Xref: helios.physics.utoronto.ca sci.med.physics:2460 sci.answers:1479 news.answers:27583
From: firstname.lastname@example.org (John Moulder)
Subject: Powerlines & Cancer FAQs 4/6: FAQ 3
Date: Mon, 15 Aug 1994 16:34:41 -0600
Organization: Medical College of Wisconsin
Expires: 12 Sep 1994 00:00:00 GMT
Reply-To: email@example.com (John Moulder)
Summary: Q&As on the connection between powerlines, electrical
occupations and cancer. Discussion of the biophysics of
interactions with EM sources, summaries of the laboratory
and human studies, information on standards, and references.
Keywords: powerlines, magnetic fields, cancer, EMF, non-ionizing
FAQs on Power-Frequency Fields and Cancer (Q&A, Part 3 of 3)
21) If exposure to power-frequency magnetic fields does not explain the
residential and occupations studies which show increased cancer
incidence, what other factors could?
There are basically four factors that can result in false associations
in epidemiological studies: inadequate dose assessment (Question 21A),
confounders (Question 21B), inappropriate controls (Question 21C), and
publication bias (Question 21D).
21A) Could problems with dose assessment affect the validity of the
epidemiological studies of power lines and cancer?
If power-frequency fields are associated with cancer, we do not know
what aspect of the field is involved. At a minimum, risk could be
related to the peak field, the average field, or the rate of change of
the field. The duration of exposure could also be a factor. It has
even been suggested that harmonics and/or interactions with the earthıs
static magnetic fields are involved. If we do not know who is really
exposed, and who is not, we will usually (but not always) underestimate
the true risk [C14].
21B) Are there other cancer risk factors that could be causing a false
association between exposure to power-frequency fields and cancer?
Associations between things are not always evidence for causality.
Power lines (or electrical occupations) might be associated with a
cancer risk other than magnetic fields. If such an associated cancer
risk were identified it would be called a "confounder" of the
epidemiological studies of power lines and cancer. An essential part of
epidemiological studies is to identify and eliminate possible
confounders. Many possible confounders of the powerline studies have
been suggested, including PCBs, herbicides, ozone and nitrogen oxides,
traffic density, and socioeconomic class.
- PCBs: Many transformers contain polychlorinated biphenyls (PCBs) and
it has been suggested that PCB contamination of power-line corridors
might be the cause of the excess cancer. This is unlikely. First,
there is little evidence for widespread PCB contamination of powerline
corridors. Second, transformers are found along distribution lines, but
not high-voltage transmission lines, so PCBs could not account for the
linkage of childhood leukemia with transmission corridors [B5]. Three,
the evidence that PCB exposure causes or promotes cancer in people is
weak [E9,L1]. Lastly, PCBıs predominantly cause and promote liver
cancer in animals; leukemia, brain and breast cancer have not been
- Herbicides: It has been suggested that herbicides sprayed on the
powerline corridors might be a cause of cancer. This is also an
unlikely explanation. First, herbicide spraying would not affect
distribution systems in urban areas (where 3 of 5 positive childhood
cancer studies have been done), and would not explain the reported
increase in cancers in electrical occupations. Second, evidence that
herbicides are carcinogens in humans is weak [L6]. Third, the
epidemiology which suggests that the phenoxy herbicides might be
carcinogens indicate that the increased risk is for lymphomas and soft-
tissue sarcomas [L6]; only one study implicates leukemia [D3], and none
implicate brain cancer.
- Ozone and nitrogen oxides: It has been suggested that ozone and
nitrogen oxides created when high voltage lines arc might be responsible
for the increased cancer along powerline corridors. This is another
unlikely explanation. First, while ozone is a cellular genotoxin, there
is no evidence that it causes cancer in humans, and only ambiguous
evidence that it causes lung cancer in rats [L5]. There is essentially
no evidence that the nitrogen oxides are carcinogens. Second, this
potential confounder would apply only to corridors containing high-
voltage lines and would not explain reports of excess cancer along
distribution systems or in electrical occupations.
- Traffic density: Transmission lines frequently run along major roads,
and the "high current configurations" associated with excess childhood
leukemia in the US studies [C1,C6,C10] are associated with major roads.
It has been suggested that power lines might be a surrogate for exposure
to cancer-causing substances in traffic exhaust. This may be a serious
confounder of the residential exposure studies, since traffic density
has been shown to correlate with childhood leukemia incidence [E5].
Note that this would explain only the reported increase in cancer along
power-lines; it would explain the reports of increased cancer in
- Socioeconomic class: Socioeconomic class may be an issue in both the
residential and occupational studies, as socioeconomic class is clearly
associated with cancer risk, and "exposed" and "unexposed" groups in
many studies are of different socioeconomic classes [C14]. This is of
particular concern in the US residential exposure studies that are based
on "wirecodes", since the types of wirecodes that are correlated with
childhood cancer are found predominantly in older, poorer neighborhoods,
and/or in neighborhoods with a high proportion of rental housing [C19].
- Other factors: If "other" factors exist that increase the incidence of
cancer they need to be controlled for in studies. In other words, you
have to make sure that the "exposed" and "unexposed" groups have the
same risk factors. Every time a new risk factor is discovered, previous
studies need to be reexamined. Thus the "hot dog factor"! The same
investigators who reported an increased incidence of childhood leukemia
along powerline corridors in Los Angeles [C10], have recently reported
an much higher leukemia risk factor for hot dog consumption (RR of 9.5)
in the same group of children [E10].
21C) Could the epidemiological studies of power lines and cancer be
biased by the methods used to select control groups?
An inherent problem with many epidemiological studies is the difficulty
of obtaining a "control" group that is identical to the "exposed" group
for all characteristics related to the disease except the exposure.
This is very difficult to do for diseases such as leukemia and brain
cancer where the risk factors are poorly known. An additional
complication is that often people must consent to be included in the
control arm of a study, and participation in studies is known to depend
on factors (such as socioeconomic class, race and occupation) that are
linked to differences in cancer rates. See Jones et al [C19] for an
example of how selection bias could effect a powerline study.
21D) Could analysis of the epidemiological studies of power lines and
cancer be skewed by publication bias?
It is known that positive studies in many fields are more likely to be
published than negative studies. This can severely bias meta-analysis
studies such as those discussed in Questions 13 and 15. Such
publication bias will increase apparent risks. This is a bigger problem
for the occupational studies than the residential ones.
Several specific examples of publication bias are known in the studies
of electrical occupations and cancer (see Doll et al [B4], page 94). In
their review Coleman and Beral [B1] report the results of a Canadian
study that found a RR of 2.4 for leukemia in electrical workers. The
British NRPB review [B4] found that further followup of the Canadian
workers showed a deficiency of leukemia (a RR of 0.6), but that this
followup study has never been published. This is an anecdotal report,
but publication bias, by its very nature, is usually anecdotal.
It is also a clear problem for laboratory studies -- it is much easier
to publish studies that report effects than studies that report no
effects. An example of this can be seen in work by Cain and colleagues.
In a 1993 they published a report [G26] that 60-Hz fields were a co-
promoter in a cell transformation system. Also in 1993 the same authors
reported at meetings that they could not replicate the co-promotion, and
that subsequent experiments showed a decrease in transformation when 60-
Hz magnetic fields were present. However, the later data is not
published, so only the positive report is currently in the peer-reviewed
There is also the related issue of "reporting bias", which refers both
to situations where multiple studies are done but only some are
reported, and to situations where abstracts and/or press reports
emphasize unrepresentative subsets of the actual study. The "Swedish"
studies [C13,C18] provide an case example. The original report used a
number of different definitions of "exposure", and studied both children
and adults. Of all the comparisons, the most significant correlations
were found for childhood leukemia and calculated fields. The published
Swedish version [C13] omits details of some of the exposure definitions
that showed no relationships, and the published English-language version
[C18] omits the adult studies. The abstract of the English-language
version emphasizes the groups, exposure definitions and cancer types for
which there were significant relationships. The press reports were
based largely on that abstract. The result is that a handful of
significantly-positive associations are picked for emphasis from a much
larger group of overwhelmingly non-significant associations.
22) What is the strongest evidence for a connection between power-
frequency fields and cancer?
The best evidence for a connection between cancer and power-frequency
fields is probably:
a) The four epidemiological studies that show a correlation between
childhood cancer and proximity to high-current wiring [C1,C6,C10,C18],
plus the meta-analysis of the Scandinavian studies [B5].
b) The epidemiological studies that show a significant correlation
between work in electrical occupations and cancer, particularly leukemia
and brain cancer [B2,B3,B4,D12,D14].
c) The lab studies that show that power-frequency fields do produce
bioeffects. The most interesting of the lab studies are probably the
ones showing increased transcription of oncogenes at fields of 1-5 G
(100-500 microT) [H4,H6,L2].
d) The two laboratory studies that provide evidence that power-frequency
magnetic fields can promote chemically-induced breast cancer [G14,G23].
23) What is the strongest evidence against a connection between power-
frequency fields and cancer?
The best evidence that there is not a connection between cancer and
power-frequency fields is probably:
a) Application of the Hill criteria (Question 20) to the entire body of
epidemiological and laboratory studies.
b) The fact that all studies of genotoxicity, and most study of
promotion have been negative (Question 16).
c) Adairıs [F2,F8] biophysical analyses that indicates that "any
biological effects of weak (less than 40 mG, 4 microT) ELF fields on the
cellular level must be found outside of the scope of conventional
d) Jacksonıs [E8] and Olsenıs [C16] epidemiological analysis that shows
that childhood and adult leukemia rates have been stable over a period
of time when per capita power consumption has risen dramatically. This
argument presumes that ³exposure² has risen in parallel with
³consumption²; there is little relevant historical data, and there are
technical reasons to question the validity of this assumption.
24) What studies are needed to resolve the cancer-EMF issue?
In the epidemiological area, more of the same types of studies are
unlikely to resolve anything. Studies showing a dose-response
relationship between measured fields and cancer incidence rates would
clearly affect thinking, as would studies identifying confounders in the
residential and occupational studies.
In the laboratory area, more genotoxicity and promotion studies may not
be very useful. Exceptions might be in the area of cell transformation,
and promotion of chemically-induced breast cancer. Long-term rodent
exposure studies (the standard test for carcinogenicity) would have a
major impact if they were positive, but if they were negative it would
not change very many minds. Further studies of some of the known
bioeffects would be useful, but only if they identified mechanisms or if
they established the conditions under which the effects occur (e.g.,
thresholds, dose-response relationships, frequency-dependence, optimal
25) Is there any evidence that power-frequency fields cause any human
While this FAQ sheet, and most public concern, has centered around
cancer, there have also been suggestions that there might be a
connection between non-ionizing EM exposure and birth defects. This
concern has focused as much on video display terminals (VDTs) as on
power lines. Little epidemiological or laboratory support for a
connection between non-ionizing EM exposure and birth defects has been
found. [J1,J2,J4,J5,J6]. Cox et al [J3] and Chernoff et al [K5] have
recently reviewed this field.
26) What are some good overview articles?
There are really no up-to-date reviews of power-frequency fields and
human health. The reviews by Davis et al [A2] and Doll et al [B4] are
good, but were published before many of the important epidemiological,
genotoxicity, and promotion studies were available.
27) Are there exposure guidelines for power-frequency fields?
Yes, a number of governmental and professional organizations have
developed exposure guidelines. These guidelines are based on keeping
the body currents induced by power-frequency EM fields to a level below
the naturally-occurring fields (Question 8). The most generally
- National Radiation Protection Board (UK) [M5]:
50 Hz: 1.6 mT (16 G) and 12 kV/m
60 Hz: 1.33 mT (13.3 G) and 10 kV/m
This document also contains guidelines for other ELF frequencies.
- American Conference of Governmental Industrial Hygienists [M6]:
At 60 Hz: 1 mT (10 G); 0.1 mT (1 G) for pacemaker wearers
This document also contains guidelines for other ELF frequencies
- International Commission on Non-Ionizing Radiation Protection [M7]
24 hr general public: 0.1 mT (1 G) and 5 kV/m
Short-term general public: 1 mT (10 G) and 10 kV/m
Continuous occupational: 0.5 mT (5 G) and 10 kV/m
Short-term occupational: 5 mT (50 G) and 30 kV/m
28) What effect do powerlines have on property values?
There is very little hard data on this issue. There is anecdotal
evidence and on-going litigation (Wall Street Journal, Dec 9, 1993).
There have been "comparable property" studies, but any studies done
prior to about 1991 (when London et al [C10] was published) might be
irrelevant. One comparable value study has been published recently
[L4], and another has been presented at a meeting [L7]. Neither study
shows hard evidence for an impact of power lines on property values.
However, both studies indicate that many owners think that there will be
an impact, particularly if concerns about health effects become
It appears possible that the presence of obvious transmission lines or
substations will adversely affect property values if there has been
recent local publicity about health or property value concerns. It
would appear less likely that the presence of "high current
configuration" distribution lines of the type correlated with childhood
cancer in the US studies [C1,C6,C10] would affect property values, since
few people would recognize their existence. If buyers start requesting
magnetic field measurements, no telling what will happen, particularly
since measurements are difficult to do (Questions 29 & 30), and even
more difficult to interpret (Question 14).
29) What equipment do you need to measure power-frequency magnetic
Power-frequency fields are measured with a calibrated gauss meter. The
meters used by environmental health professionals are too expensive for
A unit suitable for home use should meet the following criteria. First,
it must have a reasonable degree of accuracy and precision (plus/minus
20% seems reasonable for home use). Second, it should have true RMS
detection, otherwise readings might be exaggerated if the waveform is
non-sinusoidal. Third, it should have a tailored frequency response,
because if the unit is too broad-band, higher frequency fields from
VDTs, TVs, etc. may confound the measurements. Fourth, it should have
the correct response to overload; if the unit is subjected to a very
strong field, it should peg, not just give random readings. Fifth, the
presence of a strong electrical field should not affect the magnetic
Meters meeting these requirements are quite expensive, $600 would
probably be the bare minimum. These meters are not suitable for the
There is an understandable reluctance to recommend any unit with unknown
characteristics to a person whose technical abilities are also unknown,
and no peer-reviewed articles on inexpensive instruments appear to be
available. A recent issue of Consumerıs report found that the
inexpensive (under $200) meters available in the USA were unreliable.
The suggestions that one wind a coil and use headphones or a high
impedance multimeter are misguided. A clever physicist or engineer can
anticipate and correct for nonlinearities and interferences, but for the
average person, even one technically trained, this is unreasonable.
30) How are power-frequency magnetic fields measured?
Measurements must be done with a calibrated gauss meter (Question 29) in
multiple locations over a substantial period of time, because there are
large variations in fields over space and time. Fortunately, the
magnetic field is far easier to measure than the electrical field. This
is because the presence of conductive objects (including the measurer's
body) distorts the electrical field and makes meaningful measurements
difficult. Not so for the magnetic field.
It is important for the person who is making the evaluation to
understand the difference between an emission and exposure. This may
seem obvious, but many people, including some very smart physical
scientists, stick an instrument right up to the source and compare that
number with an exposure standard. Also, if the instrument is not
isotropic, measurement technique must compensate for this.
In the case of power distribution line and transformer fields, the
magnetic fields will probably vary considerably over time, as they are
proportional to the current in the system. A reasonable
characterization needs to be done over time, with anticipated and actual
electricity usage factored in. It may seem to be as simple as walking in
and reading the meter, but it's not.
31) Do the issues discussed in this FAQ sheet apply to EM fields other
than power-frequency fields?
This FAQ sheet concerns itself primarily with sinusoidal fields at
frequencies of 50 or 60 Hz. However, the basic principles and data
discussed in the FAQ sheet are generally applicable to ELF sources with
frequencies between 1 Hz and 30,000 Hz (30 kHz). Below 1 Hz, one should
also consider issues associated with static magnetic fields [M8]. Above
30 kHz, one is moving into the radiofrequency (RF) range, and other
biophysical and biological issues arise that are not within the scope of
this document [M2,M4].
The major issue encountered when dealing with other ELF EM sources is
that the currents induced by ELF magnetic fields depend on the frequency
and the wave-form as well as intensity. As the frequency increases, so
do the induced currents. Thus safety guidelines that are based on
induced currents change with frequency [M5,M6]. For example, the NRPB
magnetic field exposure guideline [M5] which is 1.33 mT (13.3 G) at 60
Hz, rises to 80 mT (800 G) at 1 Hz and falls to 80 microT (800 mG) at 3
Estimating the currents induced by non-sinusoidal ELF wave forms is more
complex, because the magnitude of the induced current depends on the
rate at which the magnetic field changes. Thus a square wave of the
same frequency and amplitude of a sinusoidal wave will induced a much
Copyright (C) by John Moulder