Overview of EMF Research
The transport of electricity is described in terms of both its voltage and current flow. Using these terms, the transport of electricity is analogous to the flow of water through a pipe. The pressure driving the water is the counterpart to the voltage on the power line, and the amount of water flowing in the pipe is the counterpart to the amount of electric current on the line.
Electric Field. Electrical lines and equipment produce an electric field as a result of the voltage applied to their wiring. The strength of the electric field is expressed in terms of volts per meter (V/m) or kilovolts per meter (kV/m).
The electric field strength falls off sharply with distance. Objects such as houses or trees shield electric fields. Thus, even in proximity to power lines or substations, the electric field in nearby residences is largely a result of internal sources; external sources of electric fields are effectively shielded from indoor environments.
Magnetic Field. Current flow in electrical lines and equipment produces a magnetic field. The strength of the magnetic field is measured in units called Gauss or milligauss (mG) (one one-thousandth (1/1000) of a Gauss). Most often, magnetic field intensities encountered in daily life are expressed in milligauss.
The field intensity varies with the amount of current flow. Like electric fields, the intensity of a magnetic field decreases as distance from the source increases. But, unlike electric fields, buildings, trees and most other objects do not provide shielding from magnetic fields.
Magnetic Field Sources. Electrical transmission, power and distribution systems are not the only sources of magnetic fields. Within homes and work places, local sources of magnetic fields include building wiring and plumbing, electric blankets, electric stoves, computer terminals, bedside clocks, ceiling fans, and other appliances that people may use for prolonged periods.
It is noteworthy that some of the common sources of higher magnetic fields are appliances and electrical devices found within the home. The magnetic field levels from such sources in typical use can range up to thousands of milligauss or higher; however, the duration of exposure from many appliances is typically much shorter than that from other sources.
Thus, exposure to both electric and magnetic fields occurs continuously, and is not simply a function of living or working near a power line or facility. Exposure depends upon the many sources and field strengths that are present where a person lives, works and otherwise spends time.
1960s to 1970s. In the late 1960s and early 1970s the possibility of adverse health effects resulting from exposures to electric fields received considerable attention. This attention was motivated by reports from the Soviet Union of various health complaints among utility workers in high-voltage switchyards.
Subsequent research on electrical utility workers in Europe and North America failed to confirm the presence of such complaints; Soviet investigators later indicated that their earlier concerns had been "overstated."
1980s. In the 1980s, interest shifted primarily to magnetic fields, for two major reasons. First, Wertheimer and Leeper published in 1979 a paper reporting a statistical association between childhood cancer and the apparent current-carrying capacity of the power lines near their residences.
Second, it was recognized that exposure to electric fields from outside sources is limited because of effective shielding by building materials. This was confirmed in studies that failed to find associations between the capacity of outside power lines and electric field levels within homes.
This resulted in a shift in interest to the study of magnetic fields. The shift away from electric fields has been further justified by subsequent residential studies that failed to report even a weak association between measured electric fields and cancer in either children or adults (Savitz et al, 1988; Severson et al, 1988; London et al, 1991).
- EMF Health Research
To assess potential health risks from an environmental agent such as power frequency electric and magnetic fields (EMF), interdisciplinary groups of scientists must consider the results from epidemiologic investigations and laboratory studies on animals, tissues and cells.
What is epidemiology? Epidemiology investigates the patterns and potential causes of disease within human populations. The objective of epidemiology is to evaluate and measure the associations between exposures to environmental factors (e.g., asbestos, benzene) and health outcomes (e.g., lung disease, leukemia). Epidemiological studies look for associations between the exposure of a group of people to an agent (possible risk factor) and the occurrence of disease in that group.
Epidemiology deals with people in their natural environments, so exposures cannot be controlled or limited to the factors being studied. Thus, epidemiology addresses associations with disease outcomes; it does not establish whether a particular agent causes disease.
Epidemiological studies. Some epidemiological studies conducted in community settings have reported weak associations between childhood cancer and estimates of exposure to magnetic fields;. others have reported no association. Those reporting associations are not consistent with respect to cancer type. A universal deficiency in the epidemiological literature concerns exposure assessment.
The ability of surrogate measures to predict power-frequency magnetic field exposures is quite limited. Improved methodology in recent studies has failed to show a commensurate strengthening of the evidence relating to health risks. In the occupational setting, some studies have reported weak associations between work in electrical occupations and leukemia or brain cancer, but other studies have not.
- Laboratory Studies
A wide range of magnetic field intensities at extremely low frequencies (ELF) have been studied in the laboratory to attempt to elicit biological responses and identify the conditions and mechanisms under which they can be produced. At present, there is no accepted biophysical mechanism that can readily explain how a cell could respond to low intensity, low frequency magnetic fields.
Any imposed external electric and magnetic fields must compete with fundamental physical fluctuations (e.g., thermal noise) and endogenous background biological fields (e.g., those generated by the normal activity of the heart, brain, skeletal muscle, and smooth muscle in the gut and airways). Most of the laboratory studies have involved exposures which are hundreds to thousands of times higher than those typically found in residential backgrounds and some occupational settings.
From several thousand studies in the literature, relatively few biological responses are confirmed to occur with exposure to time varying magnetic fields at intensities less than 1,000 mG, and those that have been confirmed have not been clearly linked to adverse health effects.
Although there is considerable interest in determining whether there is any biological basis for a cause and effect relationship between power frequency fields and cancer, the available laboratory data have not provided substantive support for this hypothesis.
Scientific reviews. Numerous internationally recognized scientific organizations and independent regulatory advisory groups have conducted scientific reviews of the EMF research literature. It is their ability to bring together experts from a variety of disciplines to review the full body of research on this complex issue, that gives their reports the credibility and recognition they have received.
Without exception, these major reviews have reported that the body of data, as large as it is, does not demonstrate that exposure to power-frequency magnetic fields causes cancer or other health risks, although the possibility cannot be dismissed.
Most reviews recommend further research. The weakness of the reported associations, the lack of consistency and the severe limitations in exposure assessment in the epidemiology studies together with the lack of support from laboratory studies were key considerations in the findings of the scientific reviews.
Panels charged with recommending exposure limits for environmental levels of electric and/or magnetic fields have concluded that no meaningful experimental data exist (e.g., no dose-response information is available) on which to base standards or limits to which the public is exposed.
- NIEHS RAPID Program