Harvard Reports on Cancer Prevention
Volume I: Human Causes of Cancer
Cancer Causes & Control:
An International Journal of Studies of Cancer in Human Populations
Official Journal of the International Association of Cancer Registries
Volume 7 Supplement November 1996 ISSN 0957-5243

Electromagnetic Fields

Introduction
A number of epidemiologic studies have reported small excesses of disease associated with proximity to electric power lines. Yet, many of the same studies have found that risks do not correlate with measured levels of electric and magnetic fields (EMF). The specific nature and intensity level of EMF underlying any association with adverse health effects is not known. Moreover, the epidemiologic correlations with utility-line proximity and size are tentative because the increases in risk are low and inconsistent, and the number of cases of disease, such as leukemia, is small. Based on current research, a cause and effect relationship between EMF and cancer, or any other disease, has not been established but cannot be definitively ruled out.

Sources of electric and magnetic field exposure
All matter is composed of charged particles, usually with charges of opposite polarity present in equal numbers. When positive and negative electric charges become separated, we experience electrical effects, such as drawing sparks after walking on a synthetic rug in the wintertime. The work required to produce the electric charge separation is measured by the voltage and the units are volts (V) or kilovolts (kV; one kV = 1,000 V). Electric charges attract (or repel) each other, and the strength of this force at any point is given by the electric field, which decreases with distance from the charges. The units of electric field are volts per meter (V/m) or kilovolts per meter (one kV/m = 1,000 V/m).

An electric current results from electric charges in motion, and units of electric current are amperes (A). The moving charges in an electric current produce magnetic fields, which exert force on other moving charges and decrease with distance away from the electric current. The units used to express the size of magnetic fields are gauss (G), milligauss (mG), or tesla (T), and microtesla (µT) (one mT = 10 µG).

In summary, electrically charged objects (charged positive or negative) produce electrical fields which exert force on other electrically charged objects. Oppositely charged objects attract, like-charged objects repel. Likewise, magnetic fields express the forces between current-carrying objects. Two wires carrying currents in the same direction attract, and two wires carrying currents in opposite directions repel. Permanent magnets contain electrical currents at the atomic level and produce strong magnetic fields (e.g., 100,000 mG). Permanent magnets respond to magnetic fields, such as when a compass needle orients with the earth's magnetic field.

Humans are exposed to a wide variety of natural and anthropogenic electric and magnetic fields. The earth's atmosphere produces slowly varying electric fields (0.1 to 10 kV/m), and the earth's core produces a steady magnetic field, which ranges in strength from about 470 mG to 590 mG over the United States. Steady electric current—direct current (DC)—and nonmoving magnets produce steady, or DC, magnetic fields. With alternating electric current (AC), time-varying magnetic fields are produced that can change both in size and direction. Electric power transmission lines, distribution lines, and electric appliances are sources of electric and magnetic fields that vary in time at a rate of 60 times a second, measured in Hertz (Hz), in North America and 50 times per second (50 Hz) in Europe.

Sixty Hz EMF can be found in the vicinity of electrical wiring and all electrical appliances (Table 1). Near appliances, the magnetic fields can be high (greater than 100 mG), but diminish sharply with distance. In residences, ubiq uitous 60-Hz magnetic fields are found at levels of about 0.5 to 2.5 mG and are produced primarily by electric wiring and the flow of electric currents into the earth, called `grounding currents'.^1,2 Because electric fields are shielded by common building materials and are interrupted by the electrically conducting nature of the body, health-effects studies have centered primarily on magnetic fields, which are not easily shielded and which were first thought to be associated with childhood cancer. Consequently, the term EMF exposure usually refers to a 50- or 60-Hz magnetic field exposure.

Table 1.

A. 60-Hz Magnetic fields at varying distances from household appliances⁴¹

B. Typical 60-Hz magnetic fields in homes, not in the vicinity of appliances

——————————————————————

Scientific reviews of EMF health effects
A number of scientific groups and regulatory agencies have investigated the question of possible injury to health arising from exposure to power-line EMF.^3-10 Three US federal agencies have produced review documents on EMF health effects over the past seven years. The first document was the Congressional Office of Technology Assessment's Biological Effects of Power Frequency Electric and Magnetic Fields,¹⁰ which concluded that while no clear-cut evidence for health effects was to be found, it might be wise to practice a `prudent avoidance' strategy to minimize exposure in cases where the economic costs are low. A prudent avoidance strategy is difficult to apply in practice because scientists have been unable to identify what aspect, if any, of EMF is to be avoided.

The second document, Evaluation of the Potential Carcinogenicity of Electromagnetic Fields,⁸ appeared in 1990 when, after two years of study, the Environmental Protection Agency (EPA) produced a draft review of the evidence for the carcinogenicity of EMF. Subsequently, EPA's Science Advisory Board, concluded that data avail able at the time of the draft report were not sufficient to show that EMF are carcinogenic and recommended that the EPA report be rewritten in a more balanced fashion.^11,12 The revised report has not yet appeared.

A third analysis was requested by the US Department of Labor and conducted through the federal Committee on Interagency Radiation Research and Policy Coordination.³ This analysis found no evidence that EMF generated by sources such as video display terminals, local power lines, and household appliances are health risks. In spite of the fact that a number of scientific review groups have failed to establish a hazardous EMF level or even a definitive EMF-disease link, the issue continues to capture the public interest.

Human epidemiology studies of EMF
Some epidemiologic studies suggest EMF health effects, but uncertainties about exposure assessment, confounders, bias, and statistical significance have caused the results to be questioned. There is a lack of support from laboratory experiments with cells and animals. Moreover, epidemiology has only shown statistically significant correlations with surrogates of EMF exposure and not with actual EMF measurements. Statistically significant associations are not sufficient to establish a cause and effect relationship. Other factors, notably confounders and bias, may be responsible for the cause of the observed association. A confounder can be any other environmental influence that may be linked with both the exposure under examination and the disease outcome. For example, in studies of any association between power lines and childhood cancer, a confounding factor could be the existence of characteristics common to neighbor hoods with heavy-duty power lines, for instance, fumes from vehicle traffic, nighttime street illumination, age of homes, and socioeconomic status. Bias can result when the comparison or control group used in a study is not similar enough to the affected group, for example, if there is an underrepresentation of homes near heavy electrical wiring in the comparison group.^13,14

Although epidemiology has the advantage of dealing with effects in humans, it is difficult to assess a dose response relationship when we cannot accurately quantify EMF exposure. In some cases of occupational exposure to chemicals (e.g., inhalation of airborne arsenic, nickel, and coke-oven emissions), it has been possible to estimate exposure adequately, and epidemiologic studies have provided quantitative estimates of risk. For EMF, success in this regard has not been achieved. The complexity of the home and office EMF environment suggests a multitude of different parameters by which EMF exposure could be defined,¹⁵ and the lack of knowledge regarding the correct EMF parameters relevant to biologic effects makes it difficult to identify, let alone test, dose-response relationships using epidemiologic studies.

Cancer risks related to EMF exposure have been studied in children, adults, and workers in occupational settings. A number of detailed reviews of the epidemiologic literature have been published.^{13, 16-20} Although childhood leukemia studies provide some support for the hypothesis that surrogates of exposure to magnetic fields are associated in some cases with increases in the risk of cancer, reviews of the EMF epidemiology studies have emphasized the difficulty of drawing conclusions about causation in the absence of stronger results and stronger biologic plausibility.^17,21 The epidemiology to date has shown correlations for the childhood cancer studies, but then only with surrogates of EMF exposure. Also, historical trends in childhood leukemia incidence^22-24 show no correlation between historical increases in per capita electric power consumption and childhood leukemia; absence of such an ecologic correlation is reassuring but not conclusive of EMF safety. Usually, occupational epidemiology provides the strongest signal for toxins produced by human technology, but in the case of EMF, the occupational risks are low despite high exposure, and study findings are not consistent.²⁵

EMF interactions with biological systems
Although numerous biological effects of EMF have been suggested in cell and animal experiments, reproducible effect has yet to be identified. For example, the way cells use genetic information had long been reported as a `real' effect of EMF, but now these results have been shown to not be reproducible.²⁶

A mechanism of action by which ambient levels of power-line EMF may produce biologic effects has not been found. A problem with any EMF mechanism is the small magnitude of the EMF produced in the body by outside sources relative to the much larger electrical activity naturally occurring in the body. At a fundamental level, the interactions of electric charges on ions, molecules, proteins, and membranes are integral to many biologic phenomena. It is plausible to speculate that exposure to environmental electric and magnetic fields, which can exert forces on fixed and moving charges, may have the potential to modulate the function of biologic systems. However, unlike other kinds of radiation, which can break apart biologic molecules, the energy contained in power-line electric and magnetic fields is too feeble to alter biologic molecules in any direct way.²⁷ EMF oscillating at 50 to 60 Hz can induce weak currents and voltages in the body, but scientists have yet to identify the mechanism by which these weak currents and voltages could make their presence felt in the midst of the much greater electrical activity that naturally occurs in living organisms.

Cancer in animals
The search for EMF carcinogenic effects in animals has been wide-ranging, but despite the importance of such experimental results, experimental studies using EMF have not addressed carcinogenicity in a systematic manner. Cancer is a multistep process, and several stages are commonly identified in the development of malignant tumors:

Available evidence indicates that EMF cannot initiate cancer, and research has focused primarily on whether EMF can influence the other steps of carcinogenesis. Reviews of the area have concluded that EMF does not function as a complete carcinogen.²⁸ The low energy of 60-Hz EMF makes it unlikely to act as an initiator, and in vitro cellular studies of initiation are predominantly negative.^29,30 Some studies of co-promotion are suggestive of an EMF effect, but these results have yet to be confirmed.³¹

Although some tumors have been reported in animals, no coherent picture has emerged.^32,33 The evidence, such as it exists, suggests that EMF does not appear to act as an initiator, promoter, or co-promoter or to stimulate tumor progression and growth.³⁴ No thorough carcinogenesis study in animals has yet been completed, but one is currently underway as part of the National Toxicology Program under the auspices of the National Institute of Environmental Health.

The secretion of melatonin by the pineal gland has been suggested as a pathway for EMF effects. The pineal gland is an endocrine organ located near the center of the brain. It secretes the hormone melatonin, which provides important time-of-day (circadian) information to various organs in the body. Changes in blood levels of melatonin occur on a circadian basis with low levels prevailing during the day and high levels at night.³⁵ This variation has been found to be a response to light falling on the retina, with exposure to light causing a depression in melatonin production.³⁵ It has been suggested that electric fields and extremely low-frequency magnetic fields as well as static magnetic fields also may depress melatonin.^36-38

Regulatory guidelines for EMF exposure
Because there is curre ntly no firm indication that electric and magnetic fields cause health effects at any particular level, there is little in the way of a quantitative basis by which to regulate EMF exposure. Nevertheless, public concern, sparked by media reports of the adverse effects of EMF, has resulted in some state guidelines based on maintaining the status quo for EMF exposure. Several states have adopted as guidelines the electric and magnetic field levels that historically have been present at ground level in the corridors for transmission lines. Table 2 illustrates guidelines that limit field strengths within the strip of land bordering transmission lines, called the `right-of-way' (RoW).

Table 2. State guidelines (a) for EMF

The International Radiation Protection Association (IRPA) has published interim guidelines on limits of exposure to 50- to 60-Hz electric and magnetic fields (5). The guidelines are based on two earlier review articles published by the World Health Organization (WHO),^7,39 where WHO concluded that health effects could not be expected for magnetic fields smaller than 50,000 mG. The IRPA guidelines suggest that continuous occupational exposure during the working day should be limited to electric field strengths no greater than 10 kV/m. Short term occupational exposures to electric field strengths between 10 and 30 kV/m are permitted, provided the product of electric-field strength (in kV/m) and duration of exposure (hours) does not exceed 80 for the whole working day. Magnetic fields for workers should not exceed a magnetic flux density greater than 5,000 mG, and for short-term exposures (under two hours) should not exceed 50,000 mG. The guidelines also state that members of the general public should not be exposed on a continuous basis to electric field strengths exceeding five kV/m. Exposure to fields between five and 10 kV/m should be limited to a few hours a day. The general public should not be exposed to magnetic flux densities exceeding 1,000 mG, and magnetic field exposure between 1,000 and 10,000 mG should be limited to a few hours per day.

The American Conference of Governmental Industrial Hygienists (ACGIH)⁴⁰ publishes Threshold Limits Values, which are levels that nearly all workers may be exposed to repeatedly without adverse health effects. ACGIH has published a permissible magnetic field values of 10,000 mG at 60 Hz. However, for workers wearing a cardiac pacemaker, the ACGIH recommends a magnetic field limit of 1,000 mG at 60 Hz. ACGIH limits occupational exposures to electric fields to below 25 kV/m over the frequency range zero Hz to 100 Hz.

Conclusions
The EMF exposure to most individuals in the US is dominated by background EMF in the home and work place environment (Table 3). Ambient 60-Hz fields are generally below 10 mG, and it would seem unlikely that EMF health effects can be expected. Such levels are far below any regulatory levels. Current research has not established a link between EMF and cancer or any other disease, but the possibility of some association cannot yet be definitively ruled out.

Table 3. Comparison of sources of electric and magnetic fields

Summary Points

Suggestions

Suggested Further Reading

References