This manuscript was prepared as part of the Environ-mental
Epidemiology Planning Project of the Health Effects Institute, September
1990 - September 1992.
This paper was undertaken at the request of the Health
Effects Institute in conjunction with a working group. I gratefully acknowledge
the contributions of Aaron Cohen, Bill Kaune, Nigel Paneth, David Savitz,
Gary Shaw, and Richard Stevens. I am also grateful for critical reviews
by Charles Poole and Michel Camus.
Few issues have excited as much public health concern and controversy
in the past decade as the alleged harmful health effects of extremely low
frequency electric and magnetic fields (EMFs). The controversy does not
show any signs of abating. The set of papers in this volume provides not
only a review of the scientific evidence concerning the possible human health
effects of exposure to EMF, but more importantly, they provide a number
of prescriptions for future research in this area (1-6). All
of these authors has drawn their own conclusions about future research needs
based on the evidence they presented. This paper serves to provide an evaluation
of the research priorities across the various areas covered by the authors
of the accompanying papers based on the background data they have assembled.
Also, this paper will reflect on the reasons for controversy in this area
and discuss the implications for both scientific research and public health
practice.
By way of introduction, it is useful to summarize briefly, albeit rather
simplistically, the current state of knowledge regarding health effects
of EMFs (2-7). Based on retrospective case-control studies,
associations have been reported between type of electrical wiring configuration
in the vicinity of the household (referred to as wiring code) and risk of
childhood cancers, notably leukemia and brain cancer. [See (2) for
an explanation of wiring code.] Some studies have reported relative risk
estimates in the range of 1.5 to 3.0 among subjects classified as very highly
exposed, with lower 95% confidence limits near 1.0. Other studies have found
no association. While the evidence is not strong, it is suggestive. There
is some correlation between type of wiring code and levels of magnetic fields
in the home, but the relationships still are poorly understood. While electric
and magnetic fields can be measured with relative ease, it is not clear
whether contemporary measurements in homes have much relevance to the estimation
of past EMF levels. The few attempts to relate contemporary measured fields
to cancer risk have produced equivocal or null findings. It is possible
that a true association with EMFs has not been detected because the etiologically
relevant EMF exposure variable has not been assessed. Based on the epidemiologic
studies alone, the statistical evidence is stronger for an association with
wire codes than it is for an association with measured fields.
Many studies, most of them based on death certificate notation of the
decedent's occupation, have examined relationships between cancer risk among
adults and so-called electrical occupations. The interpretation of these
studies is complicated by countervailing biases. On one hand, there may
have been biases in reporting results from such surveillance-type studies,
namely, positive findings are more likely to be reported than null findings.
Also, these studies generally involved posthoc delimitation of certain job
titles as exposed (i.e., the investigators usually patched together a grouping
of electrical occupations after examining the results for individual job
titles). On the other hand, there were biases in the opposite direction,
including the questionable validity of the job title and cause of death
data and the crudeness of the job title designations as indicators of occupational
exposure. Overall, there is a modest degree of consistency among these studies
that shows a slight excess of leukemia and brain cancer in such workers.
There has been suggestive evidence of a link between long-term use of
certain appliances in the household and childhood leukemia risk. There also
is some concordance between the target organs apparently related to domestic
wire code in children and to so-called electrical occupations in adults.
The most obvious common factor, if both of these associations were confirmed,
would be extremely low frequency electric and/or magnetic fields.
The biological plausibility of significant human health effects due to
electric power sources remains controversial. Studies have identified various
physical and biological mechanisms that might explain such effects, if real,
but these are considered speculative. One of the possible mechanisms proposed,
mediated by inhibition of pineal melatonin, would predict the greatest effect
of EMFs on hormone-dependent tumors such as breast tumors. Research to test
this prediction adequately has not been conducted yet, though there are
some hints of excess male breast cancers in some electrical occupations.
Because of the frequently close relationship between carcinogenesis and
teratogenesis, it is instructive and prudent to question the reproductive
effects of EMF. While plausible mechanisms can be envisaged, the evidence
is still too scant to provide the basis for any inference.
There has been concern that the central nervous system would be particularly
susceptible to perturbation. Although there have been several studies purporting
to show some effects of EMFs or their correlates on neurobehavioral outcomes,
such as suicide, these have, for the most part, been flawed or inadequately
reported studies.
Priorities
The listing of priorities is based on the following considerations: What
issue is driving the scientific controversy and concern? What is the strength
of evidence for different health effects? What types of studies would be
needed to evaluate different associations? What methodologic advances would
help most in resolving the uncertainties?
In the history of medicine, by far the most important basis for the discovery
of true etiologic relations has been, and probably will continue to be,
empiric evidence rather than deduction from biologic principles. As Doll
(8) illustrated in the area of occupational carcinogenesis, most
true associations were discovered first as a result of the chance observation
of a cluster of like-exposed cases. If and when a human risk factor has
been identified, basic research can be helpful in elucidating its mechanism
of action. But, as exemplified by the procedures and experience of the International
Agency for Research on Cancer (IARC) Monograph Program for Evaluating Human
Carcinogens (9), epidemiologic evi-dence continues to be the cornerstone
of the process for determining whether a given agent causes a given disease
among humans.
The driving force in this whole controversy has been the observation
initially made by Wertheimer and Leeper in Denver (10), then by Savitz
et al. in Denver (11), and by London et al. in Los Angeles (12)
of an association between childhood cancer, notably leukemia and brain cancer,
and the type of wiring distribution in the vicinity of the home, referred
to as wiring code. A fourth study, in Rhode Island, found no association
(13), but it has been criticized as having used methods that biased
the results towards the null (14). A fifth study, in Sweden, reported
no association for leukemia but a positive association for central nervous
system tumors (15). This was based on a simplified method for assessing
wiring code. In aggregate, this body of evidence supports the hypothesis
of an association between wiring code and childhood tumors. There are four
possible interpretations: a) a positive association that reflects
a true association between EMFs and cancer has been identified correctly,
b) wiring code was confounded by a non-EMF risk factor for childhood
cancer that was not adequately controlled, c) there was a bias generated
by the study design or data collection method, or d) it was a statistical
fluke.
While most attention has focused on the first interpretation, the others
also merit consideration. Although there is no documentation currently available
on the issue, it is unlikely that wire codes are randomly distributed through
a city. There must be many social and geographic correlates of different
wiring codes such as spacing of houses, distribution of single versus multidweller
units, and age of the housing development. Perhaps the true risk factor
is a characteristic of the neighborhood, such as air pollution, population
density, or levels of local immunity to infectious agents; possibly it is
a characteristic of the home related to its age or building materials, or
a characteristic of the family, such as residential mobility. The complex
pattern of leukemia risk as a function of crowding and mobility, possibly
mediated by infectious agents, that was hypothesized by Kinlen (16,17)
in Britain illustrates the kinds of complex and subtle factors that must
be considered. While the reports by Savitz et al. and by London et al. made
some efforts to take socioeconomic status into account, those attempts were
far from comprehensively accounting for the range of possible social and
geographic confounders.
Another objection to the Denver and Los Angeles studies that has been
raised, and that has not been answered satisfactorially, has to do with
the possibility of a biased population control group. Namely, it is alleged
that nonresponse, including noneligibility and nonparticipation, may differ
among cases and controls and may also be correlated with wire code exposure.
For certain parameters, such as use of appliances, the results may also
have been biased by differential quality of response from cases and controls.
Finally, the interpretation of the positive findings as a set of statistical
flukes cannot be dismissed, because the handful of relative risk estimates
has been of borderline statistical significance and have not shown clear
dose-response patterns. Even if we could be satisfied that biases were not
responsible for those findings, the strength of the accumulated body of
evidence (in the sense of a metaanalysis) would not justify concluding that
there is an association before additional studies were consistent in demonstrating
an association.
Although the investigators used wiring code as a proxy for EMFs and subsequent
work has shown that wiring code is correlated with EMFs, the attempts to
relate childhood cancer to EMFs directly have not succeeded yet. This is
not to say that the available evidence disproves an association with EMFs,
but it does not support such an association even to the modest degree that
it supports an association with wiring codes. The physical and biological
mechanisms that have been postulated to explain the alleged harmful effects
of EMFs on humans are largely speculative. The two legitimate reasons for
according this issue a high level of attention are the epidemiologic evidence
linking wiring code to childhood cancer and the fact that EMF exposure is
so pervasive. Were it not for the empiric observations of an association
between childhood cancer and wiring code, the issue of EMF and health would
merit little more scientific attention than the potential harmful effects
of many other common physical and chemical exposures. It might be argued
that its apparent association with wiring code has served to open the issue
of EMF exposure and cancer risk and that it is now appropriate to study
more credible measures of EMF exposure. However, it is not opportune to
put all the research eggs in the EMF basket, and it would be more cost-effective
to give some priority to establishing the validity of the Wertheimer-Leeper
observation. In part, this judgment is based on the fact that the Wertheimer-Leeper
observation is a fairly straightforward hypothesis to evaluate; conversely,
the investigation of one or another of the measured EMF metrics would be
much more expensive and time-consuming to evaluate, and any null finding
in respect to a given measure of EMF exposure will be unconvincing, because
it will be argued inevitably that the wrong metric of EMFs was studied.
If the Wertheimer-Leeper observation were confirmed, it would reinforce
the high priority of research in this general area and it would suggest
two lines of research, the effects of EMFs on cancer risk and the significance
of other correlates of wiring codes as possible risk factors. The combination
of positive findings on the Wertheimer-Leeper observation and null results
on attempts to correlate measured EMFs with childhood cancer would provide
an extremely important lead in searching for the etiology of childhood cancer.
If the association with wiring codes is not confirmed, then the general
priority level for research in this area would be lowered, and the issue
of exploring non-EMF correlates of wire codes would be eliminated.
Thus, if the problem is formulated as the search for the etiology of
childhood cancer rather than the search for the health effects of EMF exposure,
the top priority is to determine whether there is an association between
childhood cancer, notably leukemia and brain cancer, and wiring code. Of
slightly lower priority is the closely related question of whether other,
more direct measures of EMFs are associated with cancer risk. In evaluating
these related issues, there are several facets that require specification,
including the types of cancer on which to focus, whether to assess an effect
in children or in adults, and how to measure the exposure variable. Finally,
the last priority would be to evaluate the effects of EMFs on other health
outcomes. Methodologic developments would be needed at several steps.
Revisit the Available Case-Control Data Sets on Childhood Cancer and
Wire Codes
It is important to try to address the confounding and selection bias
issues in Denver and Los Angeles more thoroughly than previous studies.
It should be possible to ascertain that the observed findings are not due
to uncontrolled confounding by the types of neighborhood characteristics,
dwelling characteristics, and family and/or social characteristics mentioned
above. There may be relevant data already available to the investigators
of the Denver and Los Angeles studies. If not, it would be desirable to
conduct some additional data collection in these areas to detect and to
deal with potential confounders. Ideally, this could involve visits to the
homes of study subjects already interviewed. If this is not feasible, it
could involve examinations of the social and geographic correlates of wiring
codes in representative samples of households in these cities (i.e., to
characterize the exposure-confounder arm of the conventional confounding
triangle).
In the studies of Savitz et al. (11) and of London et al. (12),
random digit dialing (RDD) was used to ascertain eligible controls. It would
be informative to compare, on a sample basis, the kinds of households elicited
by an RDD procedure as compared with those elicited by different procedures
and then to estimate the prevalence of different wiring codes in these cities.
Because of the possibly idiosyncratic nature of telephone coverage and social
behavior, it would be preferable to carry out these methodologic studies
in Denver and Los Angeles rather than trying to transfer inferences from
another locale.
Studies to Replicate Cancer Found in Children
Even if the supplementary studies recommended above confirmed the associations
with wire codes initially reported, these findings would have to be replicated
elsewhere to provide some assurance that they were not statistical flukes
or products of uncontrolled bias. Fortunately, wiring code is a relatively
easy exposure variable to assess and it does not require access to households.
It might be possible for investigators who have previously carried out leukemia
or brain cancer case-control studies to piggyback a new evaluation of wire
codes onto their previous studies.
Although the evaluation of the childhood cancer and wire code associations
is a sufficient motivation for additional case-control studies by itself,
if feasible, it would be important to use that opportunity to evaluate again
the role of measured EMFs and appliance use. Because the nature of the risk
factor is completely unknown, it would be prudent to include an exposure
assessment protocol in as many different types of exposure metrics as possible,
including various functions of spot measurements in subjects' places of
residence, work, or school and continuous monitoring of personal exposure.
It also would be useful to explore alternative exposure metrics such as
the resonance model and exposure to transient fields, though methods for
so doing in an epidemiologic study are not apparent (2).
Any new studies undertaken to tackle this issue ideally should be carried
out in areas that have had relatively stable populations to minimize complication
of the retrospective exposure assessment because of immigration, and in
areas that contain an adequate proportion of high wire code homes. Also,
they should involve larger numbers of study subjects than previous studies.
Because of the rarity of childhood cancers and the desire to subdivide them
by histological subtypes, it may be necessary to resort to multicenter studies.
A population-based, case-control approach with data collected on social
and geographic characteristics of neighborhoods, dwellings, and households
(so that those factors can be incorporated into analyses) would be the design
of choice.
The issue of control group selection is important not only in this area
of EMF exposure and cancer but also in any case-control studies. Thus, a
brief digression is in order. While convention holds that a set of population
controls selected from the free-living general population represents the
optimal choice in a population-based case-control study, the practical aspects
of implementing this strategy may render it decidedly less attractive than
alternatives. Obtaining a sampling frame is not straight forward, since
in North America, at least, there are few, if any, continuously updated
population registers. Random digit dialing has become a popular method for
control selection, but there is little understanding of the biases that
might ensue from noncoverage due to having no telephone service, being unavailable
to answer an initial call, or being unwilling to respond honestly to the
most elementary eligibility questions. Once subjects are eligible, their
willingness to participate may differ between cases and controls, and once
they are willing to participate, the quality of their participation may
differ. Differential losses at any stage can result in bias, as can differential
quality of response. Alternative methods of population control selection
(e.g., birth certificates, immunization rosters, school lists, utility company
lists, address directories) also may have problems with differential losses
(selection bias) and differential quality information (information bias).
A properly chosen disease control group (e.g., selected from the same hospitals
as the cases and among diseases having similar referral patterns) may minimize
selection bias and avoid information bias. Because it is impossible to be
certain of the relative pros and cons of different potential control groups
within a given study, it is prudent and efficient to use two control groups,
one a so-called population control group and the other a so-called hospital
control group. This design is shunned sometimes for fear that it might produce
conflicting, and therefore difficult to interpret, results. However, this
argument is flawed; its logical conclusion is that there should not be more
than one study on any issue because of the possibility of conflicting results.
The use of multiple control groups can be seen as a way of carrying out
multiple (albeit not independent) studies at a small additional cost. In
any case, the verdict on these hypotheses regarding wire code or EMFs will
not be based on any single study but on the body of studies, some already
complete, some now in progress, and some perhaps coming later. It would
be a pity if all of these studies used basically the same control group
strategy and thus were open to the same set of criticisms. If an association
were found using different types of control groups, this would be the most
powerful way to disarm critics.
In this section on studies of childhood cancer and domestic exposure,
case-control rather than cohort-type studies are recommended. It is unlikely
that lists to constitute a retrospective cohort of exposed children for
follow-up are available anywhere. The possibility of establishing a prospective
cohort of children, with baseline information on exposure to wire codes
and possibly other exposure variables, and follow-up through morbidity or
mortality registers is a daunting prospect when the outcome of interest
is as rare as childhood cancer. Furthermore, for a variable with long-term
stability such as wiring code, it is likely that a properly conducted case-control
study would provide results equivalent to those of a properly conducted
cohort study. For a variable that is very unstable over time, as some of
the EMF metrics may very well be, a measure taken at the outset of a prospective
cohort study may be no more meaningful than a measure taken in a retrospective
case-control study.
Exposure-Related Methodology
The development of general epidemiologic methodology, including insights
into design, fieldwork methods, or analysis, may not be specific to this
particular problem, but it would benefit research in this area. Of specific
relevance to research in this area are methodologic studies focusing on
the measurement and meaning of the exposure variables. There are many sources
of EMF exposure and many approaches to measuring it. Among the most prominent
approaches to measuring nonoccupational exposure are spot measurements in
the home; personal, portable dosimeters; and wiring code. The relationships
among these and their stability over time are crucial to planning and interpreting
epidemiological studies but are understood poorly.
Given the current available epidemiologic literature, the top priority
is to investigate the significance of wire codes. This should include some
general description of the historic evolution and current urban geography
of wire codes. Empirical studies should be undertaken to correlate spot
measurements of electric and magnetic fields in houses with wire code. Various
exposure metrics should be examined to determine which are best correlated
with wire code.
Temporal stability of spot measurements, both short-term and long-term,
requires further documentation. Ideally, there should be a representative
panel of households monitored over a long period, perhaps 5 years, with
spot measurements, personal dosimetry, and wire code data collected periodically.
The relative importance of at home versus away from home EMF exposure should
be evaluated in a general population, as should relative contribution of
appliances in the home to the overall burden of domestic EMF exposure. Such
panel studies should be conducted in at least two geographically separate
locales so an estimate of the generalizability of these interrelationships
may be determined.
Occupational Studies of Cancer Occurrence
Despite the problems with the occupational studies alluded to above,
the relatively consistent evidence of slightly increased risk of brain tumors
and leukemia in so-called electrical occupations deserves further evaluation.
A possible advantage of occupational studies over residential studies is
that they may provide clearer exposure differentials. However, there needs
to be better characterization of the exposure factor than the job title.
This could be accomplished by using some sort of mechanism that measures
exposure levels either by means of a job-exposure matrix or by measurement
protocols. Studies should be devised to obtain exposure information not
only on EMF but also on occupational and nonoccupational exposures that
may confound the association between EMF and cancer. Useful studies could
be carried out in the context of cohort studies of workers known to be highly
exposed (some major studies of utility workers are already in progress)
or in the context of population-based case-control studies. While most interest
should be in leukemia and brain tumors, there is sufficient uncertainty
about other cancers to justify examining many cancer sites, especially skin
melanoma, lymphoid tissue, breast, and prostate.
Databases for Attributing Occupational Exposure to EMF
It is possible for experts in hygiene to make useful estimates of past
occupational exposures to chemicals (18). It has also been shown
that experts can make useful estimates of EMF exposure ranking in a cohort
of utility workers (19). But the validity of such expert judgments
depends on the availability of some exposure measurements in various occupations.
There is very little information available on the relative levels of EMF
exposure in different occupations. Most measurements have been made among
utility workers. There is only a scattering of data on other occupations
(20), and no indication of the relative stability of occupational
levels of EMFs over long time periods. Surveys should be carried out to
document EMF levels in representative samples of many occupational groups.
The development and availability of data bases on relative EMF exposure
levels in different occupations and on the temporal stability of such levels
would aid in the interpretation of past occupational studies and in the
conduct of new ones.
Animal Carcinogenicity Studies
Although uncertainty about the ability to extrapolate evidence of carcinogenicity
across species still exists, it is widely accepted that evidence of carcinogenicity
in one species increases the plausibility of carcinogenicity in another.
Thus, some evidence concerning the animal carcinogenic potential of these
exposure variables would benefit the planning and interpretation of epidemiologic
studies in this area. The wiring code variable, which may represent a complex
of sociological as well as physical parameters in epidemiological studies,
has no meaningful equivalent in animal experiments. If an animal model of
EMF carcinogenesis can be developed, it would help greatly in elucidating
which exposure metrics might be useful to assess in epidemiologic studies.
Animal experiments to investigate various exposure metrics should be set
up. The most interesting end points would be leukemia and brain tumors.
If there is any further support for the hypothesis that melatonin levels
are affected by EMFs, then mammary tumors also would be worth examining.
Different types of experimental protocols would be needed to evaluate different
possible mechanisms of action (e.g., complete carcinogen versus promoter).
National Survey of Domestic Exposure and Ecological Studies
Little information is available on how the exposure variables, wiring
code, or the various EMF metrics may vary from region to region and from
city to city. Such information would, at the very least, be useful in the
selection of appropriate sites for carrying out the case-control studies
mentioned above. But even further, such information may be useful in assessing
the feasibility of, and eventually the implementation of, ecological studies.
Although the limitations of ecologic studies are not to be minimized (21,22),
these limitations should not prohibit the use of such studies where they
may be useful. The opportunity for distortion in ecologic studies is minimized
when the intercommunity variation in the exposure variable is relatively
large compared to the intracommunity variation. Wiring characteristics and/or
electric and magnetic fields in buildings may be factors that vary substantially
from county to county and from city to city. If so, these would be beneficial
variables to include in a geographic correlation study of childhood cancer
risk, notably leukemia and brain cancer. Because mortality rates are readily
accessible for all causes, many types of cancer and even noncancer death
rates can be assessed. Also, it would be desirable to use incidence rates,
but this would limit the outcomes and the possible geographic areas to those
covered by tumor registries. Such a study would relate some aggregate measure
of exposure to wire codes and/or EMFs to rates of any type of cancer or
any other health outcome. In fact, for a couple of reasons, it would probably
be more successful in detecting an association with a childhood tumor than
with an adult tumor. First, the induction period probably would be shorter
for childhood tumors; thus, the current aggregate exposure index would be
more etiologically relevant for childhood tumors. Second, the opportunity
for confounding by other factors is probably greater for adult tumors than
for childhood tumors because the web of causation is probably more complex
and drawn out in time, which would make it more liable to vary from place
to place.
Because information on wiring characteristics and fields is not readily
available at the aggregate (e.g., city, or county, or state) level, it would
require some effort to carry out representative field surveys in selected
areas. It would be desirable to collect information on potential confounding
factors among the aggregate units. Some would be available from sources
such as the census bureau. Some confounder data, notably social characteristics
of the families residing in different homes, could be collected in conjunction
with the field surveys of wiring codes and EMFs. The collection of such
data would allow estimation of exposure-confounder associations at both
the individual and aggregate levels.
Surveys of as few as 30 to 100 representative households per area may
be enough to address the issue of interarea versus intraarea variability
in wiring code distribution. If the interarea variability in wiring code
is large compared to the intraarea variability, then an ecologic study with
as few as 10 to 20 ecologic units may provide useful results. Such a study
could be quite easy and not too expensive to mount. For relatively low marginal
cost, it also would be possible to evaluate some simple measured EMF metrics.
Neurobehavioral Effects
The biologic plausibility for neurobehavioral effects is somewhat higher
now than it is for other disease outcomes (5). However, despite a
substantial amount of literature, the evidence for such effects is too flimsy,
and the biologic rationale is insufficiently compelling to justify giving
high priority to research in this area. Nevertheless, methodologically sound
studies should be encouraged in this area, and the findings should be monitored.
For short-term effects, it should be possible to generate fairly clear indications
of whether there is an effect.
Reproductive Effects
The evidence concerning reproductive effects is inconclusive and inconsistent.
Because of the public's concerns and because of the possible link between
carcinogenicity and teratogenicity, it would be justifiable to pursue studies
in this area. Because the induction period between exposure and disease
may be shorter than the period for carcinogenic effects, it may be easier
to relate exposure to reproductive effects than to cancer, if there are
such effects. The disadvantage of studying reproductive outcomes as opposed
to cancer is the notorious difficulty of ascertaining outcomes. The reproductive
effects may be nonspecific; thus, it may be appropriate to focus on such
things as birth weight, congenital malformations as a class, and perhaps
even sperm quality. As the impetus for such studies would come from the
analogy between carcinogenicity and teratogenicity, rather than from evidence
about reproductive effects of EMFs, many of the arguments used to prioritize
cancer studies would prevail here as well. Outcomes should be studied in
relation to domestic wire code, measured EMFs, electrically related occupations,
and appliance use.
Adult Cancer and Nonoccupational Exposure
There is weak evidence of a differential cancer risk in adults from residential
wire codes. Additional case-control studies could be carried out along the
lines of the childhood cancer studies. It would be worthwhile to examine
risks of leukemia, brain cancer, and following Stevens' (4) conjecture,
cancers of hormone-dependent tissue. However, such studies would be difficult
to carry out properly since the etiologically relevant exposure period might
be many decades before disease onset (e.g., in puberty for breast cancer),
and it would be desirable to obtain at least wire code information on all
homes inhabited since childhood--a daunting prospect. If an ecologic study
in which information on wire codes and EMFs are collected from different
areas is carried out and if the interarea variation is large, then it would
be interesting to submit adult cancer rates to an ecologic correlation analysis.
As indicated above, however, the interarea variation in other risk factors
for adult cancers might be quite significant and would confound the observed
associations. It appears that the investigation of adult cancers in relation
to EMF exposure would be more effectively conducted in occupational studies,
not nonoccupational studies, because there may be a greater likelihood of
estimating past relative exposure levels.
General Comments
The above ranking is rather arbitrary. I would consider the top six items
to be of high priority and the bottom four to be of low priority. There
are studies in progress that correspond to several of the themes listed
above. For instance, there are multicenter case-control studies of childhood
leukemia in the United States, in Canada, and in New Zealand; there are
cohort studies of utility workers in the United States, in Canada, and in
France; there are long-term animal carcinogenicity studies in the United
States and in Canada, and so on. As they become known, results of these
studies may alter significantly our view of the research directions to be
fostered. In the above listing, I have included epidemiologic, measurement,
or toxicologic research whose results might have a direct impact on epidemiology.
Research on basic biological effects of EMFs will continue undoubtedly at
its own rhythm, and the findings may influence the conduct of epidemiologic
research in this area.
Problems in the Science and Public Health Aspects of
EMF Research
Unique Aspects of EMF Epidemiology
In most respects, the problems of research design, delimitation of disease
outcome categories, confounding, and defining appropriate comparison groups
are similar in EMF studies to what they would be in other environmental
epidemiology studies. However, while many exposure variables are difficult
to measure and do not lend themselves to a self-evident exposure metric,
EMFs pose particular challenges in this regard. First of all, it is uncertain
if the etiologically relevant exposure variable is electric field, magnetic
field, or some other correlate of wire code. Furthermore, there is no such
thing as a truly unexposed group. Finally, if effective exposure depends
on theories of resonance or on exposure to transients, then the notion of
monotonic dose-response, which serves with chemical exposures as an additional
means of juxtaposing exposed and unexposed, may not be operative. Many exposure
circumstances in environmental epidemiology are very difficult to characterize
or measure (e.g., air pollution, toxic waste sites), and many are so ubiquitous
that it is difficult to identify a truly unexposed group (e.g., ultraviolet
light, motor vehicle exhaust). But the peculiar hypothesized models of dose-response
(e.g., the resonance model or the transients model) may be unique to the
EMF issue. The rest of this paper does not concern specifically issues related
to EMFs or wiring code, but rather, it concerns the social and scientific
contexts in which the issue is being addressed
Structural Impediments to Environmental Epidemiology
EMFs from power sources have been part of the urban environment for most
of this century, and over a decade has passed since the initial Wertheimer-Leeper
report. The available epidemiologic evidence on this issue is still very
thin. Our species will continue to live in an environment full of potential
health hazards. The hypothetical matrix of all exposures by all diseases
is virtually limitless. Our capacity to evaluate each possible association
is very limited. These limitations affect our ability to detect and evaluate
the effects of any environmental agent, including EMF. What measures, if
any, can be taken to enhance the capacity of epidemiology to provide more
rapidly better quality evidence for a larger part of the hypothetical matrix?
The number of epidemiologists, and particularly environmental epidemiologists,
who cope with the multitude of potential environment-disease associations
is small. This can be improved by increasing the training and job opportunities
in environmental epidemiology. Although this would help, within the bounds
of feasibility, it is unlikely to make a significant dent in the problem.
Pouring more money into EMF-related research might help, but it would detract
from other equally worthy issues. We need to develop and implement more
efficient methods for studying environmental disease associations. The use
of experimental in vivo and in vitro procedures for testing
environmental agents is not capable of serving as an effective proxy for
human evidence. Epidemiologic evidence is still essential.
The traditional biomedical research paradigm of studying a single hypothesis
at a time does not serve well when addressing a problem of this magnitude.
More efforts should be devoted to large-scale data collection endeavors.
Population-based disease registries, such as tumor registries, represent
one essential element of a useful intelligence system. Systems for routine
registration of exposure are less common but also would be of use. The ideal
might be a system that goes beyond the traditional passive disease registry
system to something approaching an ongoing case-control study (18,23).
As cases are ascertained in the system, a data collection procedure can
be implemented to obtain different kinds of information such as occupational
history, residential history, and dietary habits. As time goes on, the accumulated
data can be analyzed to examine the relationships between the diseases covered
by the registry and the various exposure variables routinely collected.
A permanent infrastructure to run such a system would be an extremely cost-effective
tool that could be mobilized to generate hypotheses in periodic analyses
or to test hypotheses suggested by other evidence. The institution of such
systems, which should combine the breadth of traditional vital statistics
functions with the depth of so-called analytical epidemiology projects,
would greatly increase our capacity to confront rationally the hypothetical
matrix of all environmental exposures by all diseases. The main impediments
to such development are in the anachronistic attitudes toward research that
doesn't fit into the narrow hypothesis-testing mold of conventional biomedical
research and in the structural difficulties of funding such endeavors.
Epidemiology, Controversy, and Public Policy
Since the 1960s, there has been a growing consciousness of the potential
harm that environmental pollution can cause. It has become widely accepted
that extrinsic factors (as opposed to genetic factors or pure chance) play
a role in many, if not most, cases of disease. It has also become clear
that the benefits of modern technology have been accompanied by significant
degradation and pollution of the environment. In this context, it has been
easy for the public and for scientists to entertain, if not readily accept,
the hypothesis that pollutant X causes disease Y. The initial
reports of carcinogenic effects of EMFs were greeted with skepticism by
much of the scientific community. But there was sufficient scientific interest
to foster concern in nonscientific circles. In such a context, it was natural
for epidemiology to be called upon to carry out the studies to resolve the
issue. Like other sciences, epidemiology operates iteratively between hypotheses
and empirical evidence. Generally speaking, as valid scientific data accumulate,
the underlying truths are elucidated, and at least a consensus about which
hypotheses are untenable may develop among informed scientists. But there
is no law of nature that determines how much data on a given issue are needed
before a consensus develops. Unlike other sciences, epidemiology is frequently
drawn into a public policy terrain with little tolerance for uncertainty
or equivocation. Issues such as EMFs and human health force epidemiologists
to draw inferences concerning causal relations before the data are developed
enough to support conclusions. Furthermore, the issue may be so enmeshed
in adversary or ideological interests that it becomes difficult to address
it in the ideal scientific context of objective disinterest. Scientists
are not unwilling victims of this process. Research careers and funding
opportunities can be enhanced greatly by engaging in controversial and high-profile
research.
The rules of the game differ when scientists move onto this adjoining
turf. It is not necessarily the best science that prevails. Suboptimal science,
whether motivated by altruistic or base motives, can thrive in such a context.
Epidemiology is more susceptible to misuse than other disciplines, because
it can be carried out by persons who have little or no specialized training
in the area. Because of the invisible and ostensibly mysterious nature of
EMF, research in this area may be susceptible particularly to the social
pressures that may detract from methodological rigor. It is impossible to
establish hard and fast rules to regulate epidemiologic or scientific competence.
Certainly, science must make room for mavericks who are often at the origin
of important developments. However, even if mainstream science is sometimes
slow to accept new ideas, it is supple enough that valid ideas would not
be shunned indefinitely.
As with many environmental health issues, the scientific evidence concerning
effects of EMF is weak. This may be because the relative risk is too low
to be readily detected by the epidemiologic methods used or because there
really is no effect. For public health purposes, this is a crucial distinction,
because even a relative risk that is low by epidemiologic standards can
produce a nontrivial burden of disease if it is due to a prevalent exposure.
Perhaps this is the case with EMFs. What type of public policy should be
recommended in such a situation? Should the standards for evaluating causality
be relaxed in the case of a potentially significant pathogen? Should we
be prepared to conclude that there is an association when the evidence is
weak or even contradictory? No. This would not serve science, and in the
long run, it would not serve public health. The temptation to cry wolf on
the grounds of prudence will begin discrediting epidemiology and science
in general. We may have seen some of this already. Is a scientifically conservative
attitude tantamount to licensing pollution until the ephemeral scientific
certainty is achieved? No. Public health decisions are made with a different
set of rules than scientific ones. Scientific evidence is one of the parameters
that enters the equation, but there are also issues of social values, economic
costs, political will, technological alternatives or fixes, and so on. It
is entirely defensible and even desirable to contemplate a policy of prudent
avoidance of substances for which there is weak evidence of health effects.
Within reasonable economic constraints, we should try to minimize pollution
by any substance, irrespective of known toxic effects, because what we know
of toxic effects of environmental agents is only the tip of an iceberg.
However, we must resist the temptation to disguise a tough public policy
decision in a cloak of ostensible scientific rigor and precision, whether
this abuse of science is in defense of vested corporate interests or the
preservation of public health.
