CHARGE TO THE COMMITTEE
Public concern regarding possible health risks from residential exposures to low-strength, low-frequency electric and magnetic fields produced by power lines and the use of electric appliances has generated considerable debate among scientists and public officials. In 1991, Congress asked that the National Academy of Sciences (NAS) review the research literature on the effects from exposure to these fields and determine whether the scientific basis was sufficient to assess health risks from such exposures. In response to the legislation directing the U.S. Department of Energy to enter into an agreement with the NAS, the National Research Council convened the Committee on the Possible Effects of Electromagnetic Fields on Biologic Systems. The committee was asked "to review and evaluate the existing scientific information on the possible effects of exposure to electric and magnetic fields on the incidence of cancer, on reproduction and developmental abnormalities, and on neurobiologic response as reflected in learning and behavior." The committee was asked to focus on exposure modalities found in residential settings. In addition, the committee was asked to identify future research needs and to carry out a risk assessment insofar as the research data justified this procedure. Risk assessment is a well-established procedure used to identify health hazards and to recommend limits on exposure to dangerous agents.
CONCLUSIONS OF THE COMMITTEE
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.
The committee reviewed residential exposure levels to electric and magnetic fields, evaluated the available epidemiologic studies, and examined laboratory investigations that used cells, isolated tissues, and animals. At exposure levels well above those normally encountered in residences, electric and magnetic fields can produce biologic effects (promotion of bone healing is an example), but these effects do not provide a consistent picture of a relationship between the biologic effects of these fields and health hazards. An association between residential wiring configurations (called wire codes, defined below) and childhood leukemia persists in multiple studies, although the causative factor responsible for that statistical association has not been identified. No evidence links contemporary measurements of magnetic-field levels to childhood leukemia.
Epidemiologic studies are aimed at establishing whether an association can be documented between exposure to a putative disease-causing agent and disease occurrence in humans. The driving force for continuing the study of the biologic effects of electric and magnetic fields has been the persistent epidemiologic reports of an association between a hypothetical estimate of electric-and magnetic-field exposure called the wire-code classification and the incidence of childhood leukemia. These studies found the highest wire-code category is associated with a rate of childhood leukemia (a rare disease) that is about 1.5 times the expected rate.
A particular methodologic detail in these studies must be appreciated to understand the results. Measuring residential fields for a large number of homes over historical periods of interest is logistically difficult, time consuming, and expensive, so epidemiologists have classified homes according to the wire code (unrelated to building codes) to estimate past exposures. The wire-code classification concerns only outdoor factors related to the distribution of electric power to residences, such as the distance of a home from a power line and the size of the wires close to the home. This method was originally designed to categorize homes according to the magnitude of the magnetic field expected to be inside the home. Magnetic fields from external wiring, however, often constitute only a fraction of the field inside the home. Various investigators have used from two (high and low) to five categories of wire-code classifications. The following conclusions were reached on the basis of an examination of the epidemiologic findings:
Living in homes classified as being in the high wire-code category is associated with about a 1.5-fold excess of childhood leukemia, a rare disease.
Magnetic fields measured in the home after diagnosis of disease in a resident have not been found to be associated with an excess incidence of childhood leukemia or other cancers.
The link between wire-code rating and childhood leukemia is statistically significant (unlikely to have arisen from chance) and is robust in the sense that eliminating any single study from the group does not alter the conclusion that the association exists. How is acceptance of the link between wire-code rating and leukemia consistent with the overall conclusion that residential electric and magnetic fields have not been shown to be hazardous? One reason is that wire-code ratings correlate with many factors—such as age of home, housing density, and neighborhood traffic density—but the wire-code ratings exhibit a rather weak association with measured residential magnetic fields. More important, no association between the incidence of childhood leukemia and magnetic-field exposure has been found in epidemiologic studies that estimated exposure by measuring present-day average magnetic fields.
Studies have not identified the factors that explain the association between wire codes and childhood leukemia.
Because few risk factors for childhood leukemia are known, formulating hypotheses for a link between wire codes and disease is very difficult. Although various factors are known to correlate with wire-code ratings, none stands out as a likely causative factor. It would be desirable for future research to identify the source of the association between wire codes and childhood leukemia, even if the source has nothing to do with magnetic fields.
In the aggregate, epidemiologic evidence does not support possible associations of magnetic fields with adult cancers, pregnancy outcome, neurobehavioral disorders, and childhood cancers other than leukemia.
The preceding discussion has focused on the possible link between magnetic-field exposure and childhood leukemia because the epidemiologic evidence is strongest in this instance; nevertheless, many epidemiologists regard such a small increment in incidence as inherently unreliable. Although some studies have presented evidence of an association between magnetic-field exposure and various other types of cancer, neurobehavioral disorders, and adverse effects on reproductive function, the results have been inconsistent and contradictory and do not constitute reliable evidence of an association.
The purpose of exposure assessment is to determine the magnitudes of electric and magnetic fields to which members of the population are exposed.
The electromagnetic environment typically consists of two components, an electric field and a magnetic field. In general, for time-varying fields, these two
fields are coupled, but in the limit of unchanging fields, they become independent. For frequencies encountered in electric-power transmission and distribution, these two fields can be considered independent to an excellent approximation. For extremely-low-frequency fields, including those from power lines and home appliances and wiring, the electric component is easily attenuated by metal elements in residential construction and even by trees, animals, and people. The magnetic field, which is not easily attenuated, is generally assumed to be the source of any possible health hazard. When animal bodies are placed in a time-varying magnetic field (as opposed to remaining stationary in the earth's static magnetic field), currents are induced to flow through tissues. These currents add to those that are generated internally by the function of nerve and muscle, most notably currents detected in the clinically useful electroencephalogram and the electrocardiogram. The currents produced by nerve and muscle action within the body have no known physiologic function themselves but rather are merely a consequence of the fact that excitable tissue (such as nerve and muscle) generate electric currents during their normal operation.
General conclusions from the review of the literature involving studies of exposure assessment and the physical interactions of electric and magnetic fields with biologic systems are the following:
Exposure of humans and animals to external 60-hertz (Hz) electric and magnetic fields induces currents internally.
The density of these currents is nonuniform throughout the body. The spatial patterns of the currents induced by the magnetic fields are different from those induced by the electric fields. Electric fields generally are measured in volts per meter and magnetic fields in microtesla (µT) or milligauss (mG) (1 µT = 10 mG).
Ambient levels of 60-Hz (or 50-Hz in Europe and elsewhere) magnetic fields in residences and most workplaces are typically 0.01-0.3 µT (0.1-3 mG).
Higher levels are encountered directly under high-voltage transmission lines and in some occupational settings. Some appliances produce magnetic fields of up to 100 µT (1 G) or more in their vicinity. For comparison, the static magnetic field of the earth is about 50 µT (500 mG). Magnetic fields of the magnitude found in residences induce currents within the human body that are generally much smaller than the currents induced naturally from the function of nerves and muscles. However, the highest field strengths to which a resident might be exposed (those associated with appliances) can produce electric fields within a small region of the body that are comparable to or even larger than the naturally occurring fields, although the magnitude of the largest locally induced fields in the body is not accurately known.
Human exposure to a 60-Hz magnetic field at 0.1 µT (1 mG) results in the maximum current density of about 1 microampere per square meter (µA/m2).
The endogenous current densities on the surface of the body (higher densities occur internally) associated with electric activity of nerve cells are of the order
of 1 mA/m2. The frequencies associated with those endogenous currents within the brain range from less than 1 Hz to about 40 Hz, the strongest components being about 10 Hz. Therefore, the typical externally induced currents are 1,000 times less than the naturally occurring currents.
Neither experimental nor theoretic data on locally induced current densities within tissues and cells are available that take into consideration the local variations in the electric properties of the medium.
Because the mechanisms through which electric and magnetic fields might produce adverse health effects are obscure, the characteristics of the electric or magnetic fields that need to be measured for testing the linkage of these fields to disease are unclear. In most studies, the root-mean-square (rms) strength of the field, an average field-strength parameter, has been measured on the assumption that this measurement should relate to whatever field characteristics might be most relevant. As noted earlier, wire-code categories have been used in many epidemiologic studies as a surrogate measurement of the actual exposure.
Very little information is available on the ambient exposure levels to environmental electric fields other than the rms measurements of field strength. Those might vary from 5 to 10 volts per meter (V/m) in a residential setting to as high as 10 kilovolts per meter (kV/m) directly under power transmission lines. Likewise magnetic-field exposures are generally characterized only in terms of their rms field strengths with little or no information on such characteristics as the frequency and magnitude of transients and harmonics. Residential exposures to power-frequency electric and magnetic fields are generally on the order of a few milligauss.
Indirect estimates of human exposure to magnetic fields (e.g., wiring configuration codes, distance to power lines, and calculated historical fields) have been used in epidemiology.
These estimates of magnetic fields correlate poorly with spot measurements of residential 60-Hz magnetic fields, and their reliability in representing other characteristics of the magnetic field has not been established. Because of the many factors that affect exposure levels, great care must be taken in establishing electric-and magnetic-field exposures.
Unless exposure systems and experimental protocols meet several essential requirements, artifactual results are likely to be obtained in laboratory animal and cell experiments.
Many of the published studies either have used inferior exposure systems and protocols or have not provided sufficient information for their evaluation.
In Vitro Studies on Exposure to Electric and Magnetic Fields
The purpose of studies of in vitro systems is to detect effects of electric or magnetic fields on individual cells or isolated tissues that might be related to health hazards. The conclusions reached after evaluation of published in vitro studies of biologic responses to electric-and magnetic-field exposures are the following:
Magnetic-field exposures at 50-60 Hz delivered at field strengths similar to those measured for typical residential exposure (0.1-10 mG) do not produce any significant in vitro effects that have been replicated in independent studies.
When effects of an agent are not evident at low exposure levels, as has been the case for exposure to magnetic fields, a standard procedure is to examine the consequences of using higher exposures. A mechanism that relates clearly to a potential health hazard might be discovered in this way.
Reproducible changes have been observed in the expression of specific features in the cellular signal-transduction pathways for magnetic-field exposures on the order of 100 µT and higher.
Signal-transduction systems are used by all cells to sense and respond to features of their environments; for example, signal-transduction systems can be activated by the presence of various chemicals, hormones, and growth factors. Changes in signal transduction are very common in many experimental manipulations and are not indicative per se of an adverse effect. Notable in the experiments using high magnetic-field strengths is the lack of other effects, such as damage to the cell's genetic material. With even higher field strengths than those, a variety of effects are seen in cells.
At field strengths greater than 50 µT (0.5 G), credible positive results are reported for induced changes in intracellular calcium concentrations and for more general changes in gene expression and in components of signal transduction.
No reproducible genotoxicity is observed, however, at any field strength. Again, effects of the sort seen are typical of many experimental manipulations and do not indicate per se a hazard. Effects are observed in very high field-strength exposures (e.g., in the therapeutic use of electromagnetic fields in bone healing).
The overall conclusion, based on the evaluation of these studies, is that exposures to electric and magnetic fields at 50-60 Hz induce changes in cultured cells only at field strengths that exceed typical residential field strengths by factors of 1,000 to 100,000.
In Vivo Studies on Exposure to Electric and Magnetic Fields
Studies of in vivo systems aim to determine the biologic effects of power-frequency electric and magnetic fields on whole animals. Studies of individual
cells, described above, are extremely powerful for elucidating biochemical mechanisms but are less well suited for discovering complicated effects that could be related to human health. For such extrapolation, animal experiments are more likely to reveal a subtle effect that might be relevant to human health. The obvious experiment is to expose animals, say mice, to high levels of electric or magnetic fields to observe whether they develop cancer or some other disease. The experiments of this sort that have been done have demonstrated no adverse health outcomes. Such experiments by themselves are inadequate, however, to discount the possibility of adverse effects from electric and magnetic fields, because the animals might not exhibit the same response and sensitivities as humans to the details of the exposure. For that reason, a number of animal experiments have been carried out to examine a large variety of possible effects of exposure. On the basis of an evaluation of the published studies in this area, the committee concludes the following:
There is no convincing evidence that exposure to 60-Hz electric and magnetic fields causes cancer in animals.
A small number of laboratory studies have been conducted to determine if any relationship exists between power-frequency electric-and magnetic-field exposure and cancer. In the few studies reported to date, consistent reproducible effects of exposure on the development of various types of cancer have not been evident. One area with some laboratory evidence of a health-related effect is that animals treated with carcinogens show a positive relationship between intense magnetic-field exposure and the incidence of breast cancer.
There is no evidence of any adverse effects on reproduction or development in animals, particularly mammals, from exposure to power-frequency 50- or 60-Hz electric and magnetic fields.
There is convincing evidence of behavioral responses to electric and magnetic fields that are considerably larger than those encountered in the residential environment; however, adverse neurobehavioral effects of even strong fields have not been demonstrated.
Laboratory evidence clearly shows that animals can detect and respond behaviorally to external electric fields on the order of 5 kV/m rms or larger. Evidence for animal behavioral response to time-varying magnetic fields, up to 3 µT, is much more tenuous. In either case, general adverse behavioral effects have not been demonstrated.
Neuroendocrine changes associated with magnetic-field exposure have been reported; however, alterations in neuroendocrine function by magnetic-field exposures have not been shown to cause adverse health effects.
The majority of investigations of magnetic-field effects on pineal-gland function suggests that magnetic fields might inhibit nighttime pineal and blood melatonin concentrations; in those studies, the effective field strengths varied from 10 µT (0.1 G) to 5.2 mT (52 G). The experimental data do not compellingly
support an effect of sinusoidal electric field on melatonin production. Other than the observed changes in pineal function, an effect of electric and magnetic fields on other neuroendocrine or endocrine functions has not been clearly shown in the relatively small number of experimental studies reported.
Despite the observed reduction in pineal and blood melatonin concentrations in some animals as a consequence of magnetic-field exposure, studies of humans provide no conclusive evidence to date that human melatonin concentrations respond similarly. In animals with observed melatonin changes, adverse health effects have not been shown to be associated with electric-or magnetic-field-related depression in melatonin.
There is convincing evidence that low-frequency pulsed magnetic fields greater than 5 G are associated with bone-healing responses in animals.
Although replicable effects have been clearly demonstrated in the bone-healing response of animals exposed locally to magnetic fields, the committee did not evaluate the efficacy of this treatment in clinical situations.