J Clio Epidemiol Vol. 44, No. 9, pp. 895-906, 1991

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THE EPIDEMIOLOGY OF EXPOSURE TO ELECTROMAGNETIC FIELDS: AN OVERVIEW THE RECENT LITERATURE JAMES R. JAUCHBM*

OF

and JAMES H. MERRITT

Directed Energy Division, Occupational and Environmental Health Directorate, Armstrong Laboratory for Human Systems, Brooks Air Force Base, TX 78235, U.S.A. (Received in revised form 6 February 1991)

Abstract-Epidemiologic studies of exposure to electromagnetic fields (EMF) have been reviewed. Possible links to incidences of cancer and abnormal fetal development have been suggested by some investigators. In general, the results have been inconsistent. There are many deficiencies in the studies, and many questions have been raised about the validity of some of the conclusions proposed. There is currently no definitive evidence of an association between exposure to EMF and the alleged risks. Due to problems and limitations inherent in future studies (misconceptions about exposure levels, uncertainty about field variability, criticisms of surrogate measures), this question is unlikely to ever be answered with certainty. Unfortunately, many highly-publicized accounts of speculative and unsubstantiated claims have caused undue concern among the general public. Electromagnetic

fields

Electrical workers

Cancer

Fetal development

speculative notions have become entrenched in the popular press; some unsubstantiated claims in the scientific literature are widely referenced and often misquoted in the lay literature. Bridges and Preache [l] concluded that “certain sensationalized media accounts have heightened public fears and clouded real scientific issues. Thus the public. . . is largely unaware of the significant difficulties in developing definite knowledge of the effects of extra high voltage lines on humans”. Several review articles concerning epidemiologic studies of health effects of EMF have appeared in the literature during the past several years. Often, articles and letters that refute previous studies are not cited as frequently as the original article. Feinstein and Spitzer [2] addressed the problem of errors of omission when listing references. Although limited journal space may prevent authors from listing all articles or letters responding to previous papers,

INTRODUCTION

There has been increased concern recently about

potential health problems related to exposure to nonionizing electromagnetic fields (EMF). Although some questions have been raised concerning exposure to radiofrequency radiation (at frequencies up to 300 GHz), the major focus has been on fields of extremely low frequency (especially 60 Hz). The most recent epidemiologic studies have dealt mainly with effects of extremely-low-frequency EMF on fetal development and with the initiation or promotion of cancer. The populations studied have included people living in the vicinity of transmission and distribution lines, workers in the electric utility industry, and workers in other industries associated with electrical environments. In general, the biological effect of EMF has been a controversial subject. Many highly *Author for correspondence. 895

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some of these omitted responses are important to the questions at hand. The purpose of this paper is to point out some of these responses, to briefly summarize some noteworthy aspects of the review articles, and to discuss some recent studies and commentaries that have appeared since the previous review articles were published. Since statements are often misquoted in this field, much of the information in this paper is presented by citing direct quotations from the appropriate literature. REVIEW

ARTICLES

Concerning exposures to radiofrequency radiation, Roberts and Michaelson [3] noted that despite the careful review of epidemiologic data, connections that have been effectively dismissed by scientific analysis often continue to be reported in the nonscientific literature and in uncritical scientific reviews. Such reports frequently receive more widespread attention and generate more publicity than warranted. Well-conducted studies, on the other hand, extend previous investigations and conclude that earlier unsubstantiated claims of associations between exposure and adverse effects are not valid. Sheikh [4] summarized his review of EMF and leukemia by stating that “the results of the reported epidemiologic studies are inconsistent. In the few studies showing an association between exposure to EMF and the risk of leukemia, the temporal relationship between exposure and effect was not established, the observed associations were weak, the doseresponse relationships were based on qualitative levels of exposure. . . , and the observed excess of risk in the exposed groups may be a consequence of different sources of bias”. The author also remarked that “of the four case-control studies of non-occupational exposure to EMF, an association with leukemias was demonstrated in only one study . . .. A weak association (crude risk ratio between 2 and 3) coupled with obvious flaws in the design do not make the results of this study definitive evidence of an association between exposure to EMF and the risk of leukemia”. Kavet and Banks [5] provided an excellent overview of the characteristics of the electric and magnetic fields in question, and reviewed cell and tissue studies and whole animal studies, in addition to epidemiologic studies. The authors pointed out several deficiencies of these epidemiologic studies in general, including:

“failure to include accurate measurements of the magnitude and duration of field exposure, lack of adequate control groups, small numbers of subjects, nonspecific symptoms, and absence of documentation of potential exposures to other agents in the workplace (e.g. chemicals)“. The authors also noted that “comparing field exposures between occupations would be difficult even if known, since different occupations involve different mixes of electric and magnetic fields in different frequency ranges”. Kavet and Banks also raised questions “about using outdoor wiring configurations as surrogates for indoor magnetic field exposures. Preliminary data showed that (household) devices produce electric and magnetic fields much higher than the indoor background”. Michaelson and Lin [6] called attention to some of the numerous problems that arise in designing EMF epidemiologic studies. These authors stated that “even if these problems could be overcome, it might involve an inordinate, even astronomic, cost”. Savitz and Calle [7] reviewed epidemiologic studies of leukemia and occupational exposure to EMF, and noted the lack of specificity of risk for leukemia. The data led them to conclude that “unless electromagnetic fields are thought to be general human carcinogens, some other risk factor or study bias must be operating . . . The likelihood of such fields causing leukemia is diminished by the observation of other cancer associations of equal or greater magnitude”. They stressed the importance of exposure identification and pointed out that “none of the studies of leukemia have validated the assumption that the men presumed to be exposed to such fields are, in fact, exposed”. The authors also discussed potential confounding risk factors and methodologic limitations of some of the studies. In a review of cancer incidence among workers exposed to extremely-low-frequency EMF, Stern [8] observed an absence of increased risk for leukemia. A biostatistical review of 32 publications on bioeffects of electromagnetic energy, including epidemiologic studies, found “no conclusive evidence of harmful effects” [9]. Ahlbom [lo] reviewed studies of residential exposure to EMF and concluded that “the results of studies on adult cancer and residential exposure, when combined, do not provide much evidence for an association with all cancers together or with leukemia”. Anderson and Kaune [1 11, referring to occupational

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studies of EMF, noted that “because of limitations in such studies, particularly those based on an analysis of cancer registries or death certificates, one cannot be sure of the significance of these results”. Brown and Chattopadhyay [12], in their review of EMF and cancer (including epidemiologic studies), frequently cited a book by Brodeur [ 131 as presenting scientific evidence of hazardous effects of EMF. The appearance of a more recent book and three articles by Brodeur in a popular magazine have prompted responses from the scientific community. Readers should be aware of these responses [14-201, which point out how the book and articles oversimplify the experimental evidence, contain problems of interpretation, and ignore the body of work in this area in the scientific literature. Brown and Chattopadhyay assessed an article by Steneck [21] as being a “cool-handed essay” on microwave bioeffects, but did not mention responses such as that of Merritt [22], who refuted a number of Steneck’s invalid and misleading statements. Brown and Chattopadhyay remarked that “it is now apparent and uncontested that many nonionizing radiation biological effects are ‘not thermal’ but rather result from a direct interaction between the EM-field and part of the molecule, entire molecule, or molecular micelles”. The existence of nontherma1 effects, however, has not been established and is certainly not accepted by a majority of investigators. Brown and Chattopadhyay cited Robinette et al. [23], who reported on health effects of occupational exposure to microwave radiation. Subsequent responses to this work were not mentioned. For example, Morton [24] pointed out an inadequate selection of a control (or “low exposure”) group; Robinette [25] responded that no better control group existed. Coleman and Beral [26] indicated that “even if it were confirmed that particular groups of electrical workers experience excess rates of acute myeloid leukemia, it would not necessarily follow that the cause is exposure to electromagnetic fields. Electrical workers may be exposed to other physical and chemical agents, some of which may be leukemogenic, and there is no simple way of distinguishing the different potential causes”. Savitz et al. [27] cited the work of Wertheimer and Leeper [28], who claim a connection between electrical wiring configurations outside residences and childhood cancer. One criticism

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of Wertheimer and Leeper’s work, in the form of a Letter to the Editor [29], was not mentioned by Savitz et al. In this letter, Miller criticized the indirect measures of exposure to EMF and pointed out that a dose-response relationship, which had been suggested by Wertheimer and Leeper, was not present. Wertheimer and Leeper [30] stated that even if direct measurements were made, the interpretation of EMF effects “is further complicated by our not yet knowing whether it is a high peak dose or continuity over long periods of some lower-dose exposure that is important”. Savitz et al. also cited a reanalysis of Severson et aZ.‘s [31] study of leukemia and exposure to magnetic fields. The reanalysis, performed by Wertheimer and Leeper [32], supposedly resulted in a strengthening of the association between acute nonlymphocytic leukemia and residential exposure to magnetic fields. Savitz et al. [27] did not mention the response to Wertheimer and Leeper that came from Severson et al. [33] in the same issue of the journal. The response indicated that the post hoc analyses of Wertheimer and Leeper were inappropriate and led to erroneus conclusions. Severson et al. noted that reanalysis of the data suggested that, “if anything, risk decreases somewhat as exposure increases”. In epidemiologic studies reviewed by Silverman [34], relative risks for an association with EMF were generally between 1.2 and 2 for all cancers or for specific types such as leukemia and nervous system tumors. This risk was contrasted with the IO- to 30-fold difference between cigarette smokers and nonsmokers concerning lung cancer. Roeleveld et al. [35] performed a review of occupational exposure to a wide range of agents and defects of the central nervous system in offspring. These authors found no association between exposure to nonionizing radiation and congenital defects. Gamberale [36] noted the highly contradictory nature of epidemiologic studies of exposure to extremely-low-frequency EMF. Adey [37], in a review of EMF and possible effects on cancer promotion, stated that Savitz et al. [38] “replicated” the work of Werteimer and Leeper [28]. This was a misleading statement; Savitz and his team demonstrated only a negligible correlation of cancer incidence with either electric or magnetic fields. Adey further stated: “a conclusion of this study . . . is that 10 to 15% of childhood cancer may relate to environmental EM field exposure”. Although

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Adey attributed this conclusion to Savitz et al. [38] a statement to this effect did not appear in their report. It was actually a panel of the New York State Power Lines Project that presented this type of interpretation. In addition, the panel stated that this conclusion would be valid on/y if: (1) the wiring-code distribution observed by Savitz in Denver was consistently present at other locations; and (2) the cancer-causing properties of EMF could be proven. These two conditions have not been met. Adey, referring to a study by Thomas et al. [39], stated that “microwave workers with more than 20 years’ job experience . . . have a IO-fold risk of brain tumors (astrocytomas) . . . the risk was related to duration of cumulative microwave exposure”. Adey failed to mention an important point: Thomas et al. noted that workers with microwave exposure in jobs “not involving electronic and electrical equipment design, manufacture, installation, or maintenance did not have an excess brain tumor risk. In addition, assemblers,who held electronics jobs but were presumably not exposed to MW/RF radiation, experienced excess brain tumor risk”. Thus, the risk was related to duration employed in electronics manufacture and repair jobs, and not (as Adey erroneously stated) to microwave exposure. RECENT STUDIES

Since the reviews mentioned above were published, other reports of epidemiologic studies have appeared. Coleman et al. [40] found little association between leukemia and residing near electricity transmission equipment. PrestonMartin et al. [41] reported an association of gliomas with duration of employment in jobs supposedly “involving more than the usual exposure to electric and magnetic fields”. These occupations included “electrical engineers, radio operators, telegraph operators, electricians, electrician apprentices, data processing machine repairmen, household appliance and office accessory installers and mechanics, machine mechanics and repairmen, radio and television repairmen, and telephone linemen and splicers”. The authors stated that “although it is tempting to assume that this increase is attributable to increased exposure to extremely low frequency electromagnetic radiation, the mechanism for such an effect is not understood and the possible association of brain tumor risk with other exposures common to this industry (such as organic solvents) must be considered”.

Pearce et al. [42] reported on a series of case-control studies of cancer in electrical workers. Although some increased odds ratios for leukemia risk in “electrical workers” were found, according to the authors, “the findings for specific categories of electrical work should . . . be regarded with caution due to the small numbers involved”. Pearce et al. also mentioned an error in identifying occupation categories in one of their previous reports. Reif et al. [43] found that, out of 18 occupational categories, the odds ratio for brain cancer in electrical workers was ranked 15th. Johnson and Spitz [44] assessed the association between childhood nervous system tumors and paternal employment in occupations involving potential exposure to EMF. The authors concluded that “it is conceivable that EMF exposures could play a role in the genesis of mutations or tumors, although convincing evidence has yet to be reported”. They mentioned their concern of “confounding due to unmeasured lifestyle and exposure variables. One group of known exposures consists of the diverse chemicals utilized in electrical and electronics occupations”. The highest risk estimate in their analysis was for construction electricians, who work mainly with unenergized wiring, thereby seeming to have limited exposure to EMF. Bunin et al. [45], in an attempt to replicate earlier results of Spitz and Johnson [46], found no significant associations between neuroblastoma and parental employment in jobs with EMF exposure. Another case-control study of the association between paternal occupation and neuroblastoma in offspring was performed by Wilkins and Hundley [47]. In interpreting their results, the authors cautioned that “the number of case and control fathers classified as occupationally exposed to EMF by any definition is relatively small, and the number varies considerably from definition to definition”. They concluded that “for two of the exposure defiodds nitions employed. . . the corresponding ratios were modestly elevated” and that “additional studies are warranted, even though the biologic plausibility of the association is tenuous”. Wertheimer and Leeper [48] studied fetal loss in families living with, or without, ceiling cable electric heat. Overall fetal loss was essentially the

same for the two groups. However, the authors then presented the data in a different format-“a monthly running ratio of abortions

Electromagnetic Field Exposure

occurring among those exposed to electromagnetic field sources (ceiling cable electric heat), divided by abortions occurring in unexposed controls”. The authors claimed that the data were analyzed in this way to cope with unaddressed confounders, and that “one group (e.g. the group exposed to ceiling cable heat) can serve as both the exposed and the unexposed group, depending on time of year”. In spite of this statement, however, the data were still presented as ratios of homes with ceiling cable heat to those without. Presenting the data in this manner does not adequately deal with unaddressed confounders. What the authors did not consider is that ceiling cable heat may differ from conventional heat in factors other than EMF levels; whether or not indoor climatic conditions in homes with ceiling cable heat differed from those in other homes was not addressed. Socioeconomic characteristics are often associated with prenatal outcomes [49] and can affect birth rates [50]. It has been suggested that socioeconomic conditions could influence home heating situations [50,51]. Kevan [52] noted that seasonality of live births could be related to seasonality of home heating conditions. With the exception of mothers’ ages and years of mothers’ education, Wertheimer and Leeper did not consider possible differences in socioeconomic factors between the two types of homes. Wertheimer and Leeper stated that “a seasonal variation in that ratio (mentioned above) indicates that the exposed group has a seasonal pattern that differs from the ‘normal’ seasonal pattern of the unexposed group”. In their study, however, while the seasonality of spontaneous abortions in homes with ceiling cable heating was similar to that reported in other studies (which did not deal with different types of heating) [53, 541, seasonality in other homes was not. Thus, homes without ceiling cable heat did not necessarily show a “normal” seasonal pattern. In addition, their results of live birth data did not follow the normal seasonal pattern seen in other studies [55, 561. In general, since the majority of spontaneous abortions are not detected in restrospective studies, the results are usually ambiguous and contradictory [57]. After completing their data analyses, Wertheimer and Leeper reported that the ratio of abortions in the exposed group to the unexposed group was correlated with “change in heating degree days”. As Bailar and Mosteller

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[58] have pointed out in other studies, “the authors’ industriousness or skill in fishing through the data for a favorable result” must be considered by the reader. Results of a study by Verreault et al. [59] suggested that there was little evidence for an association between the use of electric blankets and the risk of testicular cancer. Savitz et al. [60] reported a weak positive association between prenatal exposure to electric blankets and childhood cancers. As the authors noted, “results are limited by nonresponse and imprecision resulting from the rarity of appliance use”. Garland et al. [61] reported that, in an analysis of 16 different U.S. Navy occupations (compared with both the National Cancer Institute’s Surveillance, Epidemiology, and End Results Program, and with the whole U.S. Navy population), electrician’s mates exhibited an excess incidence of leukemia (standardized incidence ratio of 2.5). Their statement that “electrician’s mates operate in an environment in which the 60-Hz electric power is generated, transmitted, and transformed”, was not supported with data on direct measurements of exposure. Indeed, estimates of exposure were not even presented. These authors suggested that their finding “should be considered in the context of the literature supporting an association between exposure to electromagnetic fields and increased risk of leukemia”. With this context in mind, the study, like others supposedly supporting an association between EMF exposure and cancer, is not definitive. Loomis and Savitz [62] reported that men employed in electrical occupations had odds ratios of 1.4 for brain cancer and 1.O for leukemia, compared with workers in all other occupations. Juutilainen et al. [63] reported that the incidences of leukemia and central nervous system tumors were slightly elevated in workers presumably exposed to extremely-lowfrequency magnetic fields. These investigators grouped occupations into three exposure categories and stated that “the percentage of exposed workers is certainly higher in the categories of probable and possible exposure than in the category of no exposure”. As Savitz and Calle [7] have pointed out (as mentioned above), however, there is no evidence that the presumed exposure of workers has, in fact, occurred. Regarding central nervous system tumors, Juutilainen et al. [63] remarked that “the dose-response gradient does not appear as steep as that of leukaemia”. In fact, the relative

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risks for the “probable” and “possible” exposure categories were identical, indicating no gradient. The association between exposure of 50- or 60-Hz EMF and the risk of cancer is currently being reexamined in several large epidemiologic studies [64]. CURRENT STATE OF EPIDEMIOLOGIC STUDIES

Silverman, in 1979 [65], noted that, concerning epidemiologic studies of microwave effects, “it is difficult to identify exposed populations, select suitable controls, and obtain exposure data”. The same can be said today of the recent EMF studies completed. It is Shore’s [66] opinion that “given these intrinsic methodologic problems and the questionable biologic plausibility of the relationship, it seems premature to conclude that exposure to EMF causes leukemia based on the generally small elevations in risk that have been found”. Knave and Floderus [67] stated that “for many of these occupations with presumed exposure, we do not know whether there is, in fact, exposure to magnetic fields . . . and whether or not there is exposure to other known carcinogens. . . . There is no solid evidence from current experimental studies to support the hypothesis of carcinogenicity”. Other misconceptions about exposure levels have been mentioned by Osepchuk [68]. Savitz and Chen [69], in a general review of parental occupation and childhood cancer, pointed out that “The associations are virtually all between occupation and childhood cancer, not actually between an occupational exposure and childhood cancer. Job titles may not accurately identify exposures due to errors in reporting, but especially due to the inherent variability in activities and environments associated with any given job title”. Bassett [70] noted that “epidemiologic studies that focus on electromagnetic factors in power transmission and use have largely ignored other sources of environmental pollution. Many of these are inseparable from the field source. For example, electrical equipment usually contains a variety of insulating materials, including PCBs, and some of these are proven carcinogens”. According to Gallagher et al. [71], “workers in electronics and electrical occupations may also be exposed to chemicals and solvents, and the effects of long-term exposure to these compounds should be considered along with exposure to electromagnetic fields”. At least 40

chemicals and mixtures have been reliably found to be human carcinogens [72]. Cartwright [73], in an editorial, mentioned several important points relative to epidemiologic studies of EMF:

(1) “At present there is no evidence for leukaemogenesis linked to EMF in whole animal models”. (2) “There is no direct evidence that any of (the ‘electrically’ related occupations) have greater EMF exposure than the general population; and there are likely to be other potentially leukemogenic exposures in these occupational groups, e.g. the inhalation of fumes from combusted solders and fluxes”. (3) “Of the five case-control studies that have attempted to link EMF exposure with childhood leukaemia, the only convincing statistically significant excess remains in the earliest paper. And even this paper suffers from serious drawbacks”. (4) “So far not enough is known about EMF variability to be able to design useful studies to investigate EMF health effects”. (5) “The majority of published studies, even though they do not show a statistically significant odds ratio, have encapsulated within each study a ratio greater than unity. . . . Such an observation cannot be distinguished from either publication biases or multiple comparison effects”. (6) “The criticisms of surrogate measures mean that no proposed study will ever directly address the issue about which most people want to be reassured”. (7) “With out present state of knowledge there is no justification for the massive expenditure consequential on this design” (of a prospective cohort of women about to become pregnant, followed with their children for many years). (8) “We are thus looking forward to more years of speculation surrounding the supposed adverse health effects of EMF with respect to leukemia, despite the fact that our present scientific knowledge points at the very best to a minute risk of EMF verging on the point of non-existence”. When analyzing epidemiologic studies of EMF and cancer, Ahlbom and Feychting [74] have noted that “the study designs display a number of unorthodox features that render them difficult to evaluate”. As Monson [75] has stated, “if electromagnetic radiation has no

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important adverse health effect, case-control and restrospective cohort studies are not likely to provide unambiguous evidence to allow for a judgment of no association”. Savitz and Chen [69] have pointed out that “the rarity of childhood cancer precludes conducting true prospective studies in which exposures of parents are carefully monitored from before conception to the time of diagnosis”. According to Park [76], “The controversy over the supposed hazards of low-frequency electromagnetic fields began in 1974 with a single epidemiological study in Denver indicating, by a whisker-thin margin of statistical significance, that exposure to 60-Hz magnetic fields from power lines is associated with an increased risk of childhood leukemia. That association simply has not stood up”. As Angel1 [77] has noted: “When an epidemiologic study describing a new risk factor passes critical peer review and is published, how should clinicians respond to it? First, they should probably not advise their patients to change their lifestyles on the basis of one study, no matter how well executed, unless the risk is large”. Bell and Coleman [78] have mentioned that “the epidemiological evidence needs to be particularly strong as there is no clear biological evidence of a mechanism” for EMF effects. As another author wrote: “as for the supposed dangers of electromagnetic radiation, there is patently insufficient evidence, statistical or otherwise” [79]. Moore [80] characterized the evidence for EMF hazards as “a maze of fragmentary, vague and sometimes contradictory observations”. One of the authors of the study of EMF and cancer reported in 1988 by Savitz et al. [38] has said: “It is very noisy data. It’s noisier than anything I’ve ever had a part in publishing, and it’s quoted more than anything I’ve ever published” [81]. Misconceptions about the work of Savitz et al. have been considered previously [82,83]. Feinstein and Esdaile [84] discussed some of the problems that can occur in epidemiologic studies of cancer occurrence. Feinstein [85] wrote about many highly-publicized accounts of substances used in daily life that have been associated with cancer. The author pointed out how scientific standands have often been applied incorrectly to such studies. Some of the problems of epidemiologic studies in general and EMF studies in particular include: the incorrect practice of generating hypotheses retrospectively; the recall bias which can occur regarding exposures that took place many years

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previously; the fact that few agents are received in isolation and that many agents may be difficult to identify reliably; and defining “exposure” only after the data have been collected and analyzed. These problems are apparent when reviewing epidemiologic studies of EMF. Bowman et al. [86] indicated that “the wide variability in field exposures over time and between workers will necessitate better exposure measurements to assess more rigorously the association between leukemia rates and electromagnetic fields”. (The consistent positive association between socioeconomic status and risk of leukemia in children [87] must also be considered.) Feinstein [88] has also noted that epidemiologic studies “are almost always aimed at implicating an agent rather than demonstrating safety or finding beneficial effects. . . . In a datadredging expedition, if agent X seems to promote disease D, the results may be highly publicized; but if the agent seems protective, the finding will often be dismissed as a statistical artifact of multiple comparisons”. Harley [89] indicated that “the shortcoming of risk projection based on retrospective studies is that some of the input data used in the models (such as exposure data) may be poor”. The statements of Feinstein and Harley would seem to also apply specially to EMF studies. Mayes et al. [90] reported on 56 topics in which epidemiologic studies indicated contradictory results, which may have been due, in part, to arbitrary decisions by investigators. The topic of EMF effects could be added to this list. Kupper et al. [9 l] explained how proportional mortality ratios, especially those that are cancer-specific, often give misleading information concerning relative risks. Relative risks on the order of 1.2-2.0 are often difficult to interpret due to confounding with general cultural, socioeconomic, or geographical factors [92]. In studies of EMF and cancer, most of the ratios are below the levels that would indicate a relative excess risk [4]. Yet, Nair and Morgan [93] illogically consider a relative risk ratio as low as 1.4 to indicate a “much increased risk”. Researchers claiming a cause-effect relationship of EMF and cancer on the basis of epidemiologic studies should be aware that statistically significant results may not necessarily support such a relationship. An example of an implausible conclusion based on statistical significance was presented by Feinstein [94]: “Changes in the

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birth rate in Copenhagen once showed a positive correlation with the number of storks sighted flying over the city. . . . The reader may accept wildly implausible conclusions, such as the idea that storks bring babies, because the results are statistically significant”. Regarding studies of EMF and cancer, Marino and Morris [95] have suggested that “when studies are done by different investigators at different times and places using different study groups, the likelihood of a common confounding variable (or other study bias) is proportionately reduced. Thus, the whole is more valuable than the sum of the parts”. This philosophy of “meta-analysis”, however, may be flawed since “although pooled analyses provide a single quantitative result, the combined studies may be methodologically insufficient” [96]. As Feinstein [97] has noted: “big samples sizes often convert small errors into large blunders”. Meta-analysis has been used mainly to assess the effectiveness of treatments or interventions in disease states [98]. In these cases, the independent variable is usually a controlled factor, such as administration of a drug or other therapy. The dependent variable is often quantitative, such as a blood factor which is easily measured. The validity of EMF cause-effect comparisons using meta-analysis, however, where the independent variable is uncontrolled and the dependent variable is not easily measured, must be questioned. John Boice Jr, chief of the National Cancer Institute’s Radiation Epidemiology Branch, has stated: “There’s been an enormous increase in our consumption of electrical power over the last 30 years, so we’re all constantly pervaded by EMFs. During the same period, the incidence rates for childhood leukemias have been relatively stable, not what one would expect to see if increasing EMF exposure were having a major effect” [99]. In the words of The International Non-ionizing Radiation Committee of the International Radiation Protection Association [loo]: “Although some epidemiological studies suggest an association between exposure to 50/60 Hz fields and cancer, others do not. Not only is this association not proven, but present data do not provide any basis for health risk assessment useful for the development of exposure limits”. Although many measurements of magnetic fields and electric fields in residential and occupational settings have been performed [loll, the relative contributions of these different fields in

postulated EMF bioeffects have not been elucidated. As Savitz and Calle [7] noted, “without a firm biologic rationale, it is difficult to specify which type of field (electric or magnetic) is critical or the appropriate dose measure (peak exposure, time-weighted average, etc) to examine”. A preliminary draft of a report on EMF and cancer risks, prepared by the U.S. Environmental Protection Agency (EPA), was mentioned in Nature by Shulman [102]. According to this news item, the director of EPA’s Office of Health and Environmental Assessment, when assessing epidemiologic studies suggestive of a causal relationship, noted uncertainty about the related biological mechanisms and the absence of a dose-response relationship. The EPA report, as quoted in Nature, mentions that “the ruling out of several confounding exposure factors in the Savitz et al. study [American Journal of Epidemiology 128, 21; 19881argue in favor of a causal link between these tumor types in children and exposure to ELF magnetic fields or electric fields”. In another study of Savitz and Feingold [ 1031, however, incidence of childhood cancer was associated with residential traffic density; increased risks for total number of cancers and leukemias were related to increased traffic densities. These data were obtained during the same study of EMF and childhood cancer cited in the EPA report. The odds ratios for this association were greater than those reported earlier for EMF and cancer. As the authors [103] pointed out, one potential consequence of traffic density is a high level of benzene [104], which is a well-established cause of leukemia [105]. (For additional comments on Shulman’s article, see additional reference

[1061-) The EPA report was released as a “Workshop Review Draft” and contained, on each page, the admonition “DRAFT-DO NOT QUOTE OR CITE”. However, in addition to Nature, the popular news media quoted other citations from the draft before scientific review was completed, prior to release of a final version. Nature [IO71 apparently accepted the draft as being a final report. Other misconceptions about the EPA report have been considered previously [log]. An important point concerning studies of EMF is that biological effects are not necessarily biological hazards. As Cox [109] has stated: “Electric and magnetic fields may have real effects on a broad spectrum of biological systems but. . . the fields from power frequency

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systems do not have any significant health effects for the general public”. Cox [ 1lo] also noted that “the public and the press, unfortunately, pay much regard to work . . . which does not stand up to critical scientific analysis”. Articles containing misinformation on the alleged hazards of EMF have recently been published in the lay press (e.g. in the New Yorker, Time, Newsweek, Family Circle, Discover, East West Journal and Woman’s Day). Highly speculative and unsubstantiated claims such as those put forth in these articles should not be accepted as representing the scientific literature of the field. Doll [l 1 l] commented on the relevance of epidemiology to policies for the prevention of cancer. He noted that “the risk may be shown to be, at the most, so small that its absence can effectively be regarded as having been demonstrated . . . although proof of a negative is not theoretically possible”. Aldrich and Easterly [112] indicated that “a reasonable amount of data has been gathered to support the hypothesis that electric fields in and of themselves are not carcinogenic. However, essentially everyone is exposed”. Thus, the authors reasoned that although “no clear public health impact is evident, . . . the simple ubiquity of the exposure (at some degree, low or high) argues for further investigations”. Meyer et al. [ 1131reiterated this belief. Yet Foster and Pickard [114], when considering microwave bioeffects, have said that “such searches for hazards can go on too long, and guidelines for ending them must be established”. The same may be true for studies of EMF at lower frequencies as well. The disabling defect in studies that attempted to relate EMF to health hazards and risk is the lack of assessment of EMF exposure. It is obvious that epidemiologic studies will only make sense when the epidemiologist can identify an exposed population, or at least populations with different exposures. Indeed, due to the ubiquity of EMF in modern society, it may not be possible to find any cohort of unexposed humans. The use of job titles, in this instance, cannot serve as indices of exposure, since jobs listed as “electrical” may not expose the incumbents to EMF any more than the rest of the population. The use of “surrogates” of exposure (e.g. Wertheimer and Leeper [28]) has been shown not to be an accurate indicator of actual fields [38]. From the unscientific epidemiologic studies, and the unwarranted speculations engendered by them, has come an awareness in the public that exposure to EMF may be hazardous to

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human health. The articles in the lay press cited above, and the numerous newspaper stories that appear almost daily, have elicited what Adair [ 181,Osepchuk [ 151,and others call “electrophobia”. Matters have not been helped much by some scientists and policy analysts who have suggested such strategies as “prudent avoidance” [115] to mitigate risks that are not at all apparent. At the very most, accepting at face value the extant studies, the risk of health hazards from EMF exposure as experienced by the general public is vanishingly small. Feinstein [85] has pointed out that the public is uncertain about how to distinguish the false alarms from the real thing in health hazard issues and will have to rely on common sense and their own scientific concepts to evaluate the reported evidence. Epidemiologists can be of value to society by helping such common sensical evaluations by speaking out against the barrage of unscientific studies that have appeared. Rothman and Poole [116] have said that “scientists should ignore policy questions to persevere in pursuit of their objective, which is knowledge”. As explained by Foxman [I 171, however, epidemiologic data regularly influence public policy. Even if an epidemiologist is not a policy-maker, citations of his work can indirectly influence policy. The difficulties associated with epidemiologic methods are often misunderstood by the public. Foxman [ 1171mentioned a classic example of this, in which a case-control study was mentioned in a National Enquirer article. Even though subsequent cohort or case-control studies provided no evidence to support the original claim, “the controversy and subsequent litigation were sufficient to remove a well understood and apparently effective drug from the market. . . . Although the epidemiologic community understands that newsworthy hypothesisgenerating studies, leading to intriguing hypotheses . . . are not designed to test hypotheses, but merely suggest areas for future research, this distinction may be invisible to the public and policy-maker alike”. According to The Lancet [118], “concentrating attention on small risks that are politically charged or that attract media attention” can cause the decisionmaking process to become unbalanced. “As a result, morbidy increases because technological benefits are held up”. We suggest that epidemiologists should attempt to place public health risks in the proper perspective and communicate information on risks and priorities to the public and political leaders.

JAMESR.

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JAUCHEMand JAMESH. MERRI~

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