Magnetic Fields: Possible Environmental Health Effects

Magnetic Fields: Possible Environmental Health Effects PA Valberg, Gradient, Cambridge, MA, USA & 2011 Elsevier B.V. All rights reserved.

Abbreviations ac ACGIH ARPANSA CADHS dc DNA DOE ELF EMF HPA

IARC ICES ICNIRP IEEE MRI NAS NIEHS NRC NRPB

NTP ORAU RAPID WHO

alternating currents American Conference of Governmental Industrial Hygienists Australian Radiation Protection and Nuclear Safety Agency California Department of Health Services direct currents deoxyribonucleic acid Department of Energy extremely low frequency electric and magnetic fields Health Protection Agency (used to be the National Radiation Protection Board) (UK) International Agency for Research on Cancer International Committee on Electromagnetic Safety International Commission on Non-Ionizing Radiation Protection Institute of Electrical and Electronics Engineers magnetic resonance imaging National Academy of Sciences (USA) National Institute of Environmental Health Sciences (USA) National Research Council (USA) National Radiation Protection Board (now the Health Protection Agency) (UK) National Toxicology Program (USA) Oak Ridge Associated Universities Research and Public Information Dissemination World Health Organization

Electromagnetic Fields Describe Forces on Electrical Charges ‘Magnetic fields’ is the name given to one type of interaction that occurs among electrically charged objects. The other type of interaction between charged

objects involves ‘electric fields,’ which will also be mentioned in this article by way of contrast with magnetic fields. Electric charges are present in all matter, and most objects are electrically neutral, because positive and negative charges are present in equal numbers. When the balance of electric charges is altered, electrical effects are experienced, such as the attraction between a comb and the hair or the drawing of sparks after walking on a synthetic rug in the wintertime. The voltage on an electrical wire is caused by electric charges, which can exert forces on other nearby charges, and this force is called an ‘electric field.’ When charges move, they produce an electric current that can exert forces on other electric currents, and this force between electric currents is called a ‘magnetic field.’ Electric and magnetic fields (sometimes abbreviated ‘EMF’) are invisible lines of force that surround electric charges, for example, those in electrical appliances or in wires conducting electricity. Everyone is exposed to these fields both in nature and by virtue of the use of electric power, for example, in homes when a lamp is turned on or an electric oven or microwave is used for cooking. All encounter a wide variety of natural and man-made EMF. The earth’s atmosphere produces slowly varying electric fields, and thunderstorms produce very intense electric fields that are occasionally discharged by a lightning bolt. The earth’s core produces a steady magnetic field, which can easily be demonstrated with a compass needle. The earth’s magnetic field has strength of approximately 550 mG, which can be expressed in other units such as 55 mT. The earth, permanent magnets, and steady electric (or ‘dc’) currents produce steady magnetic fields. Magnetic fields from the earth or from small magnets exert forces on electric currents or on other magnetic objects, for example, when a compass needle orients toward a magnet. Magnets that may be used to hold items on metal surfaces have nearby magnetic fields of approximately 100 000–500 000 mG. An increasingly common diagnostic procedure, magnetic resonance imaging (MRI), uses fields of approximately 20 000 000 mG. Moving magnets or wires carrying alternating currents (ac) create time-varying magnetic fields. If, for example, a magnet spins at a rate of 60 times per second, an alternating magnetic field is produced, which is like the magnetic fields produced by electric utility lines. Time-varying magnetic fields also induce current in electrically conducting materials, and this is the principle used in the generation of electrical power from mechanical energy.

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Magnetic Fields: Possible Environmental Health Effects

This article provides an overview of scientific research on biological effects of low-frequency magnetic fields. Because living organisms are generally ‘transparent’ (i.e., interact only weakly) with low-frequency magnetic fields, the existence of biological effects (other than compass orientation) would not be expected. However, since the 1980s controversy has existed as to whether exposure to magnetic fields might contribute to deleterious effects such as increasing human cancer risk due to magnetic fields created by operating power lines. Both electric and magnetic fields are produced by the currents and voltages associated with society’s use of electric power, and these fields are sometimes designated as extremely low frequency (ELF), or 60-cycle ac (60 Hz) EMF. A significant fraction of the subsequent discussion relates to this ongoing controversy about the existence or absence of health effects of EMF. EMF are expressions of the long-standing and fundamental observations by physicists that electric charges, even though separated by distance, (1) exert attractive or repulsive forces on each other (through the electric field) and (2) when in motion (e.g., as in an electric current) exert force on other electric currents and moving electric charges (through the magnetic field). In addition, (3) time-varying magnetic fields can induce time-varying electric fields and electric currents (Faraday’s Law). Because of the relationship between time-varying magnetic fields and the induced electric fields and currents, this article frequently includes discussion of both electric and magnetic fields. Electrical potential (e.g., on electrical conductors) is expressed in volts (V) or kilovolts (1 kV ¼ 1000 V). Voltage can be thought of as electrical ‘pressure,’ and the voltage difference between conductors results in an electric field, usually expressed in units of kilovolts per meter (kV m1). The size of the electric field depends on the conductor voltage, the separation between conductors and ground, and other factors. The ‘electric field’ is essentially a measure of the force per unit electrical charge, with the force being in opposite directions for positive versus negative charges and with the size of the force being proportional to the amount of charge being acted on by the electric field. Conductors that convey moving charges (an electric current) create a magnetic field. The unit for electric current is amperes (A) and is a measure of the electricity ‘flow.’ The size of the magnetic field is proportional to the size of the electric current, but changes with the distance from the wire and the layout of the wire. The ‘magnetic field’ is essentially a measure of the force per unit current, with the direction of the force depending on the direction of the electric current, and with the size of the force proportional to the amount of electric current being acted on by the magnetic field. Two units of measurement are used for the magnetic field, and they

can be interconverted as follows: Unit type CGS (Gauss)

MKS (Tesla)

1G 1 G ¼ 1000 mG 10 mG 10 mG

0.000 1 T 0.1 mT 1 mT 1 nT

The types of scientific investigation that have been used to probe the possible health effects of EMF fall into three main categories. Understanding the strength and basis for possible health effects requires integration of these three main lines of scientific evidence: 1. Epidemiology 2. Laboratory animal studies 3. Mechanisms of action One central focus of research on power-line magnetic fields and health endpoints is related to possible cancer risk, specifically childhood leukemia. The following discussion provides summaries of these main lines of scientific evidence and offers a chronology of how the understanding of health effects of magnetic field has evolved over time. Although the effects of magnetic fields from various sources have been studied in biological systems for a long period of time, a significant impetus to health effect of magnetic field concerns occurred in 1979, when an epidemiology study by Wertheimer and Leeper reported a statistical association between ‘wire codes’ and childhood cancers in certain residential neighborhoods of Denver, Colorado. What these investigators did was to identify childhood leukemia cases and then match each case with a control child that did not have leukemia. The investigators then visually inspected electric power-line structures in the vicinity of each child’s home and assigned a ‘wire code’ wherein increasing numbers were assumed to designate increasingly heavy-duty electrical wiring configurations. Their statistical analysis reported that the odds of finding heavy-duty power lines in the vicinity of case homes was greater than the odds of finding such lines in the vicinity of homes of control children. With the additional assumption that the ‘wire code’ was a surrogate for ac power-line magnetic fields, Wertheimer and Leeper suggested that this correlation might be caused by magnetic-field exposure acting to increase the risk of childhood cancers. The Wertheimer and Leeper report triggered laboratory investigation as well as further epidemiological studies. Since 1979, a multitude of laboratory and correlative studies have investigated the questions raised by

Magnetic Fields: Possible Environmental Health Effects

the Wertheimer and Leeper hypothesis that power-line configuration was linked to risk of childhood cancers. In 1992, the U.S. Congress authorized the Electric and Magnetic Fields Research and Public Information Dissemination (EMF RAPID) program in the Energy Policy Act. In the RAPID program, the National Institute of Environmental Health Sciences (NIEHS), National Institute of Health (NIH), and the Department of Energy (DOE) were designated to fund, direct, and manage research and analysis aimed at providing scientific evidence to clarify the potential for health risks from exposure. A great quantity of scientific data have been reported, assembled, reviewed, analyzed, and often re-reviewed by many independent scientific consensus groups of research, government, and public health experts.

Lines of Scientific Inquiry into Health Effects of Magnetic Fields Epidemiology Because of the aforementioned epidemiology studies, the International Agency for Research on Cancer (IARC), which is part of the World Health Organization (WHO), classified power-line magnetic fields as a ‘possible’ (Group 2B) carcinogen in 2002. Even though epidemiology continues to provide the strongest suggestions of magnetic-field health risk, the results among the studies remain weak and inconsistent, and poorly linked to actual exposures to magnetic fields. Although epidemiology provides statistical, correlative results in human populations, such associations are not able to establish causation. That is, although a laboratory scientist can precisely set exposure conditions, randomly allocate groups to be exposed or nonexposed, do careful pathology on the outcome, and can read the results blindly (i.e., without knowing the exposure history), epidemiology is an observational science and cannot apply such rigorous scientific methods. Interpretation of the magnetic-field epidemiology is subject to additional problems. Few of the epidemiology studies used actual magnetic-field measurements, and none of the exposure assessments was based on mechanisms of interaction, or on possible choices for mitigation of magnetic-field levels. Also, an epidemiologic study that reports ‘statistically significant’ associations is only testing that significance against the role of random chance, given the size of the population studied. If other sources of uncertainty in epidemiologic studies were to be quantitatively included in the confidence interval (e.g., confounding factors, measurement error, selection bias, and misclassification), the error bars would become wider and may well overlap a null outcome (i.e., ‘no association’). Reviews of magnetic-field epidemiology emphasize this point, namely, that the error bars in reported

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results do not reflect these other sources of uncertainty, and consequently, the results are less indicative of an actual link that one might at first conclude. Laboratory Animal Studies Laboratory animal studies have examined the biological effects of exposure to magnetic fields in mammalian species expected to have reactions similar to humans. Interpretation of power-line magnetic-field epidemiology can be made less clouded and uncertain, by comparison to other lines of scientific evidence. To date, these other lines of evidence weigh against assigning a causal basis to the associations reported by epidemiology. Scientists have not been able to identify an established laboratory bioassay or animal model by which power-line magnetic fields can be shown to consistently initiate or accelerate biological changes related to cancer risk. Although lifetime exposure to high levels of 60 Hz magnetic fields has been tested in numerous animal studies (using different species), the results have failed to show that 60 Hz magnetic fields can initiate or exacerbate any disease or precancerous condition, even in genetically modified and susceptible animals. For example, the National Toxicology Program (NTP) research campaign extensively tested exposures to elevated levels of 60 Hz magnetic field and remains the largest laboratory study, because its scope and quantity of animals tested is unlikely to ever be duplicated. The NTP study found no cancer risks at high magnetic-field exposure levels (10 000–20 000 mG). Such animal testing is the foundation (or ‘gold standard’) for probing health effects, because it is often through such exhaustive animal studies that regulators can determine what (if any) aspect of an exposure (e.g., chemicals or ‘magnetic fields’) should potentially be regulated. Mechanisms of Action Studies of ‘mechanisms of action’ utilize well-established laws of physics, chemistry, and biology to predict and understand how a magnetic field can alter the function of biological structures such as cell membranes or genetic (DNA) molecules. Mechanistic magnetic field research to date, reporting extensive efforts by scientists worldwide, has not been able to identify plausible mechanisms or causal pathways by which typical levels of power-line magnetic fields can cause adverse health effects. Magnetic field interactions with biological systems have been analyzed carefully in light of the biophysics of electromagnetic field interactions with matter. The applicability of fundamental physics to all systems, and to biology in particular, permits evaluation of the interaction of magnetic fields with ions, molecules, cells, and organisms. The conclusions are that typical power-line magnetic fields do not create disturbances that are detectable

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Medicine

Biology

Chemistry Physics

Figure 2 Each scientific discipline rests on the underlying laws of a more basic discipline. Figure 1 Three-legged stool: health-effects research looks at three independent lines of evidence: cell studies (mechanism of action), laboratory animal studies, and population studies (epidemiology). To understand toxicity, support is required in each area.

above the internal biological sources of disturbance (electrical and other ‘noise’) that naturally occur in living systems. Examinations of all possible aspects of exposure to magnetic fields have revealed no firm basis on which to attribute a potential for adverse biological effects to the specific values of, for example, any of the following EMF metrics: (1) electric field or magnetic field magnitudes, (2) the fundamental frequency or harmonic frequencies, (3) continuous exposure versus intermittent exposure, (4) time-averaged fields versus peak fields, and (5) constant-amplitude magnetic fields versus transient magnetic fields. A mechanism recently under investigation is that of ‘contact currents,’ which are the grounding currents that people might experience when touching electrical appliances or metal plumbing fixtures. If true, this hypothesis would suggest that the epidemiological associations arise through factors that are not magnetic fields at all, but happen to vary with magnetic fields. Over the years, many ideas have been proposed in this area, and many analyses have been performed, but diligent attention by scientists has not yielded identified aspects, levels, or durations of exposure to magnetic fields that can biophysically be traced to increased cancer risk. Without any understanding of mechanism, it remains unknown as to what, if any, aspect of exposure to magnetic fields should be controlled to reduce risks. Integration of Lines of Evidence Biological effect evidence that may establish the existence of a health impact is often illustrated as a ‘threelegged stool’ (Figure 1), where strength in each line of evidence (each leg) is required for strength and stability, and weakness in any one leg makes the stool unstable.

That is, lack of support from all three lines of evidence restricts the conclusions that can be drawn. The three legs are (1) exposure/disease correlations in human populations (epidemiology), (2) empirical laboratory animal studies at controlled and elevated levels of exposure, and (3) in vitro and mechanistic studies of the agent’s mode of action. For low-frequency magnetic fields, evidence for adverse health effects derives primarily from leg (1), but there is a profound lack of support from animal studies and mechanistic studies (legs (2) and (3)). In fact, much of the evidence from legs (2) and (3) suggests an absence of health risks. Mechanistic evidence (leg (3)) highlights what is known about the physics of the interaction of electromagnetic fields with matter. The applicability of fundamental physics to all systems, and to biology in particular, permits conclusions to be drawn about the interaction of EMF with ions, molecules, cells, and organisms. The basic interactions of electromagnetic fields with matter involve force on fixed and moving charges. Living organisms rely on the same laws that govern all systems, and hence, mechanistic considerations are crucial. As shown in Figure 2, physics forms the basis of chemistry, which forms the basis of biology, and which then forms the basis of medicine. Hence, even though there is an increase in complexity as one moves up in this illustration, each successive layer must obey the fundamental laws found to be valid for the layer below. At the most fundamental level are the laws of physics, which have been validated by experiment and internal consistency. Maxwell’s laws of electromagnetism are accepted to be invariant in time and space, and their accuracy in describing the interactions between electromagnetic fields and matter underlies the functioning of virtually all technology. No exceptions have been found, despite constant challenges and tests. Likewise, physics has been found to be valid in complex systems, encompassing chemistry, biology, technology, and medicine. Simple conservation laws (e.g., energy, motion, charge, and momentum) are universally applicable, without exception.

Magnetic Fields: Possible Environmental Health Effects

For magnetic fields to exert an influence on living cells, the fields must in some manner modify molecules or structures in the organism. By their very definition, magnetic fields interact with matter only by exerting force on stationary or moving electric charges. At sufficiently high levels, these forces will add thermal energy or change the configuration of a charged biological molecule or structure. However, the magnitudes of natural forces that cells use (and are sensitive to) have been measured, and the results demonstrate that biological structures can withstand forces far larger than can be generated by typical magnetic fields. Cells and organs function properly despite many internal sources of interfering chemical and force effects, which exceed by a large factor the forces that can be caused by magnetic fields. In summary, for magnetic fields to alter physiological function, initiate dysfunction, or cause the onset of disease in humans or animals, there must exist a mechanism by which magnetic forces alter molecules, chemical reactions, cell membranes, or biological structures. A magnetic field is not a chemical agent, and biological plausibility must be assessed with this in mind. The initial physical step is illustrated in the following causal chain required by any connection to disease: Magnetic fields ) matter ðphysicsÞ ) molecules ðchemistryÞ ) organisms ðbiologyÞ ) disease

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Some agencies have provided guidelines for acceptable, continuous exposure of the general public to power-line magnetic-field levels, and these values range from approximately 800 to 9000 mG. It is helpful to review some of the history (particularly before the 2002 IARC report) as to the public health agencies that have addressed, analyzed, and provided conclusions on the issue of magnetic fields. Oak Ridge Associated Universities Expert Panel (1992) In the early 1990s, the U.S. Department of Labor commissioned an expert panel to study reports that attributed adverse health effects to magnetic fields from power lines, household appliances, and video display terminals. For this report, Oak Ridge Associated Universities coordinated the analysis and established a panel of expert scientists, who published their conclusions in 1992. The executive summary of the Oak Ridge report states that: This review indicates that there is no convincing evidence in the published literature to support the contention that exposures to extremely low-frequency electric and magnetic fields (ELF-EMF) generated by sources such as household appliances, video display terminals, and local power lines are demonstrable health hazards.

American Medical Association (1994) A necessary condition for magnetic fields to impact on human or ecosystem biology is that the magnetic fieldsinduced changes have to exceed chemical changes from natural or background influences. Changes in biology are coupled to magnetic fields through changes in forces on charged structures, which in turn, must be coupled to metabolically important chemical processes (e.g., reaction or transport rates).

Expert Panel Reviews of Magnetic Fields and Environmental Effects Various international scientific ‘blue-ribbon’ panels have reviewed, and continue to review, researches on health effects of magnetic fields. Overall, the absence of robust findings from careful, replicated laboratory studies causes health agencies to be cautious about relying on the reported epidemiological links. The statistical, epidemiologic results are suspected to arise from such factors as selection bias and unmeasured or uncontrolled confounding. Scientific guideline-setting committees do not consider the epidemiologic evidence to be adequate for guideline development. No major public health agency has set guidelines based on distance from power-line rights-of-way (ROWs).

In 1994, the American Medical Association adopted recommendations regarding health effects of magnetic fields. In addition to encouraging research and continuing to monitor developments, the adopted recommendations stated: No scientifically documented health risk has been associated with the usually occurring levels of electromagnetic fields.

Swedish National Board of Health and Welfare (1995) In January 1995, the Swedish National Board of Health and Welfare released a report that noted the lack of substantial evidence regarding health effects of magnetic fields: The existing epidemiological data cannot be used to support any definite conclusions as to whether exposure to electromagnetic fields increases the cancer risk in any organ system.

American Cancer Society (1996) The Epidemiology and Surveillance Research group of the American Cancer Society reviewed the epidemiology

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data suggesting increases in cancer risk from exposure to power-line magnetic fields. They noted that, in 1996, the idea that exposure to power-frequency magnetic fields might contribute to cancer causation had been under investigation for nearly two decades. They reviewed all the available epidemiologic observations suggesting that exposure to magnetic fields may or may not promote human carcinogenesis. They concluded that: While [epidemiological] observations may suggest a relationship [to risk] for leukemia and brain cancer in particular, the findings are weak, inconsistent, and inconclusive. The weakness and inconsistent nature of epidemiologic data, combined with the continued dearth of coherent and reproducible findings from experimental laboratory research, leave one uncertain and rather doubtful that any real biologic link exists between EMF exposure and carcinogenicity.

National Academy of Sciences (1997) In 1996, the U.S. Congress requested that the National Academy of Sciences (NAS) of the National Research Council review the scientific literature regarding effects of magnetic fields on health. In 1997, the NAS issued a 356-page report, concluding that the NAS review of the evidence did not indicate that power-line magnetic fields was a human health hazard, stating: Based on a comprehensive evaluation of published studies relating to the effects of power-frequency electric and magnetic fields on cells, tissues, and organisms (including humans), the conclusion of the committee is that the current body of evidence does not show that exposure to these fields presents a human-health hazard. Specifically, no conclusive and consistent evidence shows that exposures to residential electric and magnetic fields produce cancer, adverse neurobehavioral effects, or reproductive and developmental effects.

International Commission on Non-Ionizing Radiation Protection (1998) The International Commission on Non-Ionizing Radiation Protection (ICNIRP) of the International Radiation Protection Association has published guidelines on limits of exposure to 50/60 Hz EMF. The guidelines are based on analyses of the scientific literature and on review articles published by WHO, which concluded that no biological effects could be expected for magnetic fields smaller than 50 000 mG. The ICNIRP guidelines state that occupational exposure continuing throughout the working day should be limited to 60 Hz magnetic fields below 4167 mG. The guidelines also state that for members of the general public, exposure should be limited to 833 mG, and exposure between 1000 and

10 000 mG should be limited to a few hours per day. Overall, ICNIRP concluded: In the absence of evidence from cellular or animal studies, and given the methodological uncertainties and in many cases inconsistencies of the existing epidemiologic literature, there is no chronic disease outcome for which an etiological relation to EMF exposure can be regarded as established.

NIEHS (1999) and NAS (1999) In 1999, the NAS/National Research Council evaluated the scientific and technical content of research projects conducted under the U.S. EMF RAPID program. Results of the EMF-RAPID program do not support the contention that the use of electricity poses a major unrecognized public-health danger. No finding from the EMF-RAPID program alters the conclusions of the previous NRC review on the Possible Effects of [EMF] on Biologic Systems (NRC 1997). In view of the negative outcomes of EMF-RAPID replication studies, it now appears even less likely that [EMF] in the normal domestic or occupational environment produce[s] important health effects, including cancer.

The report ‘‘recommends that no further special research program focused on possible health effects of power-frequency magnetic fields be funded.’’ The NIEHS, in a 1999 report to the U.S. Congress, summarized the 6 years of accelerated laboratory magnetic fields research conducted under the U.S. EMF RAPID program: [EMF] exposure cannot be recognized as entirely safe because of weak scientific evidence that exposure may pose a leukemia hazard y. The NIEHS does not believe that other cancers or non-cancer health outcomes provide sufficient evidence of a risk to currently warrant a concern. Virtually all of the laboratory evidence in animals and humans, and most of the mechanistic studies in cells fail to support a causal relationship y. The lack of consistent, positive findings in animal or mechanistic studies weakens the belief that this association is actually due to [EMF y.

California Department of Health Services (2000) The ‘California EMF Program,’ which operated within the CA DHS, developed a fact sheet entitled ‘Electric and Magnetic Fields: Measurements and Possible Effect on Human Health – What We Know and What We Don’t

Magnetic Fields: Possible Environmental Health Effects

Know in 2000.’ The fact sheet concluded: Public concern about possible health hazards from the delivery and use of electric power is based on data that give cause for concern, but which are still incomplete and inconclusive and in some cases contradictory. A good deal of research is underway to resolve these questions and uncertainties. Until we have more information, you can use ‘‘no and low cost avoidance’’ by limiting exposure when this can be done at reasonable cost and with reasonable effort, like moving an electric clock a few feet away from a bedside table or sitting further away from the computer monitor.

U.K. National Radiological Protection Board (2001) The National Radiological Protection Board (NRPB) has a statutory responsibility for advising U.K. government departments on standards of protection for exposure to nonionizing radiation. This covers static and low-frequency EMF and radiofrequency radiations, and as far as power-line magnetic fields, the NRPB concluded that: Laboratory experiments have provided no good evidence that extremely low frequency electromagnetic fields are capable of producing cancer, nor do human epidemiological studies suggest that they cause cancer in general. There is, however, some epidemiologic evidence that prolonged exposure to higher levels of power frequency magnetic fields is associated with a small risk of leukaemia in children. In the absence of clear evidence of a carcinogenic effect in adults, or of plausible explanation from experiments on animals or isolated cells, the epidemiological evidence is currently not strong enough to justify a firm conclusion that such fields cause leukaemia in children. Unless however, further research indicates that the finding is due to chance or some currently unrecognized artifact, the possibility remains that intense and prolonged exposures to magnetic fields can increase the risk of leukaemia in children.

Minnesota Department of Health (2002) In 2002, the Minnesota Department of Health reviewed the EMF literature, and the report stated that: MDH concludes that the current body of evidence is insufficient to establish a cause and effect relationship between EMF and adverse health effects. However, as with many other environmental health issues, the possibility of a health risk from EMF cannot be dismissed. [y] Based on its review, the policy recommendations of the Work Group include: apply low-cost EMF mitigation options in electric infrastructure construction projects; encourage conservation; encourage distributed generation; continue to monitor EMF research; encourage

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utilities to work with customers on household EMF issues; and provide public education on EMF issues.

California Department of Health Sciences Evaluation (2002) Three California Department of Health Services (CADHS) epidemiologists evaluated the overall EMF epidemiology as it existed in 2002. Their report stated: Three epidemiologists who worked for the California Department of Health Services (CADHS) reviewed studies about possible health problems from electric and magnetic fields (EMFs) [y] To one degree or another, all three of the DHS scientists were inclined to believe that EMFs can cause some degree of increased risk of childhood leukemia, adult brain cancer, Lou Gehrig’s Disease [ALS], and miscarriage. They strongly believe that EMFs are not universal carcinogens, since there are a number of cancer types that are not associated with EMF exposure. To one degree or another they are inclined to believe that EMFs do not cause an increased risk of breast cancer, heart disease, Alzheimer’s Disease, depression, or symptoms attributed by some to a sensitivity to EMFs.

The CADHS epidemiologists did not quantify what they mean by ‘some degree’ of increased risk, and subsequent to the report, the state of California did not issue any general-public guidelines on allowable magneticfield exposure. It seems that this report did not allow for the nonsupportive, negative results from animal studies or mechanistic studies, because the epidemiologist felt that epidemiology was ‘observational,’ whereas laboratory results were more ‘theoretical’ as to their relevance to humans. However, the constraints imposed on interpretation of epidemiologic associations by physical law and experimental studies are real and cannot be easily disregarded, and physical laws and laboratory studies are firmly based on reproducible and validated ‘observations.’ International Agency for Research on Cancer (2002) In its 2002 review of EMF, IARC (an agency of WHO) noted that considerably greater EMF exposure occurs in some occupations, yet the agency concluded that for workers, ‘‘There was no consistent finding across studies of an exposure–response relationship and no consistency in the association with specific sub-types of leukaemia or brain tumors.’’ IARC classified power-frequency magnetic fields as ‘possibly carcinogenic,’ based on ‘limited’ evidence from humans concerning childhood leukemia, ‘inadequate’ evidence from humans concerning all other cancer types, and ‘inadequate’ evidence from animals. Power-frequency electric fields were judged ‘not

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classifiable’ (Group 3) on the basis of ‘inadequate’ evidence from both humans and animals. Australian Radiation Protection and Nuclear Safety Agency (2006) A draft power-line EMF standard was proposed in December 2006 by ARPANSA entitled ‘Radiation Protection Standard: Exposure Limits for Electric & Magnetic Fields – 0 Hz to 3 kHz.’ The ELF-EMF standard was addressed again in 2008, in a ‘Forum on the Development of an ELF Standard.’ The ARPANSA proposed value for acceptable ‘general public’ exposure to 50 Hz EMF is 5 kV m1 for the electric field and 3000 mG (300 mT) for the magnetic field, the magneticfield guideline being somewhat higher than the ICNIRP guideline, where the general public magnetic-field guideline is 1000 mG for 50 Hz EMF (833 mG for 60 Hz EMF). It is somewhat lower than the ICES/IEEE magnetic-field guideline for public exposure to 50 Hz EMF, which is 10 000 mG (9040 mG for 60 Hz EMF).

give confidence that magnetic fields do not cause the disease. (p. 12)

Furthermore, in their ‘Summary and Recommendations for Further Study’ WHO emphasized that: the limit values in [EMF] exposure guidelines [not] be reduced to some arbitrary level in the name of precaution. Such practice undermines the scientific foundation on which the limits are based and is likely to be an expensive and not necessarily effective way of providing protection. (p. 12)

WHO concluded that: given both the weakness of the evidence for a link between exposure to ELF magnetic fields and childhood leukaemia, and the limited impact on public health if there is a link, the benefits of exposure reduction on health are unclear. Thus, the costs of precautionary measures should be very low. (p. 13)

World Health Organization (2007) WHO undertook a comprehensive and critical review of all researches on environmental effects of low-frequency magnetic fields. In their 2007 publication on ‘Extremely Low Frequency Fields’ WHO stated: Uncertainties in the hazard assessment [of epidemiological studies] include the role that control selection bias and exposure misclassification might have on the observed relationship between magnetic fields and childhood leukaemia. In addition, virtually all of the laboratory evidence and the mechanistic evidence fail to support a relationship between low-level ELF magnetic fields and changes in biological function or disease status. Thus, on balance, the evidence is not strong enough to be considered causal, but sufficiently strong to remain a concern. (p. 12)

Also, regarding disease outcomes, aside from childhood leukemia, the WHO in 2007 reached similar conclusions to the IARC 2002 conclusions, in that WHO stated that: A number of other diseases have been investigated for possible association with ELF magnetic field exposure. These include cancers in both children and adults, depression, suicide, reproductive dysfunction, developmental disorders, immunological modifications and neurological disease. The scientific evidence supporting a linkage between ELF magnetic fields and any of these diseases is much weaker than for childhood leukaemia and in some cases (for example, for cardiovascular disease or breast cancer) the evidence is sufficient to

Scientific Committee on Emerging and Newly Identified Health Risks (2008) Upon request of the European Commission, an opinion on the possible health effects of EMF on health was recently formulated by the Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR). The [IARC] opinion, that ELF magnetic fields are a ‘possible’ carcinogen, chiefly based on childhood leukaemia results, is still valid. There is no generally accepted mechanism to explain how ELF magnetic field exposure may cause leukaemia. Animal studies have not provided adequate evidence for a causal relationship.

The SCENIHR committee concluded that (1) there was no consistent relationship between ELF exposure and self-reported symptoms, (2) it was unlikely that breast cancer risk or cardiovascular disease risk was linked to ELF-EMF exposure, and (3) the effects of ELFEMF on neurodegenerative disease and brain tumors remain uncertain. In summary, even though the sum of the epidemiology correlations continues to provide a suggestion of health risk of EMF, the results among the studies remain weak and inconsistent, and poorly linked to actual EMF exposures. Importantly, scientists have been unable to establish a laboratory animal or mechanistic model relevant to human cancer risk that reliably demonstrates biological changes in response to powerline EMF exposure that leads to an increased tumor risk.

Magnetic Fields: Possible Environmental Health Effects

Current Quantitative Guidelines on Magnetic Fields (and Electric Fields) Most countries, and specifically the United States, have no federal or legally enforceable standards limiting occupational or residential exposure to 60 Hz EMF. Table 1 shows guidelines suggested by national and world health organizations. The table shows levels that are designed to be protective against any adverse health effects but that should not be viewed as demarcation lines between safe and dangerous levels of EMF. The second table shows (state) guidelines that have been adopted to maintain the status quo of typical EMF on and near transmission line ROWs and are not health-based.

Ecological Effects of Magnetic Fields Electrical Aspects of Behavior/Navigation/ Migration of Organisms Both terrestrial (e.g., birds and honeybees) and aquatic organisms (e.g., bacteria, finfish, eels, sharks, and sea turtles) likely use naturally occurring electric and magnetic fields (e.g., the earth’s dc magnetic field) for certain specific purposes such as orientation, navigation, prey location, and long-distance migration. Table 2 gives examples of effects of electric and magnetic fields that have been reported in marine animals. The mechanisms underlying these electric and magnetic field senses are primarily limited to slowly varying fields and are not

Table 1

expected to respond to rapidly varying ac electric and magnetic fields (e.g., at power-line frequencies, 50–60 Hz). Aside from orientation and navigation, other potential effects of low-frequency EMF on ecological systems have been investigated, but the findings on ecological effects have been equivocal in the sense that there is no consistent evidence to establish an adverseeffect level. Because time-varying magnetic fields can cause electric currents in salt water, effects of magnetic fields should also consider electrosensory capabilities. A considerable amount of research has aimed to elucidate the mechanism of electrosensory ability in aquatic organisms and the potential utility of this sensory modality for navigation and prey detection. This research has, to date, demonstrated that, although electrical sensitivity is common, very few species have been shown to possess an electrosensory system whose signal processing is sophisticated enough to differentiate the different frequencies that might be present in time-varying magnetic fields. However, electrosensitivity appears developed enough to allow detection and orientation toward bioelectric fields of prey organism, which are at frequencies from approximately 0 to 15 Hz. In particular, some elasmobranchs demonstrate a range of electrosensory detection capabilities. Sensitivity to steady magnetic fields is reasonably well established. In contrast to the weak electrical fields present in the ocean, the relative strength of the earth’s steady north–south magnetic field (B500 mG) appears to

Magnetic and electric field guidelines established by health and safety organizations

Organization

American Conference of Governmental and Industrial Hygienists (ACGIH) (occupational) International Commission on Non-Ionizing Radiation Protection (ICNIRP) (general public, continuous exposure) Non-Ionizing Radiation (NIR) Committee of the American Industrial Hygiene Association (AIHA) endorsed (in 2003) ICNIRP’s occupational EMF levels for workers Institute of Electrical and Electronics Engineers (IEEE) Standard C95.6 (general public, continuous exposure) U.K. National Radiological Protection Board (NRPB) (now Health Protection Agency (HPA)) Australian Radiation Protection and Nuclear Safety Agency (ARPANSA), Draft Standard, December 2006c Comparison to steady (dc) magnetic/electric fields Earth’s magnetic field and atmospheric electric fields, steady levels, typical of environmental exposure Magnetic resonance imaging scan, static magnetic field intensity a

553

Values for ‘60-Hz’ frequencies Magnetic field (mG)

Electric field (kV m1)

10 000,a 1000b

25,a 1b

833

4.2

4170

8.3

9040

5.0

833

4.2

3000

5.0

550d

0.2 to 412

20 000 000d

–

ACGIH guidelines for the general worker. ACGIH guideline for workers with cardiac pacemakers. c ARPANSA – http://www.arpansa.gov.au/pubs/comment/dr_elfstd.pdf and http://www.arpansa.gov.au/News/events/elf.cfm. d These are steady ‘dc’ fields and do not vary in time at the characteristic 60 cycles per second that power-line fields do. However, if a person moves in the presence of these fields, the body experiences a time-varying field. b

554 Table 2

Magnetic Fields: Possible Environmental Health Effects Reported effects of electric and magnetic fields on marine animals

Marine animal

Electric/magnetic field threshold

Organism behavior effect

Catshark, small-spotted

103 V m1, determined using constant electric field generated by dc electrodes in seawater 105 V m1 1–2  105 V m1

Avoidance

Catshark, small-spotted Dogfish, smooth Dolphins, Atlantic white-sided, common, and Risso’s Elasmobranchs Elasmobranch, Black Sea Skate

Sharks Sharks Sharks and rays Sharks and rays

Magnetic minima in the Earth’s field. Intensity variations of less than B0.5 mG of the earth’s field 5  107 V m1, 0.1–10 Hz Reducing 39 000 mG smoothly to 0 over 1 s, i.e., a changing magnetic field evoked the response, a constant field failed to do so 8 mA current, approximately r1/8 to 8 Hz operating range 0.37 mG km1 (earth’s magnetic field gradient – steady dc field) 5  107 V m1, o8 Hz 25 nV membrane potential change (in vitro)

Sharks (theoretical calculation)

12 mG km1 magnetic intensity gradient (steady dc field) B102 V m1, o5 Hz

Skate

4  105 V m1 at 5 Hz square-wave

Stingray Turtles

5  107 V m1 0.1% total of the earth’s steady dc magnetic field intensity fluctuations 0.1–2 mG (steady dc field)

Sharks, scalloped hammerhead

Whales

play a part in the directional orientation of some marine animals. The mechanism underlying the magnetic sense in marine animals is limited to steady or slowly varying fields (dc magnetic fields of 0–15 Hz) and would not be expected to respond to more rapidly timevarying fields. Also, as illustrated by the following quotation, the use of magnetic field ‘maps’ for long-distance navigation, rather than simple orientation, has been difficult to prove:

More generally, the present results call for new hypotheses about the navigational tools and mechanisms of marine animals. So far, research in the field has been influenced by two recurrent assumptions. The first is that the Earth’s magnetic field, being a potential source of positional information, is actually used by animals as a navigational map. This hypothesis, suggested in its original form long ago, faces a burden of theoretical problems and has received very little support from the many experiments prompted by the enthusiasm for the findings demonstrating magnetic orientation (but not navigation) in many animals. (Papi et al., 2000)

Attraction, at 0.1 m from the source Orientation to the bioelectric fields of prey, initiation of dives Statistical correlations with stranding locations Attraction, but avoidance at 103 V m1 Evocation of a neuronal response in the area acoustico-lateralis of the medulla oblongata recorded using glass electrodes Turning, acceleration, attack on stimulating electrodes Movements associated with areas of highintensity slope in the earth’s magnetic field Orientation Response of ampullae of Lorenzini (pulses of an afferent nerve passing a signal to the central nervous system) Orientation change Afferent nerve pulses in response to an electrical field signal Respiratory rhythm affected, slowing down of the heart beat Orientation response Theoretical threshold of behavioral response Behavioral threshold to magnetic intensity changes necessary to form a useful magnetic map

Marine Organism Sensitivity to Weak Electric Fields Threshold levels and associated effects of electric fields are presented in Table 2. Available data concerning measured, numerical threshold levels are limited largely to elasmobranchs. It is widely accepted that at least some sharks, skates, and rays have developed the ability to perceive and interpret weak electric fields (of 380 shark species, only 9 have been tested for electroreceptive response). Their sensitivity is developed enough to allow them to detect and orient toward the bioelectric fields of prey organisms. Sharks and rays are able to detect the movement of hidden prey animals that generate electric fields as low as 106 V m1, and their electric pattern recognition is tuned to field frequencies caused by the movement of a gill or beating of a heart in the prey (r1/ 8 s to 8 Hz, e.g., a heartbeat of 120 beats per minute would result in electric fields at a frequency of 2 Hz). It therefore seemed likely that sharks and rays are able to detect moving animals in their surroundings on a purely electrical basis because the threshold of sensitivity

Magnetic Fields: Possible Environmental Health Effects (0.01 mV/cm) [1 mV/m] is much lower than the amplitude of the muscle potentials generated by the sea water by, for example, the gill movements of a plaice (Pleuronectes platessa), which can exceed 1 mV/cm [100 mV/m]. (Kalmijn, 1966)

Much like the human sense of hearing, a shark’s ability to sense electrical fields in a useful way requires not only acute sensitivity but also the ability to discern and interpret a signal of interest within a cacophony of background noises. The dc electric fields induced by global ocean currents range up to approximately 5  105 V m1, yet at least one shark species is able to discriminate the smaller (1–2  105 V m1) bioelectric field of prey. As Kalmijn points out, Therefore the question is not whether the individual receptor cells are sensitive enough to detect the extremely weak electric fields to which the animals respond behaviorally, but how the animals can distinguish the signal from the noise and how it can profitably maintain such a high electrical sensitivity under real-life conditions.

The answer is that sharks are able to interpret the perceived signal by integrating it over space and time. It is this signal separation and interpretation ability that allows animals that can sense electrical fields to obtain useful information and ignore background signals. One other important consideration relative to the discriminatory ability of electrosensitive fish is the form of the electric field. It is not only the size of the field that matters but also the frequency. For example, the response of ampullary nerves of thornback rays in a uniform electric field is limited to the near-dc frequency range from below 1/8 Hz to an upper cutoff of approximately 8.0 Hz. The ability of these same animals to detect, process, and meaningfully interpret electrical stimuli in the frequency range of 50–60 Hz, such as those fields produced by, for example, submarine electric power systems, has not yet been demonstrated. Sensitivity to Weak Magnetic Fields Threshold levels and associated effects of magnetic fields are also presented in Table 2. The literature on magnetic field supports several general findings: Although the basic magnetic field sense of marine • organisms has not been well studied, the thresholds of magnetic intensity changes required to form a magnetic map in some birds and honeybees (and inferred to exist in whales) are 0.1–2 mG (steady-state dc fields). Reported detection thresholds for steady (dc) electric fields (induced by time-varying magnetic fields) range from 106 to 103 V m1 (i.e., microvolts to millivolts per meter).

555

Crystals of the mineral magnetite (Fe O ) have been • found to be the basis of magnetotactic bacteria being 3

•

•

•

4

able to swim along magnetic field lines. Magnetite has also been detected in honeybees, birds, salmon, sea turtles, and a number of other animals known to orient to the earth’s magnetic field. Magnetite crystals are minute, permanently magnetized bar magnets that twist into alignment with the earth’s magnetic field if allowed to rotate freely. Total magnetic intensity variations of less than 0.5 mG (steady-state dc field, changing by 0.5 mG as a function of location) have been statistically related to stranding events in several species of whales. The frequency range of a shark’s electric field receptors covers approximately 1/8 to 8 cycles per second (Hz), with discriminatory ability being a function of both the size of the field and its frequency. The detection threshold for magnetic intensity gradients (i.e., changes in magnetic field levels with distance) is postulated to be 1.2 nTm1 (0.012 mG m1) in sharks and approximately 0.1% of the total intensity fluctuation of the earth’s magnetic field in whales and turtles. These magnetic intensity gradients are all based on slow changes in the earth’s steady dc field as a function of location. Magnetic field sensitivity appears to exist for dc and near-dc frequency magnetic fields, but has not been demonstrated (and may not exist) for 50–60 Hz magnetic fields such as those associated with, say, undersea electric power cables.

The ability to orient in the earth’s magnetic field has been shown in sharks and rays. By moving through highly conductive seawater even at very slow speed (0.01 m s1), sharks generate an electric field strong enough to inform them of their compass heading. For example, by moving at 1/2 m s1 through the earth’s magnetic field, sharks experience a steady electric field of approximately 105 V m1 when moving perpendicular to the magneticfield direction (east–west), but zero electric field when moving parallel to the magnetic field (north–south). This motion-induced electric field, which is not available to organisms inhabiting (nonconducting) freshwater, terrestrial, or atmospheric media, appears to work despite the interfering and confounding effects of electric fields created by local eddies of ocean water. This magnetic sense can likely detect minor changes in the earth’s steady magnetic field. Another type of magnetic sensory function, operating with a different mechanism, which has been demonstrated in several organisms and may be present in all animals, is associated with specialized receptor cells containing biogenic magnetite. That is, micron-sized magnetic iron oxide particles synthesized (or sequestered) by living organisms may act as tiny compasses. The sensitivity to magnetic flux that has been

556

Magnetic Fields: Possible Environmental Health Effects

reported in some marine organisms could be accomplished by less than a million magnetite-containing cells. Although the detection of magnetic fields by dolphins has not been demonstrated, the observation that magnetite can be found in association with what are apparently nerve fibers suggests that these animals possess a magnetic sensory function. The ability of marine animals to orient to the earth’s steady magnetic field through the electric field generated when swimming through it suggests the possibility of navigation, but does not necessarily demonstrate it, because the second requirement for navigation is the development of a cognitive map: Electrical measurements could be used to determine a compass bearing and to determine the strength and direction of any local electric field gradient. This information is neither sufficient nor necessary for navigation, as shown by the fact that many marine species lacking an electric sense have navigational skills as impressive as those of elasmobranches. (Paulin, 1995)

The ability to sense small fluctuations in background steady dc magnetic fields (such as the earth’s magnetic field) has been implicated in navigational maps used by whales and turtles, but a clear link between magnetic sensory ability and navigation has not been demonstrated to date in marine organisms. One contrary argument for the reliance of marine organisms on magnetic navigation is the fact that any small, natural abnormality or magnetic storm (e.g., from solar flares) could circumvent directional movement. A more plausible argument is that magnetic orientation can lead to the development of a cognitive map that can be modified as required to incorporate any changes to the magnetic landscape, the way humans create visual maps. In general, ac magnetic fields acting on a bipolar magnetic sensing system would cancel, resulting in a time-average magnitude of zero; therefore, they are not detected as a magnetic-field deviation and would not be expected to interfere with the navigation sense of marine organisms. See also: Electromagnetic Fields: Environmental Exposure, Microwaves: Exposure and Potential Health Consequences, Radio Frequency Electromagnetic Fields: Health Effects.

Further Reading Adair RK, Astumian RD, and Weaver JC (1998) Detection of weak electric fields by sharks, rays and skates. Chaos 8(3): 576--587. Ahlbom A, Bridges J, de Seze R, et al. (2008) Possible effects of electromagnetic fields (EMF) on human health – Opinion of the scientific committee on emerging and newly identified health risks (SCENIHR). Toxicology 246(2–3): 248--250. Brown HR, Andrianov GN, and Ilyinsky OB (1974) Magnetic field perception by electroreceptors in Black Sea skates. Nature 249(5453): 178--179.

CADHS (2002) An evaluation of the possible risks from electric and magnetic fields (EMF) from power lines, internal wiring, electrical occupations and appliances. Report of the California Department of Health Services (CADHS), California EMF Program, Oakland, CA. http://www.ehib.org/emf/RiskEvaluation/riskeval.html (accessed November 2009). Feychting M, Ahlbom A, and Kheifets L (2005) EMF and health. Annual Review of Public Health 26: 165--189. Frankel RB and Bazylinski DA (2009) Magnetosomes and magnetoaerotaxis. Contributions to Microbiology 16: 182--193. Frankel RB and Blakemore RP (1989) Magnetite and magnetotaxis in microorganisms. Bioelectromagnetics 10: 223--237. HPA (2001) ELF electromagnetic fields and the risk of cancer. Report of an Advisory Group on Non-Ionising Radiation. http:// www.hpa.org.uk/web/HPAwebFile/HPAweb_C/1194947420620 (accessed November 2009). ICNIRP (2004) ICNIRP statement related to the use of security and similar devices utilizing electromagnetic fields. Health Physics 87: 187--196. http://www.icnirp.de/documents/EASD.pdf. ICNIRP (2009) ICNIRP statement on the ‘‘Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz).’’ Health Physics 97: 257--258. International Agency for Research in Cancer (IARC) (2002) Non-Ionizing Radiation, Vol. 80, Part 1: Static and Extremely Low-Frequency (ELF) Electric and Magnetic Fields. World Health Organization. http:// monographs.iarc.fr/ENG/Monographs/vol80/volume80.pdf (accessed November 2009). International Commission for Non-Ionizing Radiation Protection (ICNIRP) (1998) Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz). Health Physics 74: 494--522. International Committee on Electromagnetic Safety (ICES)/Institute of Electrical and Electronics Engineers (IEEE) (2002) C95.6-2002 IEEE Standard for Safety Levels with Respect to Human Exposure to Electromagnetic Fields, 0–3 kHz. Prepared by IEEE Subcommittee 3, ISBN 0-7381-3389-2. http://www.ieee.org/web/standards/home/ index.html (accessed November 2009). Kalmijn AJ (1966) Electro-perception in sharks and rays. Nature 212: 1232--1233. Kalmijn AJ (1982) Electric and magnetic field detection in elasmobranch fishes. Science 218: 916--918. Kalmijn AJ (2003) Physical principles of electric, magnetic, and nearfield acoustic orientation. In: Collins SP and Marshall NJ (eds.) Sensory Processing in Aquatic Environments, pp. 77--91. New York: Springer. Kirschvink JL (1997) Homing in on vertebrates. Nature 390: 339--340. Kirschvink JL and Kirschvink AK (1991) Is geomagnetic sensitivity real? Replication of the Waler-Bitterman magnetic conditioning experiment in honey bees. American Zoologist 31: 169--185. National Institute of Environmental Health Sciences (NIEHS) (1999) Health effects from exposure to power-line frequency electric and magnetic fields. Report of the NIEHS to the U.S. Congress. NIH Publication No. 99-4493, 67 pp. http://www.niehs.nih.gov/health/ docs/niehs-report.pdf (accessed November 2009). National Institute of Environmental Health Sciences (NIEHS) (2002) Questions and Answers About EMF. National Institute of Environmental Health Sciences and U.S. Department of Energy, 2002. http://www.niehs.nih.gov/health/topics/agents/emf/docs/ emf2002.pdf (accessed November 2009). NRC (1997) Possible health effects of exposure to residential electric and magnetic fields. Report of the National Research Council, Committee on the Possible Effects of Electromagnetic Fields on Biologic Systems. Washington, DC: National Academy Press. http:// www.nap.edu/openbook.php?isbn ¼ 0309054478 (accessed November 2009). NRC (1999) Research on power-frequency fields completed under the Energy Policy Act of 1992 National Academy of Sciences. Final Report: Evaluation of the EMF RAPID Program, June, 107 pp. National Research Council, National Academy Press. http:// www.nap.edu/catalog/9587.html (accessed November 2009). ORAU (1992) Health effects of low-frequency electric and magnetic fields. Report to the Committee on Interagency Radiation Research

Magnetic Fields: Possible Environmental Health Effects

and Policy Coordination by the Oak Ridge Associated Universities Panel. ORAU 92/F-8 and NTIS Publication #029-000-00443-9, 368 pp. Papi F, Luschi P, Akesson S, Capogrossi S, and Hays GC (2000) Opensea migration of magnetically disturbed sea turtles. Journal of Experimental Biology 203: 3435--3443. Paulin MG (1995) Electroreception and the compass sense of sharks. Journal of Theoretical Biology 174: 325--339. Valberg PA (1995) Designing EMF experiments: What is required to characterize ‘‘EMF Exposure’’. Bioelectromagnetics 16: 396--401. Valberg PA, Van Deventer TE, and Repacholi MH (2007) Base stations and wireless networks: Radiofrequency (RF) exposures and health consequences. Environmental Health Perspectives 115: 416--424. Valberg PA, Kavet R, and Rafferty CN (1997) Can low-level 50/60 Hz electric and magnetic fields cause biological effects? Radiation Research 148: 2--21.

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Walker MM, Diebel CE, Haugh CV, Pankhurst PM, Montgomery JC, and Green CR (1997) Structure and function of the vertebrate magnetic sense. Nature 390: 371--376. Wertheimer N and Leeper E (1979) Electrical wiring configurations and childhood cancer. American Journal of Epidemiology 109: 273--284. Wiltschko R and Wiltschko W (1995) Magnetic Orientation in Animals. Berlin: Springer Verlag. World Health Organization (WHO) (2007) Monograph on ‘‘Environmental Health Criteria Volume N1 238 on Extremely Low Frequency Fields,’’ 446 pp. http://www.who.int/peh-emf/ publications/elf_ehc/en/index.html; See also http://www.who.int/ mediacentre/factsheets/fs322/en/index.html (accessed November 2009).