September 04, 2001
ELECTRO-MAGNETIC FIELDS FROM POWER LINES
By: Kevin E. McCarthy, Principal Analyst
You requested a discussion of the physics and possible health effects of electromagnetic fields (EMF) produced by power lines.
Electric transmission and distribution lines, electric wires in the home, and household appliances all produce electric and magnetic fields. The strength of both types of fields decreases with distance from the source. Electric fields are stopped by such things as structures and trees, as well as human skin, while magnetic fields are not. As a result, the focus of concern regarding potential health effects of EMFs has been on magnetic fields.
Several early epidemiological studies found a correlation between magnetic field exposure and cancer rates, particularly for childhood leukemia. Most of these studies measured exposure by using a “wire code,” which calculates the distance of a home from power lines and the amount of power the lines carry. These studies found that the homes with the highest wire code rating had a childhood leukemia rate that was approximately 1.5 times as high as the expected rate. However, epidemiological studies have generally not found a correlation between actual magnetic field levels measured in the home and childhood leukemia rates.
More recent research has been conducted by federal agencies, including the National Research Council (NRC) and the National Institute of Environmental Health Sciences (NIEHS). In 1997, NRC issued a report in which a scientific committee found that that there was no conclusive and consistent evidence that exposures to residential EMFs produces cancer, adverse neurobehavioral effects, or reproductive and developmental effects The committee concluded that the evidence did not show that exposure to EMFs presents a human health hazard.
Federal law adopted in 1992 led to the creation of an interagency program investigating a wide range of possible effects of EMF. In 1999, the NRC convened a panel to review this research. It found that people are generally exposed to low levels of EMF in the home. It concluded that it is unlikely that magnetic fields in the home or in most workplaces produce important health effects, including cancer.
In 1999, the NIEHS issued a report with similar findings. It found that the probability that EMF exposure is truly a health hazard is small. However, a scientific panel that participated in the study concluded that, since EMF cannot be ruled out as a possible source of cancer, it should be considered a potential carcinogen.
Power lines, electric wiring, and household appliances all produce EMFs. The fields are invisible lines of force that surround any electrical device. The fields have very long wavelengths and do not cause heating or ionization (the breakdown of molecular bonds). In contrast, forms of energy with very short wavelengths, such as microwaves and X-rays, cause these effects. EMFs do induce electric currents within the body, but these currents are much smaller than those induced naturally through the operation of nerves and muscles. While structures, trees, and skin block electric fields, they do not shield against magnetic fields.
Both electric and magnetic fields weaken with distance from the source. In the case of magnetic fields from a standard three-phase transmission line, the strength is one divided by the distance from the line squared. While the strength of the field from such lines is about 17 milligaus (mG) at the line, it falls to about 8 mG at 10 meters from the line. (A meter is a little over three feet.) At 20 meters the exposure is less than 4 mG.
Most homes and businesses receive 0.1 to 3 mG from power lines and other sources whose magnetic fields vary over time. Much of this exposure comes from sources within the building itself. For example, some household appliances can produce magnetic fields of up to 1 mG in their immediate vicinity. In contrast, the earth's magnetic field (which does not vary over time) is about 500 mG.
POSSIBLE HEALTH EFFECTS
The research on the potential health effects of EMFs from power lines has focused on magnetic fields. As discussed below, some epidemiological studies have found links between magnetic field exposure (particularly as indicated by the proximity of individuals to power lines) and increased cancer risks, primarily with regard to childhood leukemia. No such associations have been found with the electric fields produced by power lines.
There have been three types of research conducted on the potential health effects of magnetic fields — epidemiological, animal, and cellular. Epidemiology studies patterns and possible causes of diseases among humans. The results of these studies are reported in terms of statistical associations between various factors and diseases. However, a finding that a factor is associated with a disease does not necessarily demonstrate cause and effect.
Scientists have also studied the effects of magnetic fields on animals and individual cells. In addition to studying health effects, such as cancer rates, scientists analyze whether magnetic fields change biological processes, for example the production of certain chemicals by cells.
The first study to report an association between power lines and cancer was conducted in 1979 in Denver by Dr. Nancy Wertheimer and Ed Leeper. They found that children who had died of childhood leukemia were two to three times as likely to have lived within 40 meters of a power line that were other children covered by the study. The authors suggested exposure to magnetic fields from the lines as a possible factor in this finding. However, they did not directly measure magnetic fields in the home. Instead, they used the wire code ratings, which calculated the size and number of power lines near the home and their proximity to it.
Subsequent studies in Denver (1988) and Los Angeles (1991) also found significant associations between childhood cancers and wire code ratings. Neither study produced statistically significant associations when actual EMF exposure levels were compared with cancer incidence rates. Other studies, primarily in Europe, also found a correlation between wire code ratings and childhood (and in some cases adult) cancer rates.
Another epidemiological approach estimates EMF exposure using historical data on electrical loads on transmission lines. A 1992 Swedish study found that the relative risk for childhood leukemia was greatest for children living within 50 meters of a power line, although the calculations were based a very small number of cancer cases. In 1994, the Swedish government published a public information document that stated that it suspected that magnetic fields may pose certain risks to health and that there was good reason to exercise a certain amount of caution. But, the document cautioned that “current knowledge is not sufficient for us to tell how magnetic fields affect us. So we do not have a basis on which to set limits.” These studies are discussed in greater detail on the U.S. National Institutes for Health Webpage http://www.niehs.nih.gov/oc/factsheets/emf/humanhlth.htm.
Several states have conducted their own studies. In Connecticut, the Academy of Sciences and Engineering (CASE) published a study in 1992. It concluded that the state of knowledge at that time did not permit firm judgments as to possible adverse effects of magnetic fields. It noted that the epidemiological studies did not support a conclusion that EMF exposure unequivocally increases the risk for cancer or other adverse health outcomes. It stated that if there were such effects, they were not likely to be large. Similarly, CASE found that studies of cells and organisms were difficult to interpret due to their lack of methodological consistency. The CASE report is available in the Legislative Library.
Although this memo focuses on power lines, “cancer clusters” have been found near power substations at several locations (including one in Connecticut). In addition, several epidemiological studies have found higher than expected cancer incidence levels among workers subject to high EMF levels, such as electrical linemen.
National Research Council Study
In 1991, Congress asked the National Academy of Sciences to (1) conduct a review of the literature on the possible effects of EMF exposure and (2) determine whether the scientific evidence allowed a determination of their health effects. The National Research Council (NRC), the academy's parent body, convened a committee of scientists to study the possible links between EMF exposure and cancer. Specifically, NRC asked the committee to (1) review the existing literature on possible links between EMF and cancer, reproductive and developmental abnormalities, and neurobiology; (2) conduct a risk assessment, if the scientific evidence permitted this; and (3) identify future research needs. The committee reviewed existing epidemiological studies and laboratory investigations on cells, tissues, and animals.
NRC published the committee's report in 1997. The committee found that the epidemiological studies linking EMF exposure (as measured by wire codes) and childhood leukemia were statistically significant and robust. But the committee found that the studies had not explained this relationship. It noted that wire codes and other estimates of exposure relate poorly to on-the-spot measurements of EMF exposure, although they are correlated with such things as the age of the housing and nearby traffic levels. The committee found that the epidemiological literature did not support an association between EMFs and childhood cancers other than leukemia, adult cancers, miscarriages, or adverse neurobehavioral outcomes.
According to the committee, laboratory studies on cells did not find any effects from exposure to EMF at levels commonly found in the home that had been replicated in independent studies. There were replicated findings of changes to cellular function (not necessarily adverse) at exposure levels that were 1,000 to 10,000 times higher than experienced in the home. Laboratory studies on animals did not provide convincing evidence of a link between EMF exposure at residential levels and cancer or adverse effects on reproduction or development. The studies provided good evidence of behavioral responses to EMFs at levels well above those experienced in the home, but there was no evidence of adverse neurobehavioral effects even in the presence of strong fields. The animal research did find that exposure to strong fields promotes bone healing in animals.
The committee concluded that the evidence did not show that exposure to EMFs presents a human health hazard. Specifically, it concluded that there was no conclusive and consistent evidence that exposure to residential EMFs produces cancer, adverse neurobehavioral effects, or reproductive and developmental effects. The study is available online at http://books.nap.edu/books/0309054478/html/.
Review of EMF-RAPID Program
The federal Energy Policy Act of 1992 led to the creation of the interagency EMF-Research and Public Information and Dissemination (EMF-RAPID) program. One of the major goals of this program was to determine whether exposure to EMFs from the generation, transmission, and use of electricity caused health effects. The program studied a wide range of issues, including possible links between EMF and cancer, neurobiology, reproduction and development, and cellular and genetic research.
The NRC established a committee of scientists and engineers to review the activities conducted under the program. The committee issued its report in 1999. The committee found that the research demonstrated that most people are exposed to less than 2 mG in the home and very few people are routinely exposed to more than 4 mG. The committee found that the biological research contributed little evidence linking magnetic field exposure and cancer. And it found that the largely unpublished data from the program supported NRC's 1997 conclusion that it is unlikely that magnetic fields in the home or in most workplaces contribute to produce important health effects, including cancer.
National Institute of Environmental Health Sciences (NIEHS)
In 1992, Congress asked that the NIEHS study the effects from exposure to residential EMFs. NIEHS conducted laboratory research but did not conduct new epidemiological studies because of time constraints and the existence of several well-conducted studies.
To assist NIEHS in its research, several panels of scientists reviewed the existing data in public hearings. In 1998, one of these panels concluded that EMF did not meet the criteria to be labeled as a "known" or "probable" carcinogen. But a majority of the panel said it could not conclude that EMF plays no role in cancer and thus it should be regarded as "possible" carcinogen. The panel's chair noted that the members believed that the risks posed by EMF were quite small compared to other health risks. The panel found inadequate evidence to link EMFs to adult cancers, to non-cancer diseases such as Alzheimer's, or to birth defects.
In its final report, issued in 1999, NIEHS found that the scientific evidence that EMF causes any health effects is weak. It suggested that the strongest evidence of an effect came from the epidemiological studies. According to NIEHS, these studies provide weak but fairly consistent support for a small increase in the risk of childhood leukemia. (They also provide weaker support for a link between EMF exposure in certain occupational settings and an increased risk of one form of adult leukemia.) In contrast, NIEHS found that virtually all of the research it and others conducted on animals and most of the work done on cells failed to find any biological effects of EMF. NIEHS asserts that these findings weaken, but do not completely discount, the epidemiological evidence.
Overall, NIEHS found that the probability that EMF exposure is truly a health hazard is small. The weak epidemiological associations and lack of any laboratory support for these associations provide only marginal scientific support that exposure to EMF is causing any degree of harm. The study concluded that these findings are insufficient to warrant aggressive regulatory concern. The study is available on NIEHS' Webpage http://www.niehs.nih.gov/emfrapid/html/EMF_DIR_RPT/NIEHS_Report.pdf.