Morgan, 1992) of the "prudentavoidance" concept, himself, has suggested (Morgan, 1986) that there need to be some "stopping rules" for such ambiguous research. With regard to MPT or the SPS systems, a clearly pertinent bioeffect is the effect of microwave radiation on birds. Much research (Little, 1982) has been done on such effects at 2450 MHz. The results clearly show mildly thermal effects that probably are welcomed by birds in the winter and avoided in the summer. There appears not to be any serious threat to the survival of birds exposed to microwaves from such systems. The unique finding from microwave bioeffect research that is not based on gross thermal effects is the "microwave auditory effect" (Lin, 1978). This occurs when the human head is exposed to high peak power pulses, such as occur with radar even though there may be insignificant heating. The subjects hear some-
Health and safety issues what non-specific sounds characterized as "clicks" or "buzzing". Until recently this has been considered an effect associated with highpower systems. It has recently been found, however, (Osepchuk, 1995) that such effects can also be produced by close exposure to low-power sources (< 0.5 W) with an appropriate modulation at frequencies close to acoustic resonance of the human brain. It has been demonstrated to exist only in the frequency range of roughly 0.1-3.0 GHz. Theoretically, it is not expected (if related to acoustic disturbances in the brain) to occur if the radiation is non-penetrating, e.g. above 6 GHz. An important, but often unappreciated, type of research is that on humans. Dr Eleanor Adair (Adair, 1988) has been a leader in experimental human exposures to microwaves. She has been gathering a data-base on sensation as a function of frequency and more recently is doing research on human response to high thermalizing levels of microwave radiation. It is found, at least at 2450 MHz, that the human body is remarkably adaptable to heat stress created by the microwave heating. A myth continually promulgated in the media is that there are a n insignificant number of studies of chronic exposure of animals. Sometimes the simplistic criterion for "chronic" in the media is a near-lifetime (for humans), tens of years, continuous exposure to some unchanging field. This is, of course, unrealistic. Real-life exposures are intermittent as a person moves about in his life. (Only a dead or 100% immobile person would approach an unchanging exposure level in a world where all sources of EM energy are never switched on and off). Thus if animals are exposed for a significant period of time (e.g. 0.5 h or more) each day, (or working day), for many months, this is a chronic exposure. In view of the animals' shorter and accelerated life span, it is a valid procedure to scale from an animal exposure of 1-2 yr to human exposure of many years. On this basis, many chronic exposure experiments have been reported (Guy et al., 1980; Michaelson et al., 1975; Jensh et al., 1983; Chou et al., 1992; Bonasera et al., 1988) to cite only a few exampies. This subject is slated for specific examination at the 1995 EEA BEMS Meetings. Most of the studies have been done with rodents at frequencies of 0.1-10 GHz--especially 2.45 GHz. This is the frequency range where deepest penetration and absorption occurs for rodents. At frequencies > 2.45 GHz, penetration
55
into humans is somewhat superficial, < 2 cm, and effects should be less dramatic in humans than in (the corresponding experiments with) rodents where this radiation is penetrating. On the strength of these experiments, there is no basis for a fear of significant chronic health effects. There have been claims that certain studies (Chou et al., 1992; Szmigielski et al., 1982) support a link of microwave exposure to cancer. The majority viewpoint, however, is that these inferences are isolated and unsupported by further research. Reviews of the whole literature of microwave bioeffect research over the years have consistently agreed with the above viewpoints, always with the caveat that more research is desirable or necessary. These reviews include the comprehensive one in the IEEE (Gandhi, 1980), extensive reviews in the U.K. (NRPB, 1993) and in the International Radiation Protection Association (IRPA/WHO, 1993) have highlighted papers that either support the general assessments or have been considered not useful or applicable to making judgments on safe human exposure to microwaves. Thus, by and large, the overall conclusion of bioeffect research is that microwave exposures are generally benign except for the case of penetrating exposure to intense fields far above existing safe-exposure limits. 3. REVIEW OF SAFETY S T A N D A R D S
Standard-setting for microwave exposure has a long history (Caine, 1972) that begins with the simple 10 mW/cm 2 standard (as averaged over any 6 min). To properly understand exposure standards, one needs to consult Fig. 1 which is an exposure diagram. This depicts the maximum permissible exposure (MPE) (power or power density) as a function of exposure time. This MPE is set a factor of 10-100 below a similar curve that is a threshold of some undesirable effect. The characteristic time T, which may be a thermal time constant, is also the so-called averaging time. The net result is that for exposure time > T the MPE is constant. For exposure time < T, higher MPEs are permitted as long as the total energy (integral of power) is below a set limit. Over the years the standards have changed. In the most recent U. S. standard (ANSI/IEEE C95.1, 1992) the MPEs and averaging time are functions of frequency, and also exist in two tiers--one for the controlled environment
56
J.M.
Osepchuk
use in their licensing procedures for broadcasting and other types of transmitter towers. The U. S. Environmental Protection Agency (EPA, 1994), however, has discarded its extensive preparation (EPA, 1986) for "federal guidance" on environmental levels of radiofrequency radiation, which had been based on ANSI C95.1 (1982). Instead the EPA is now planning a twophase preparation of guidance advice. The first phase is based on existing standards for the near term. The second phase may result in a tighter standard /f studies undertaken by the N C R P under EPA support find valid evidence of hazardous "nonthermal" effects. Such effects have not yet been recognized by any standards body in the world. On the other hand, NATO (NATO, 1994) has proposed dropping the lower tier in their newly proposed standard (called a STANAG). It is important to stress the world-wide consensus on safe microwave levels. This is depicted in Fig. 2. Note that in the microwave range, 1 - 1 0 G H z , even the conservative IRPA limit (IRPA/WHO, 1993) is not too far from the U. S. limit. Therefore there is now and presumably will be for the foreseeable future an international consensus even including the views of the former Soviet Union (Vanke 1994) which are likely to relax upwards with time. Even now, the value of 0.01 mW/cm 2 applies as an average over 8-24 h / d a y - - s o that for short periods of time much higher levels are permitted up to 1 mW/cm 2. Thus the official health agencies, world wide, agree that levels (e.g. < 0 . 0 1 m W / c m 2) likely outside of restricted
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Health and safety issues rectenna areas of an SPS or M P T system are safe for indefinite duration. These judgments reflect the worldwide scientific consensus that there is no scientific basis to fear long-term microwave exposure as it would exist near M P T or SPS systems. In fact, exposures in the environment from broadcasting (Tell and Mantiply, 1980) has existed for decades, with a small part of the population chronically exposed to levels >0.001 or even 0.01 mW/cm 2 with no reported associated health effects. This is to be added to the extensive exposure history of probably more than one million patients to microwave diathermy (Lehmann, 1973) with no undue effects. Recently, there has been some attention (U.S. Congress, 1994) to the existence of potentially hazardous RFI from cellular phones. This has restimulated work towards a rational separation of the need for susceptibility limits (e.g. on aircraft or medical devices) versus the need for additional limits on radiation from transmitters to protect electronic circuits which are not as rugged as the human body in the presence of electromagnetic energy (Osepchuk, 1982). A very visible current issue is the safety of cellular phones (Fischetti, 1993)--both the hand-held phone to the user and the environmental exposure from cellular towers (which produce levels generally <<0.001 mW/cm 2 in the environment). Even here there is public opposition because of perceived risks. To overcome the siting dispute dilemma, the support of standard-setting groups is essential. Increasing strength of these groups will enhance their credibility and avoid the dismal alternatives of a "non-thermal", "prudent avoidance" policy or a "risk-communication" management policy. With respect to safety of devices like the hand-held radio, the EEA (EEA, 1994) has proposed to create an accredited standards committee for the purpose of developing product performance standards for items like the cellular phone, the VDT, police radar, and electric blanket. These standards would be like the successful microwave-oven emission standard developed by the FDA or the classification standards for lasers developed by an accredited standards committee with the participation of FDA and many other groups. The IEEE has indicated a willingness to collaborate with EEA in this matter. If successful, standards for new products would ease their public acceptance just as the standards for microwave ovens and lasers have
57
greatly helped their public acceptance (nobody has raised fears of the popular laser pointers now commonly used by lecturers). The EEA/IEEE project is designed to fill the vacuum created when the FDA decided in 1980 not to develop further standards of this type.* 4. RISK COMMUNICATION
In the early eighties some attacks were mounted on the dominant role of science in setting microwave safety standards. A history professor (Steneck, 1985) believed that too much reliance on science is apparent in the U.S. C95 standards. He stated that the C95 standards had the goal of permitting as large an exposure as possible with reasonable assurance of no bodily harm. In his opinion the goal of standards, instead, should be to set limits as low as achievable at reasonable cost. When epidemiological studies linking ELF (extremely-low frequency as from power lines) exposure with leukemia were taken seriously for the first time, the concept of "prudent avoidance" was proposed by Morgan (Nair et al., 1989). The idea was to reduce non-ionizing exposures when inexpensive measures are feasiMe--like moving a clock away from the bed. At the same time the proponents of "risk communication" were teaching that in the face of public "outrage", scientific facts are irrelevant (Sandman, 1987). Ideas precipitated by the uncertainty about E L F bioeffects included probabilistic nothreshold risk studies (Florig, 1992)--thus removing the idea of safe exposures and nudging the treatment of radiofrequency (more generally non-ionizing radiation) hazards towards the usual viewpoints which address the hazards of ionizing radiation and chemicals. This type of hypothesis was even applied to the calculation of potential risk from negative evidence--e.g, if AM radio broadcasting has caused no cancer in the environment, then a proposed military radio system is predicted to cause less than so many cancers--from zero hazard is raised a finite probability of hazard (NAS/NRC, 1993). The idea of discarding standards for "prudent avoidance" has been vigorously opposed (EEPA, 1990). The existence of a vibrant science-based standards community is the best defense against wholesale adoption of "prudent avoidance" by society. *Recentlyapproved by the IEEE as Standards Coordinating Committee 34, chaired by R. C. Petersen.
58
J.M. Osepchuk 5. THE RFI PROBLEM
I have earlier (Osepchuk, 1986) suggested that the real environmental problem facing the M P T or SPS systems is "RFI" or "interference" and not bodily exposure hazards. In recent years the problem, as it relates to the 200 million microwave ovens in the world, has been publicly addressed (Nebbia, 1992) and steps are underway (CISPR, 1994) to tighten (lower) permissible limits on out-of-band (spurious) energy from microwave ovens. The spurious signals are characteristic of magnetrons, especially when run by raw rectified a.c. power supplies as is the case in the microwave oven. In the SPS, it may be possible to eliminate most if not all of this noise by careful operation of magnetrons under d.c. current and controlled emission conditions (Brown, 1994). Nevertheless, proponents of M P T and SPS should focus effort in establishing a scientific data base demonstrating ways of achieving lownoise operation in magnetrons--the likely generators in M P T systems because of their uniquely high efficiency. A related aspect of this problem is that of frequency allocation. In principle, the ISM bands (e.g. 2.40-2.50GHz) are reserved for power applications like M P T and SPS and communications systems that operate in ISM bands do so at their own risk, i.e. must accept any interference from ISM. In recent years, however, in the U.S., the FCC has encouraged communications systems to operate in the 2.45 G H z band, e.g. the proposed (and later rescinded) auction of the band 2.40-2.412 G H z (FCC, 1994). Proponents of M P T and SPS need to add their voices to those who oppose this co-use of ISM bands. Clearly M P T and SPS involves so much power and flee-space radiation that no communications system will easily operate in the 2.45 G H z band within miles of a rectenna site (for an SPS or M P T operating in that band).
6. BENEFICIAL USES OF MICROWAVES
The fear of microwaves will dissipate as more and more beneficial uses of microwaves become widespread. In the past, diathermy (Guy and Chou, 1983) implied that absorption of 100 W of microwave energy in localized parts of the human body was relatively benign and provides beneficial heating. In recent years, this modality has become less popular, possibly because of
increased perception of microwave hazard. The increasing use of wireless (Kobb, 1993) implies that absorption of up to a watt of microwave energy in localized areas of the head is safe. Future uses of microwave power include rewarming of animals (Buffler, 1988) as well as humans (Hesslink et al., 1989) from hypothermia and use of microwave heating to keep people warm while reducing energy use through lower thermostat setting for conventional heating (Pound, 1980). There are many other uses of microwaves (Osepchuk and Petersen, 1993) that will occur only if there is rational acceptance of microwave exposure and even microwave heating as safe and manageable phenomena. 7. STEPS TOWARD ACCEPTANCE OF MPT AND SPS There are distinct actions in the areas of R&D and public policy that could help acceptance of M P T and SPS. Public education towards a rational view of E M F (a generalized term meaning electric or magnetic fields at any frequency but especially those in the E L F range) and microwaves is being pursued by the IEEE Committee on Man and Radiation (COMAR) which is now under the aegis of the Engineering in Medicine and Biology Society of the IEEE, and the EEA. These are noble efforts, but much too limited in scope because of costs. What is desirable is the funding ( ~ $500K) to produce a half-hour video on the subject which could be aired on TV. One point that could be made is that microwaves deserve no less a rational acceptance than infrared and optical energy--witness the acceptance of the laser pointer which emits visible light even though visible light poses far more hazard than either i.r. or microwaves. Granted--the fact that it is visible helps while both IR and microwaves are invisible. Emphasis should be placed on accepted beneficial microwave applications that involve human exposure, e.g. diathermy. The benefits of M P T and SPS need to be stressed by comparison to alternative technologies. It would be helpful, in my opinion, if other beneficial uses for microwave beams could be demonstrated, e.g. preventing frost in orange groves in Florida by microwave irradiation of the grove and surrounding areas. Research and development should provide technically-supported answers to questions like: Is it possible for an SPS or M P T beam to stray,
Health and safety issues b e c o m e focused a n d possibly d a m a g e p e o p l e o r their electronic devices acutel? F u r t h e r m o r e , w h a t is the m a g n i t u d e of lateral scattering of m i c r o w a v e energy which is i m p i n g i n g o n a rectenna? Q u a n t i t a t i v e b o u n d s on such scattering are desirable. It is i m p o r t a n t to d e t e r m i n e w h e t h e r such scattered energy is less o r m o r e intense t h a n preexisting r a d i a t i o n levels from b r o a d c a s t i n g a n d o t h e r local sources. O n e s h o u l d note t h a t well-financed ( b y the C e l l u l a r T e l e c o m m u n i c a t i o n s Industries Association, C T I A ) research is u n d e r w a y to explore the h a z a r d s of m i c r o w a v e exposure, as they affect the user of the cellular p h o n e a n d residents n e a r a cellular tower. This research c o u l d have valuable spin-off value for M P T / S P S p r o p o n e n t s . T h e emergence of R F I is m o r e likely to be a serious technical p r o b l e m for M P T a n d S P S t h a n h a z a r d o u s exposures. At the m o m e n t ( C I S P R , 1994) there are i n t e r n a t i o n a l s t a n d a r d s being d e v e l o p e d to c o n t r o l m o r e tightly the o u t o f - b a n d energy from m i c r o w a v e ovens. It m a y be t h a t such noise u n d e r frequency-sweeping c o n d i t i o n s is m o r e a c c e p t a b l e t h a n with a tightly c o n t r o l l e d carrier frequency as in m o s t M P T o r S P S systems. T h e r e is needed R & D to unders t a n d h o w to c o n t r o l such noise, a n d to ascertain the new c o m m u n i c a t i o n s technologies (e.g. digital a n d s p r e a d spectrum) which m a y coexist better with m i c r o w a v e ovens a n d o t h e r m i c r o wave systems. T h e r e is no device on the h o r i z o n as efficient as existing c o o k e r m a g n e t r o n s , t At 915 M H z h i g h - p o w e r m a g n e t r o n s are over 90% efficient a n d the 2 4 5 0 M H z tubes for ovens ( 8 0 0 - 2 0 0 0 W ) are 80% efficient on the bench. It is clear t h a t such efficient devices are b a d l y n e e d e d at o t h e r I S M frequencies, e.g. at 5 8 0 0 M H z . Even at 915 M H z , where clothes d r y i n g is m o r e promising, there is no low-cost h i g h - p r o d u c t i o n n u m b e r m a g n e t r o n for microwave ovens. Lastly, the p o w e r a p p l i c a t i o n s c o m m u n i t y , as well as M P T a n d S P S s u p p o r t e r s , m u s t express their o p p o s i t i o n to g o v e r n m e n t a l steps ( F C C , 1994) to a u c t i o n off p a r t of the I S M s p e c t r u m for c o m m u n i c a t i o n purposes. Eventually, b u t s o o n e r if s o m e of the a b o v e actions are r o b u s t enough, the use of m i c r o w a v e energy for n o n c o m m u n i c a t i o n s p u r p o s e s will be a p p r e c i a t e d b y the average m a n a n d the p r o p h e c y of K a p i t s a (Vanke, 1977) will have been confirmed: tThere is a very recent report of a 67% efficient Klystron suitable for use in microwave ovens (Kim et al., 1975).
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...It is worth noting that, before electrical engineering was pressed into service by power engineering, it was almost exclusively occupied with electrical communication problems (telegraphy, signaling and so on). It is very probable that history will repeat itself." At present, electronics is used mainly in radio communication, but its future lies in solving major problems in power engineering... We m u s t preserve the I S M s p e c t r u m if this p r o p h e c y is to b e c o m e a reality. REFERENCES
Adair E. R. Microwave challenges to the thermoregulatory system, In Electromagnetic Fields and Neurobehavioral Function, O'Connor M. E. and Lovely R. H. (Eds). Alan R. Liss, New York (1988). Adair E. R. Nurturing electrophobia, IEEE Spectrum 27, 11-14 (1990). ANSI/IEEE C95.1-1992, IEEE Standard for safety !evels with respect to human exposure to radio frequency electromagnetic fields, 3 kHz to 300 GHz. Institute of Electrical and Electronics Engineers (1991/1992). A. D. Little Inc. Responses of airborne biota of microwave transmission from satellite power systems. Final Report to EPA (1982). Bonasera S., Toler J., Popovic V., Popovic P., Honeycutt C. and Sgoutas D. Long-term study of 435 MHz radiofrequency radiation on blood borne end points in cannulated rats. J. Microwave Power 23, 94-136 (1988). Brodeur P. The Zapping of America. William Morrow, New York (1978). Brodeur P. Currents of Death. Simon & Schuster, New York (1989). Brown W. C. A history of microwave power transmission. IEEE Trans. MTT-32, pp. 1230-1244 (1984). Burlier C. Whole-body microwave heating of humans and livestock. Harvard Graduate Society Newsletter, pp. 11-13 (1988). Caine S. Status report on activities of the American National Standards C95 Committee on radiofrequency radiation hazards. Dig. 1972 IEEE Syrup. Electromagnetic Compatibility, pp. 117-124 (1972). Carstensen E. L. Biological Effects of Transmission-Line Fields. Elsevier, New York (1987). Chou C. K., Guy A. W., Kunz L. L., Johnson R. B., Crowley J. J. and Krupp J. H. Long-term low-level microwave irradiation of rats. Bioelectromagnetics 13, 469-496 (1992). CISPR (International Electrotechnical Commission). Proposed limits for ISM equipment in the range of 1-18 GHz, Subcommittee B (1994). Dunn D. A. and Loewenstern W. Jr. Cylindrical waveguide as a power transmission medium--limitations due to mode conversion. Proc. IEEE 54, 955-968 (1966). Electromagnetic Energy Association, EEA Bulletin 6, (1994). Electromagnetic Energy Policy Alliance (EEPA), Prudent Avoidance: The Abandonment of Science. Washington, D.C. (1990). EPA, 51 Fed. Reg., 27318 et seq. Federal radiation protection guidance: proposed alternatives for controlling public exposure to radiofrequency radiation (1986). EPA, Proposal for r.f. guidelines, Private communication with the Electromagnetic Energy Association, Washington, D.C. (1994). Fischetti M. The cellular phone scare. IEEE Spectrum 30, 43-47 (1993).
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FCC. Notice of proposed rule making, guidelines for evaluating the environmental effects of radiofrequency radiation. FCC ET Docket No. 93-62. (1993). FCC, Docket ET 94-32, Notice of proposed rule making. Auctioning of Spectrum (1994). Florig H. K. Containing the costs of the e.m.f, problem. Science 257, 468-469, 488, 490, 492 (1992). Foster K. and Pickard W. Microwaves: the risk of risk research. Nature 531-532 (1987). Gandhi O. P. (Ed.). Special issue on biological effects and medical applications of electromagnetic energy. Proc. IEEE 68, (1980). Glaser P. Power from the Sun: its Future. Science 162, 857-866 (1968). Gutman C. H. SPS technical issues. The Final Proceedings of the Solar Satellite Program Review, DoE/NASA Conf. 800491, pp. 107-108 (1980). Guy A. W., Kramar P. O., Harris C. A. and Chou C. K. Long-term 2450 MHz CW microwave irradiation of rabbits: methodology and evaluation of ocular and physiologic effects. J. Microwave Power 15, 37-44 (1980). Guy A. W. and Chou C. K. Electromagnetic heating for therapy. In Microwaves and Thermoregulation, Adair E. R. (Ed.), pp. 57-93. Academic Press, New York (1983). Hesslink R. L., Pepper S., Olsen R. G., Lewis S. B. and Homer L. D. Radio-frequency (13.56MHz) energy enhances recovery from mild hypothermia. J. appl. Physiol. 67, 1208-1212 (1989). International Radiation Protection Association (IRPA). United Nations Environment Programme, and the World Health Organization (WHO), Electromagnetic Fields (300 Hz to 300 GHz), Environmental Health Criteria 137, World Health Organization, Geneva (1993). ISU, Course Project on Solar Power Satellite. Kitakyushu, Japan (1992). Jensh R. P., Vogel W. H. and Brent R. L. An evaluation of the teratogenic potential of protracted exposure of pregnant rats to 2450 MHz microwave radiation. J. Toxicol. Environ. Health II, 23-59 (1983). Justesen D. R. et al. Compilation and assessment of microwave bioeffects, DoE/NASA Report PNL-2634 (rev.) (1978, 1980). Kim G. J., Shon J. C., Kim S. G. and Park Y. S. The development of multi-beam Klystron for a microwave oven. Proc. 30th Microwave Power Symp., pp. 73-74, IMPI, Manassas, VA (1995). Kobb B. Z. Personal wireless. IEEE Spectrum 30, 20-25 (1993). Lehmann J. Letter to Senate Committee; Hearings before the Committee on Commerce, U.S. Senate on Public L a w 90-602, Radiation Control for Health and Safety, Serial No. 93-24, pp. 462-463 (1973). Lin J. C. Microwave Auditory Effects and Applications. Charles C. Thomas, Springfield, II1. (1978). Michaelson S. M. The tri-service program-a tribute to George M. Knauf, USAF(MC). IEEE Trans. MTT-19, 131-146 (1971). Michaelson S. M., Houk W. M., Lebda N. J. A., Lu S. T. and Magin R. L. Biochemical and neuroendocrine aspects of exposure to microwaves. Ann. N. Y. Acad. Sci. 247, 21-45 (1975). Microwave News, Vol. 20, New York (1990). MIT, Report of MIT students on SPS assessment, MIT (1973). Morgan M. G. Risk research: when should we say "Enough"? Science 232, 913 (1986). Morgan M. G. Prudent avoidance. Public Utilities Fortnightly 26-29 (1992). Nair I., Morgan M. G. and Florig K. F. Biological effects of power-frequency electric and magnetic fields. Report OTA-BP-E-53, U.S. Govt. Printing Office, Washington, D.C. (1989).
NAS/NRC, Commission on Life Science, National Research Council. Assessment of the Possible Health Effects of the Ground-Wave Emergency Network, Board of Radiation Effect Research, NRC, Washington, D.C. (1993). NATO, STANAG 2345 (Ed.3) (5th Draft), NATO Standardization Agreement (1994). NCRP, Biologic effects and exposure criteria for radiofrequency electromagnetic fields. National Commission on Radiation Protection and Measurement, Bethesda, MD. (NCRP Report #86) (1986). NRPB. Board statement on restrictions on human exposure to static and time-varying electromagnetic fields and radiation. Documents of the NRPB 4, Nat. Rad. Prot. Board (1993). Nebbia K. The impact of worldwide emission standards on the microwave oven community. J. Microwave Power 27, 130, 191-192 (1992). Osepchuk J. M. Microwaves in the media: truth or consequences. Dig. 1979 MTT-S Syrup, (IEEE ) 11-14 (1979). Osepchuk J. M. Dealing with misperception: on the costs of perceived versus real risks of microwave radiation. In: Risk/Benefit Analysis: The Microwave Case, Steneck N. (Ed.). San F'rancisco Press, San Francisco (1982). Osepchuk J. M. (Ed.). Biological Effects of Electromagnetic Radiation. IEEE Press, New York (1983). Osepchuk J. M. Environmental issues for the solar power satellite. Space Power 6, 165 173 (1986). Osepchuk J. M. Science-based standards: the prudent response to electrophobia. Presented to 1992 Ann. Meet. Pacific Coast Elect. Assoc. (1992a). Osepchuk J. M. Microwave radiation hazards and spacesolar power. Lecture Presented at 1992 Sessions Int. Space Univ. (ISU), Kitakyushu, Japan (1992b). Osepchuk J. M. and Petersen R. C. The role in emerging technologies played by standards for exposure to electromagnetic energy. Paper commissioned by the IEEE Standards Department (1993). Osepchuk J. M. Impact of public concerns about low-level e.m.f, on the interpretation of the e.m.f.-RFR Database. Radiofrequency Radiation Standards, pp 415-426. Plenum Press, New York (1995). Osepchuk J. M. Overcoming perceived microwave radiation hazards relative to microwave power transmission. Proc. 29th Microwave Power Syrup. (IMPI ) 132-134 (1994). Osepchuk J. M. A new view of the microwave auditory effect. Presented at the Ann. Meet. Bioelectromagnetics Soc, Boston, Mass. (1995). Pound R. Radiant heat for energy conservation. Science 208, 494-495 (1980). Sandman P. M. Risk communication: facing public outrage. EPA J. 21-22 (1987). Steneck N. H. The Microwave Debate. MIT Press, Cambridge, Mass. (1985). Szmigielski S., Szudzinski A., Pietraszek H., Bielec M. and Wrembel J. K. Accelerated development of spontaneous and benzopyrene-induced skin cancer in mice exposed to 2450 MHz microwave radiation. Bioelectromagnetics 3, 179-191 (1982). Tell R. A. and Mantiply E. D. Population exposures to VHF and UHF broadcast radiation in the U. S. Proc. IEEE 68, 6-12 (1980). U.S. Congress, Hearings on Electromagnetic Compatibility Between Wireless Technology and Medical Devices. Before the House Government Operations Subcommittee on Information, Justice, Transport and Agriculture, Washington, D.C. (1994). Vanke V. A., Lopukhin V. M. and Savvin V. L. Satellite solar power systems. Soy. Phys. Usp. 20, 989-1001 (1977). Vanke V. A., Lesota S. K., Rachnikov A. V. and Savvin V. L. An investigation on space power beaming by microwaves in Moscow university. Proc. 29th Microwave Power Syrup. (IMPI ) 114-115 (1994).
HEALTH
0038-092X(95)00083-6
Solar Energy Vol. 56, No. 1, pp. 53-60, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-092X/96 $15.00 + 0.00
AND SAFETY ISSUES FOR MICROWAVE TRANSMISSION
POWER
J O H N M. O S E P C H U K Full Spectrum Consulting, 248 Deacon Hayes Road, Concord, MA 01742, U.S.A. Abstract--A general public perception that microwaves are hazardous has been a key obstacle for acceptance of microwave power transmission (MPT). This perception will eventually dissipate and then attention will focus on a real technical problem, that of interference (RFI). This can range from perceptible through annoying to hazardous. A program of actions is proposed to accelerate the goal of public acceptance of MPT. In this paper, a historical review shows that the solar power satellite (SPS) was reviewed a number of times relative to potential microwave exposure hazards. In all cases, no "show-stopper" was found but often the shibboleth "more research is needed" was aired. It is shown that standards for safe exposure to microwaves are the most important asset in convincing an audience that microwave exposure associated with M P T or SPS is safe. Standard-setting, world-wide, is shown to converge towards rational limits that are supportive of the M P T / S P S concepts. In recent times there has been the proposed substitute of "risk communication" ("prudent avoidance"). This is an unwise substitute for standards. Other aspects of microwave exposure standards are the new interface with R F I - - h e n c e the need for a rational division of responsibility between the radiators and the victim devices, like medical electronics-using both radiation limits and susceptibility limits. Beneficial applications of microwave exposure are being developed. Several studies are recommended which could put into perspective the likelihood of improbable events that represent "catastrophe"--e.g. the inadvertent focusing of a great amount of energy into inhabited areas.
1. INTRODUCTION
1980) of microwave bioeffects as they related to the SPS. The conclusions in the official review (Gutman, 1980) were that there were no serious anticipated problems (show-stoppers), but it would be wise to continue research on longterm chronic exposures to confirm the safety of environmental levels around an SPS rectenna site. Student perceptions (cited in Gutman, 1980), however, remained typically cynical and displayed an antipathy to "expert" opinions, presumed to be controlled by the establishment. Some student perceptions (MIT, 1973), however, were supportive of the SPS. During the 1980s, as considerable research on long-term chronic exposure to microwaves continued, the perception problem was intensified into a controversy (Steneck, 1985) and a "misperception" (Osepchuk, 1982) problem with extra-scientific dimensions. In 1986, this author (Osepchuk, 1986) reviewed the environmental issues for the SPS and concluded that the "perception" problem remained a number one problem even if no scientific basis for it existed. Other environmental issues were deemed controllable although it was predicted (Osepchuk, 1986) that interference (RFI) would eventually become the most serious technical issue to be faced in the deployment of the SPS. It was stressed that increasing beneficial uses of microwaves, the attendant familiarity with the entity
The idea of microwave power transmission in recent decades was brought forth by W. C. Brown (Brown, 1984) and Dunn (Dunn et al., 1966). Ever since Peter Glaser (Glaser, 1968) proposed the idea of the solar power satellite (SPS), there has been a lingering distrust of M P T in the general public because of vague fears of the effects of microwave exposure. There sometimes is voiced the fear that somehow a malfunction might cause an M P T system to accidentally "focus" its energy onto an inhabited area. This is, of course, an extremely unlikely and maybe impossible event but it forms the basis for the fear of a "catastrophe". The more common worry is that somehow or other longterm chronic exposure to low levels of microwaves might be harmful and even cause cancer. These fears are largely based on misinformation (Osepchuk, 1979) that appears in the writings (e.g. Brodeur, 1978) of some misguided advocates for the public. These fears have risen and fallen but in general they have evolved into a generalized electrophobia (Adair, 1990). This has been intensified in recent years because of the fear of power line fields (Nair et al., 1989). In recognition of these fears, the U.S. Department of Energy (DOE), during its extensive program review of the SPS concept in the 1970s supported a substantial assessment (Justesen, 53
54
J.M. Osepchuk
and the benign nature of casual exposures would ease the problem of public acceptance. In the late eighties, the perception problem was complicated and even heightened by the emergence of a fear (Nair et al., 1989) that 60 Hz magnetic fields could cause cancer. As popularized by Brodeur (Brodeur, 1989), the danger is restricted to fields produced by power lines, distribution lines and substations under the control of the utilities, and not appliances and other common exposure sources. His stated viewpoint was that exposures to appliances are transient but exposure to power lines are chronic. Brodeur and the media (e.g. Microwave News, 1990) in this way have intensified the fear of "long-term" (time) exposure without relation to levels (intensities) and totally divorced from scientifically-supported absorbed dose (energy) considerations (Osepchuk, 1992). In 1992, even after thorough exposition (Osepchuk, 1992b) of the facts about electromagnetic energy (bioeffects and RFI), students (ISU, 1992) still voiced an apprehension of the long-term exposure problem while not appreciating the reality of the RFI problem. Opposition to electromagnetic energy has surfaced in a series of well-publicized media events in the U.S.; these have targeted (in rough historical order) radar, microwave ovens, high-voltage power lines, microwave-relay towers, videodisplay terminals, broadcast towers, distribution lines, electric blankets, police radar, hand-held cellular phones and cellular towers. Not targeted have been the children's walkie-talkie which emits more power than most police radars; most household appliances; electric trains (commuting); mobile radio (police, fire, and taxis); amateur radio; medical procedures; as well as infrared/optical sources like lasers, halogen lamps, etc. In a 1994 review (Osepchuk, 1994), the author states that the reasons for these distinctions remain extra-scientific. In fact, today because of the emergence of epidemiological studies, with their weak and inconsistent evidence, as the only credible reason for fearing low-level electromagnetic fields, there is a cry to do away with threshold-based standards. Instead, policies of prudent avoidance (Morgan, 1992) or risk communication (Florig, 1992) are proposed in which no level is assumed safe and probabilistic assessments are generated. These mimic procedures common to the assessment of chemical and ionizing-radiation hazards but without any scientific basis for applying the
non-threshold hypothesis to nonionizing electromagnetic energy. The author concluded (Osepchuk, 1994) that the best response by responsible technologists is the support of standards-setting, public education and a small amount of directed hard-science bioeffect/hazards research. The fact that many other emerging technologies are threatened (Osepchuk and Petersen, 1993) has drawn together their disparate interests into organizations (Electromagnetic Energy Association, 1994) which are devoted to working toward rational standards for safe use of electromagnetic energy, and public education on the benefits as well as the hazards of electromagnetic energy. 2. REVIEW OF B I O E F F E C T R E S E A R C H
Since the beginning of organized research programs on microwave bioeffects in the 1950s (Michaelson, 1971), there have literally been thousands of papers published over the world. By and large, robust effects occur only well above exposure limits existing anywhere in the world. There has been a history of many claims of anomalous low-level, non-thermal effects. However, most of these are found difficult to replicate or exhibit unsatisfying ambiguities--so much so that the term "Cheshire Cat" has been applied (Carstensen, 1987) to describe the elusive character of much of the research on electromagnetic bioeffects. Some, (Foster and Pickard, 1987) have questioned how long one must tolerate such ambiguous research before calling a halt. Even the author (Morgan, 1992) of the "prudentavoidance" concept, himself, has suggested (Morgan, 1986) that there need to be some "stopping rules" for such ambiguous research. With regard to MPT or the SPS systems, a clearly pertinent bioeffect is the effect of microwave radiation on birds. Much research (Little, 1982) has been done on such effects at 2450 MHz. The results clearly show mildly thermal effects that probably are welcomed by birds in the winter and avoided in the summer. There appears not to be any serious threat to the survival of birds exposed to microwaves from such systems. The unique finding from microwave bioeffect research that is not based on gross thermal effects is the "microwave auditory effect" (Lin, 1978). This occurs when the human head is exposed to high peak power pulses, such as occur with radar even though there may be insignificant heating. The subjects hear some-
Health and safety issues what non-specific sounds characterized as "clicks" or "buzzing". Until recently this has been considered an effect associated with highpower systems. It has recently been found, however, (Osepchuk, 1995) that such effects can also be produced by close exposure to low-power sources (< 0.5 W) with an appropriate modulation at frequencies close to acoustic resonance of the human brain. It has been demonstrated to exist only in the frequency range of roughly 0.1-3.0 GHz. Theoretically, it is not expected (if related to acoustic disturbances in the brain) to occur if the radiation is non-penetrating, e.g. above 6 GHz. An important, but often unappreciated, type of research is that on humans. Dr Eleanor Adair (Adair, 1988) has been a leader in experimental human exposures to microwaves. She has been gathering a data-base on sensation as a function of frequency and more recently is doing research on human response to high thermalizing levels of microwave radiation. It is found, at least at 2450 MHz, that the human body is remarkably adaptable to heat stress created by the microwave heating. A myth continually promulgated in the media is that there are a n insignificant number of studies of chronic exposure of animals. Sometimes the simplistic criterion for "chronic" in the media is a near-lifetime (for humans), tens of years, continuous exposure to some unchanging field. This is, of course, unrealistic. Real-life exposures are intermittent as a person moves about in his life. (Only a dead or 100% immobile person would approach an unchanging exposure level in a world where all sources of EM energy are never switched on and off). Thus if animals are exposed for a significant period of time (e.g. 0.5 h or more) each day, (or working day), for many months, this is a chronic exposure. In view of the animals' shorter and accelerated life span, it is a valid procedure to scale from an animal exposure of 1-2 yr to human exposure of many years. On this basis, many chronic exposure experiments have been reported (Guy et al., 1980; Michaelson et al., 1975; Jensh et al., 1983; Chou et al., 1992; Bonasera et al., 1988) to cite only a few exampies. This subject is slated for specific examination at the 1995 EEA BEMS Meetings. Most of the studies have been done with rodents at frequencies of 0.1-10 GHz--especially 2.45 GHz. This is the frequency range where deepest penetration and absorption occurs for rodents. At frequencies > 2.45 GHz, penetration
55
into humans is somewhat superficial, < 2 cm, and effects should be less dramatic in humans than in (the corresponding experiments with) rodents where this radiation is penetrating. On the strength of these experiments, there is no basis for a fear of significant chronic health effects. There have been claims that certain studies (Chou et al., 1992; Szmigielski et al., 1982) support a link of microwave exposure to cancer. The majority viewpoint, however, is that these inferences are isolated and unsupported by further research. Reviews of the whole literature of microwave bioeffect research over the years have consistently agreed with the above viewpoints, always with the caveat that more research is desirable or necessary. These reviews include the comprehensive one in the IEEE (Gandhi, 1980), extensive reviews in the U.K. (NRPB, 1993) and in the International Radiation Protection Association (IRPA/WHO, 1993) have highlighted papers that either support the general assessments or have been considered not useful or applicable to making judgments on safe human exposure to microwaves. Thus, by and large, the overall conclusion of bioeffect research is that microwave exposures are generally benign except for the case of penetrating exposure to intense fields far above existing safe-exposure limits. 3. REVIEW OF SAFETY S T A N D A R D S
Standard-setting for microwave exposure has a long history (Caine, 1972) that begins with the simple 10 mW/cm 2 standard (as averaged over any 6 min). To properly understand exposure standards, one needs to consult Fig. 1 which is an exposure diagram. This depicts the maximum permissible exposure (MPE) (power or power density) as a function of exposure time. This MPE is set a factor of 10-100 below a similar curve that is a threshold of some undesirable effect. The characteristic time T, which may be a thermal time constant, is also the so-called averaging time. The net result is that for exposure time > T the MPE is constant. For exposure time < T, higher MPEs are permitted as long as the total energy (integral of power) is below a set limit. Over the years the standards have changed. In the most recent U. S. standard (ANSI/IEEE C95.1, 1992) the MPEs and averaging time are functions of frequency, and also exist in two tiers--one for the controlled environment
56
J.M.
Osepchuk
use in their licensing procedures for broadcasting and other types of transmitter towers. The U. S. Environmental Protection Agency (EPA, 1994), however, has discarded its extensive preparation (EPA, 1986) for "federal guidance" on environmental levels of radiofrequency radiation, which had been based on ANSI C95.1 (1982). Instead the EPA is now planning a twophase preparation of guidance advice. The first phase is based on existing standards for the near term. The second phase may result in a tighter standard /f studies undertaken by the N C R P under EPA support find valid evidence of hazardous "nonthermal" effects. Such effects have not yet been recognized by any standards body in the world. On the other hand, NATO (NATO, 1994) has proposed dropping the lower tier in their newly proposed standard (called a STANAG). It is important to stress the world-wide consensus on safe microwave levels. This is depicted in Fig. 2. Note that in the microwave range, 1 - 1 0 G H z , even the conservative IRPA limit (IRPA/WHO, 1993) is not too far from the U. S. limit. Therefore there is now and presumably will be for the foreseeable future an international consensus even including the views of the former Soviet Union (Vanke 1994) which are likely to relax upwards with time. Even now, the value of 0.01 mW/cm 2 applies as an average over 8-24 h / d a y - - s o that for short periods of time much higher levels are permitted up to 1 mW/cm 2. Thus the official health agencies, world wide, agree that levels (e.g. < 0 . 0 1 m W / c m 2) likely outside of restricted
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Health and safety issues rectenna areas of an SPS or M P T system are safe for indefinite duration. These judgments reflect the worldwide scientific consensus that there is no scientific basis to fear long-term microwave exposure as it would exist near M P T or SPS systems. In fact, exposures in the environment from broadcasting (Tell and Mantiply, 1980) has existed for decades, with a small part of the population chronically exposed to levels >0.001 or even 0.01 mW/cm 2 with no reported associated health effects. This is to be added to the extensive exposure history of probably more than one million patients to microwave diathermy (Lehmann, 1973) with no undue effects. Recently, there has been some attention (U.S. Congress, 1994) to the existence of potentially hazardous RFI from cellular phones. This has restimulated work towards a rational separation of the need for susceptibility limits (e.g. on aircraft or medical devices) versus the need for additional limits on radiation from transmitters to protect electronic circuits which are not as rugged as the human body in the presence of electromagnetic energy (Osepchuk, 1982). A very visible current issue is the safety of cellular phones (Fischetti, 1993)--both the hand-held phone to the user and the environmental exposure from cellular towers (which produce levels generally <<0.001 mW/cm 2 in the environment). Even here there is public opposition because of perceived risks. To overcome the siting dispute dilemma, the support of standard-setting groups is essential. Increasing strength of these groups will enhance their credibility and avoid the dismal alternatives of a "non-thermal", "prudent avoidance" policy or a "risk-communication" management policy. With respect to safety of devices like the hand-held radio, the EEA (EEA, 1994) has proposed to create an accredited standards committee for the purpose of developing product performance standards for items like the cellular phone, the VDT, police radar, and electric blanket. These standards would be like the successful microwave-oven emission standard developed by the FDA or the classification standards for lasers developed by an accredited standards committee with the participation of FDA and many other groups. The IEEE has indicated a willingness to collaborate with EEA in this matter. If successful, standards for new products would ease their public acceptance just as the standards for microwave ovens and lasers have
57
greatly helped their public acceptance (nobody has raised fears of the popular laser pointers now commonly used by lecturers). The EEA/IEEE project is designed to fill the vacuum created when the FDA decided in 1980 not to develop further standards of this type.* 4. RISK COMMUNICATION
In the early eighties some attacks were mounted on the dominant role of science in setting microwave safety standards. A history professor (Steneck, 1985) believed that too much reliance on science is apparent in the U.S. C95 standards. He stated that the C95 standards had the goal of permitting as large an exposure as possible with reasonable assurance of no bodily harm. In his opinion the goal of standards, instead, should be to set limits as low as achievable at reasonable cost. When epidemiological studies linking ELF (extremely-low frequency as from power lines) exposure with leukemia were taken seriously for the first time, the concept of "prudent avoidance" was proposed by Morgan (Nair et al., 1989). The idea was to reduce non-ionizing exposures when inexpensive measures are feasiMe--like moving a clock away from the bed. At the same time the proponents of "risk communication" were teaching that in the face of public "outrage", scientific facts are irrelevant (Sandman, 1987). Ideas precipitated by the uncertainty about E L F bioeffects included probabilistic nothreshold risk studies (Florig, 1992)--thus removing the idea of safe exposures and nudging the treatment of radiofrequency (more generally non-ionizing radiation) hazards towards the usual viewpoints which address the hazards of ionizing radiation and chemicals. This type of hypothesis was even applied to the calculation of potential risk from negative evidence--e.g, if AM radio broadcasting has caused no cancer in the environment, then a proposed military radio system is predicted to cause less than so many cancers--from zero hazard is raised a finite probability of hazard (NAS/NRC, 1993). The idea of discarding standards for "prudent avoidance" has been vigorously opposed (EEPA, 1990). The existence of a vibrant science-based standards community is the best defense against wholesale adoption of "prudent avoidance" by society. *Recentlyapproved by the IEEE as Standards Coordinating Committee 34, chaired by R. C. Petersen.
58
J.M. Osepchuk 5. THE RFI PROBLEM
I have earlier (Osepchuk, 1986) suggested that the real environmental problem facing the M P T or SPS systems is "RFI" or "interference" and not bodily exposure hazards. In recent years the problem, as it relates to the 200 million microwave ovens in the world, has been publicly addressed (Nebbia, 1992) and steps are underway (CISPR, 1994) to tighten (lower) permissible limits on out-of-band (spurious) energy from microwave ovens. The spurious signals are characteristic of magnetrons, especially when run by raw rectified a.c. power supplies as is the case in the microwave oven. In the SPS, it may be possible to eliminate most if not all of this noise by careful operation of magnetrons under d.c. current and controlled emission conditions (Brown, 1994). Nevertheless, proponents of M P T and SPS should focus effort in establishing a scientific data base demonstrating ways of achieving lownoise operation in magnetrons--the likely generators in M P T systems because of their uniquely high efficiency. A related aspect of this problem is that of frequency allocation. In principle, the ISM bands (e.g. 2.40-2.50GHz) are reserved for power applications like M P T and SPS and communications systems that operate in ISM bands do so at their own risk, i.e. must accept any interference from ISM. In recent years, however, in the U.S., the FCC has encouraged communications systems to operate in the 2.45 G H z band, e.g. the proposed (and later rescinded) auction of the band 2.40-2.412 G H z (FCC, 1994). Proponents of M P T and SPS need to add their voices to those who oppose this co-use of ISM bands. Clearly M P T and SPS involves so much power and flee-space radiation that no communications system will easily operate in the 2.45 G H z band within miles of a rectenna site (for an SPS or M P T operating in that band).
6. BENEFICIAL USES OF MICROWAVES
The fear of microwaves will dissipate as more and more beneficial uses of microwaves become widespread. In the past, diathermy (Guy and Chou, 1983) implied that absorption of 100 W of microwave energy in localized parts of the human body was relatively benign and provides beneficial heating. In recent years, this modality has become less popular, possibly because of
increased perception of microwave hazard. The increasing use of wireless (Kobb, 1993) implies that absorption of up to a watt of microwave energy in localized areas of the head is safe. Future uses of microwave power include rewarming of animals (Buffler, 1988) as well as humans (Hesslink et al., 1989) from hypothermia and use of microwave heating to keep people warm while reducing energy use through lower thermostat setting for conventional heating (Pound, 1980). There are many other uses of microwaves (Osepchuk and Petersen, 1993) that will occur only if there is rational acceptance of microwave exposure and even microwave heating as safe and manageable phenomena. 7. STEPS TOWARD ACCEPTANCE OF MPT AND SPS There are distinct actions in the areas of R&D and public policy that could help acceptance of M P T and SPS. Public education towards a rational view of E M F (a generalized term meaning electric or magnetic fields at any frequency but especially those in the E L F range) and microwaves is being pursued by the IEEE Committee on Man and Radiation (COMAR) which is now under the aegis of the Engineering in Medicine and Biology Society of the IEEE, and the EEA. These are noble efforts, but much too limited in scope because of costs. What is desirable is the funding ( ~ $500K) to produce a half-hour video on the subject which could be aired on TV. One point that could be made is that microwaves deserve no less a rational acceptance than infrared and optical energy--witness the acceptance of the laser pointer which emits visible light even though visible light poses far more hazard than either i.r. or microwaves. Granted--the fact that it is visible helps while both IR and microwaves are invisible. Emphasis should be placed on accepted beneficial microwave applications that involve human exposure, e.g. diathermy. The benefits of M P T and SPS need to be stressed by comparison to alternative technologies. It would be helpful, in my opinion, if other beneficial uses for microwave beams could be demonstrated, e.g. preventing frost in orange groves in Florida by microwave irradiation of the grove and surrounding areas. Research and development should provide technically-supported answers to questions like: Is it possible for an SPS or M P T beam to stray,
Health and safety issues b e c o m e focused a n d possibly d a m a g e p e o p l e o r their electronic devices acutel? F u r t h e r m o r e , w h a t is the m a g n i t u d e of lateral scattering of m i c r o w a v e energy which is i m p i n g i n g o n a rectenna? Q u a n t i t a t i v e b o u n d s on such scattering are desirable. It is i m p o r t a n t to d e t e r m i n e w h e t h e r such scattered energy is less o r m o r e intense t h a n preexisting r a d i a t i o n levels from b r o a d c a s t i n g a n d o t h e r local sources. O n e s h o u l d note t h a t well-financed ( b y the C e l l u l a r T e l e c o m m u n i c a t i o n s Industries Association, C T I A ) research is u n d e r w a y to explore the h a z a r d s of m i c r o w a v e exposure, as they affect the user of the cellular p h o n e a n d residents n e a r a cellular tower. This research c o u l d have valuable spin-off value for M P T / S P S p r o p o n e n t s . T h e emergence of R F I is m o r e likely to be a serious technical p r o b l e m for M P T a n d S P S t h a n h a z a r d o u s exposures. At the m o m e n t ( C I S P R , 1994) there are i n t e r n a t i o n a l s t a n d a r d s being d e v e l o p e d to c o n t r o l m o r e tightly the o u t o f - b a n d energy from m i c r o w a v e ovens. It m a y be t h a t such noise u n d e r frequency-sweeping c o n d i t i o n s is m o r e a c c e p t a b l e t h a n with a tightly c o n t r o l l e d carrier frequency as in m o s t M P T o r S P S systems. T h e r e is needed R & D to unders t a n d h o w to c o n t r o l such noise, a n d to ascertain the new c o m m u n i c a t i o n s technologies (e.g. digital a n d s p r e a d spectrum) which m a y coexist better with m i c r o w a v e ovens a n d o t h e r m i c r o wave systems. T h e r e is no device on the h o r i z o n as efficient as existing c o o k e r m a g n e t r o n s , t At 915 M H z h i g h - p o w e r m a g n e t r o n s are over 90% efficient a n d the 2 4 5 0 M H z tubes for ovens ( 8 0 0 - 2 0 0 0 W ) are 80% efficient on the bench. It is clear t h a t such efficient devices are b a d l y n e e d e d at o t h e r I S M frequencies, e.g. at 5 8 0 0 M H z . Even at 915 M H z , where clothes d r y i n g is m o r e promising, there is no low-cost h i g h - p r o d u c t i o n n u m b e r m a g n e t r o n for microwave ovens. Lastly, the p o w e r a p p l i c a t i o n s c o m m u n i t y , as well as M P T a n d S P S s u p p o r t e r s , m u s t express their o p p o s i t i o n to g o v e r n m e n t a l steps ( F C C , 1994) to a u c t i o n off p a r t of the I S M s p e c t r u m for c o m m u n i c a t i o n purposes. Eventually, b u t s o o n e r if s o m e of the a b o v e actions are r o b u s t enough, the use of m i c r o w a v e energy for n o n c o m m u n i c a t i o n s p u r p o s e s will be a p p r e c i a t e d b y the average m a n a n d the p r o p h e c y of K a p i t s a (Vanke, 1977) will have been confirmed: tThere is a very recent report of a 67% efficient Klystron suitable for use in microwave ovens (Kim et al., 1975).
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...It is worth noting that, before electrical engineering was pressed into service by power engineering, it was almost exclusively occupied with electrical communication problems (telegraphy, signaling and so on). It is very probable that history will repeat itself." At present, electronics is used mainly in radio communication, but its future lies in solving major problems in power engineering... We m u s t preserve the I S M s p e c t r u m if this p r o p h e c y is to b e c o m e a reality. REFERENCES
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