Virginia Department of Health
Pursuant to Senate Joint Resolution No. 126 of the 1985 Session and Senate Joint Resolution No. 278 of the 1993 Session, this tenth annual report on monitoring of ongoing research on the human health effects of high voltage transmission lines is submitted to the members of the 1995 Virginia General Assembly. Pertinent journal articles, reports, and reviews published in the scientific literature, as well as reviews and documents by various scientific bodies and advisory groups that became available during 1993-1994, have been reviewed and an overview of these publications is presented.
Over the past three decades, both public controversy and scientific uncertainty have surrounded the subject of potential adverse human health effects from exposure to power frequency electromagnetic fields (EMF). The first studies of possible health effects of EMF exposure in an occupational environment were reported from the former Union of Soviet Socialist Republics (USSR) in the mid-1960s. Several subjective complaints, involving the cardiovascular, digestive, and central nervous systems, were reported by electric switchyard workers. Subsequent studies of electric utility linemen in the United States failed to observe the same adverse health effects reported by their counterparts in the former USSR. Since that time, enormous strides have been taken to explore the nature of any association between residential and occupational exposures to EMF and deleterious health effects.
Recently, there has been a growing concern about the possible carcinogenic effects of EMF associated with such exposures. This concern began with the 1979 publication of a report that children who had died from leukemia were two to three times more likely to have lived in homes within about 50-130 feet of high-current distribution lines compared to children who did not have leukemia. Subsequently, additional studies have appeared in the scientific literature that also found an association between residential exposure to EMF and increased risk of childhood cancer. Other epidemiologic studies have also examined increased incidence of leukemia and brain cancer among adults, especially with respect to occupational EMF exposure. In earlier studies there was an implicit assumption that the relevant risk factor was exposure to electric fields. However, virtually all recent epidemiologic studies of cancer have focused on magnetic field exposures as the possible etiologic determinant.
The quest to confirm a causal association between exposure to EMF, specifically magnetic fields, and purported carcinogenic effects continues in the scientific literature. Several recent epidemiologic studies have focused their attention on the occurrence of leukemia and brain tumors among residents near high voltage transmission lines and among workers who were occupationally exposed to EMF. While some studies imply a possible link between EMF and certain types of cancer, others tend to impugn such a relationship. Most of the epidemiologic studies which incriminate exposure to EMF as a cause of cancer have been only suggestive of a relatively small increase in incremental risk over the background incidence. There is an eminent lack of statistical significance in many studies. Studies in which statistical association has been reported confer only a two- to three-fold increase of certain cancers. A relative risk of this magnitude is low and does not necessarily indicate a cause-effect relationship. These reported increases in cancer incidence might be expected to occur on the basis of chance alone or as a result of some unidentified factor or etiologic agent other than EMF. There is no unifying pattern in the published studies with respect to cancer types or site. Empirical estimates of exposure have been extrapolated from the historic line loads and do not accurately represent the extent or intensity of past exposures. In view of the inconsistent and contradictory results in the literature currently available, the notable lack of a dose-response or an exposure-effect relationship, as well as the constraints of epidemiologic studies, it is not yet possible to ascribe EMF from high voltage transmission lines as an etiologic factor for cancers in humans.
Epidemiologic studies examining the possible association between EMF and cancer have some inherent strengths and weaknesses. In order to detect an association between a given risk factor and disease, an epidemiologic study must control for other potential risk factors that may be confounding this association. Even when all potential risk factors are known and controlled to the maximum extent possible, it is frequently impossible to rule out confounding when the strength of an association observed between the risk factor of interest and disease is weak. In reality, it is seldom possible to control for all other potential risk factors, because for many diseases, like various forms of cancer, those other risk factors are unknown. Some epidemiologic studies have found that exposure to EMF may confer a two- to three-fold increased risk of certain cancers. This is a fairly small increase when compared to the association between cigarette smoking and cancer, where the risk is increased by ten-fold or greater. Furthermore, exposure to EMF is universal and unavoidable. Thus, it is not possible to find a control group of individuals who would be unexposed; only populations with relatively greater or lesser exposure can be compared. Also, past exposure can only be estimated based on wiring configuration. There is no biological test to assess past exposure and current environmental measurements may be misleading. The assumption that the exposed group would have had a higher exposure to electromagnetic fields than the rest of the population may not be true and therefore, may skew the interpretation of the results of epidemiologic studies.
Although epidemiologic studies may fail to find an association between a given risk factor and disease, it is practically impossible for any epidemiologic study to rule out the possibility of a weak association. This is because the power of a study to confirm a negative association hinges on the prevalence of the disease of interest and the size of the study population. Because of the rarity of most tumors, any competent epidemiologic study that attempts to rule out very small associations between EMF and one type of cancer would have to include an exceedingly large population. Such a study would almost certainly be cost-prohibitive.
Scientific proof of a cause and effect relationship cannot be readily inferred from a single epidemiologic study. Causality is established using multiple criteria, only one of which is epidemiologic association. Other important factors in confirming a cause and effect relationship include strength of association, consistency and specificity of observations, appropriate temporal relationship, doseresponse relationship, biological plausibility, and experimental verification. None of these factors by itself is sufficient to prove or disprove that an observed association represents a true cause and effect relationship. In the case of EMF, these tests for causality have not been satisfied for the implicit deleterious effects.
Laboratory experiments conducted on cells, tissues, and whole animals have shown that under certain conditions, exposure to EMF can produce changes in behavior, changes in nervous system activity, and alterations in biological rhythms and the production of certain hormones. Biological changes such as these are not necessarily physiologically significant. Hence, it cannot be determined that these biological effects translate into adverse human health effects. The observed effects depend upon various factors, including field strength, frequency, duration of exposure, variability of exposure, rate of change in intensity, and interaction with the Earth's magnetic field. Unlike ionizing radiation, power frequency EMF do not appear to cause direct damage to DNA or other genetic material. Thus, it is believed that exposure to EMF could not, by itself, initiate cancer. However, some scientists have postulated that electric and/or magnetic fields may potentially serve as cancer promoters (an agent which facilitates the growth of a cancer which has already been initiated). These hypotheses are now being tested by researchers.
The Virginia Department of Health and the State Corporation Commission will continue to monitor the ongoing studies on the subject, and will inform the members of the General Assembly should evidence emerge establishing a clear link between adverse human health effects and EMF exposure.