From: John Moulder <jmoulder@its.mcw.edu>;
Organization: Medical College of Wisconsin
Subject: Powerlines and Cancer FAQs (2 of 2)
To: Multiple recipients of list SCIFAQ-L <SCIFAQ-L@YaleVM.YCC.Yale.Edu>;
Status: U
Archive-name: powerlines-cancer-FAQ/part2
Last-modified: 1993/12/28
Version: 2.2
FAQs on Power-Frequency Fields and Cancer (part 2 of 2)
19) What criteria do scientists use to evaluate all the confusing and
contradictory laboratory and epidemiological studies of power-frequency
magnetic fields and cancer?
There are certain widely accepted criteria that are weighed when assessing
such groups of epidemiological and laboratory studies. These are often
called the RHill criteriaS [23]. Under the Hill criteria one examines the
strength (Q19A) and consistency (Q19B) of the association between exposure
and risk, the evidence for a dose-response relationship (Q19C), the
laboratory evidence (Q19D), and the biological plausibility (Q19E). These
criteria are viewed as a whole; no individual criterion is either necessary
of sufficient for the conclusion that there is a causal relationship
between an exposure and a disease.
Overall, application of the Hill criteria shows that the current evidence
for a connection between power frequency fields and cancer is quite weak,
because of the weakness (Q19A) and inconsistencies (Q19B) in the
epidemiological studies, combined with the lack of a dose-response
relationship in the human studies (Q19C), and the negative laboratory
studies (Q19D&E).
19A) Criterion One: How strong is the association between exposure to
power frequency fields and the risk of cancer?
The first Hill criterion is the *strength of the association* between
exposure and risk. That is, is there a clear risk associated with
exposure? A strong association is one with a RR (Q12) of 5 or more.
Tobacco smoking, for example, shows a RR for lung cancer 10-30 times that
of non-smokers.
Most of the positive power-frequency studies have RRs of less than two.
The leukemia studies as a group have RRs of 1.1-1.3, while the brain cancer
studies as a group have RRs of about 1.3-1.5. This is only a weak
association.
19B) Criterion Two: How consistent are the studies of associations
between exposure to power frequency fields and the risk of cancer?
The second Hill criterion is the *consistency* of the studies. That is, do
most studies show about the same risk for the same disease? Using the
same smoking example, essentially all studies of smoking and cancer showed
an increased risk for lung and head-and-neck cancers.
Many power-frequency studies show statistically significant risks for some
types of cancers and some types of exposures, but many do not. Even the
positive studies are inconsistent with each other. For example, while a
new Swedish study [46] shows an increased risk for childhood leukemia for
one measure of exposure, it contradicts prior studies that showed a risk
for brain cancers [7, 39], and a parallel Danish study [36] shows a risk
for childhood lymphomas, but not for leukemia. Many of the studies are
internally inconsistent. For example, where the Swedish study [46] shows
an increased risk for childhood leukemia, it shows no overall increase in
childhood cancer, implying that the rates of other types of cancer are
decreased. In summary, few studies show the same positive result, so that
the consistency is weak.
19C) Criterion Three: Is there a dose-response relationship between
exposure to power frequency fields and the risk of cancer?
The third Hill criterion is the evidence for a *dose-response
relationship*. That is, does risk increase when the exposure increases?
Again, the more a person smokes, the higher the risk of lung cancer.
No published power-frequency exposure study has shown a dose-response
relationship between measured fields and cancer rates, or between distances
from transmission lines and cancer rates. The lack of a relationship
between exposure and increased cancer risk is a major reason why many
scientists are skeptical about the significance of the epidemiology.
Not all relationships between dose and risk can be described by simple
linear no-threshold dose-response curves where risk is strictly
proportional to risk. There are known examples of dose-response
relationships that have thresholds, that are non-linear, or that have
plateaus. For example, cancer induced in rodents by ionizing radiation
shows curves in which the risk rises with dose, but only up to a certain
point; beyond that point the risk plateaus or even drops. Without an
understanding of the mechanisms connecting dose and risk it is impossible
to predict the shape, let alone the magnitude of the dose-response
relationship.
19D) Criterion Four: Is there laboratory evidence for an association
between exposure to power frequency fields and the risk of cancer?
The fourth Hill criterion is whether there is *laboratory evidence*
suggesting that there is a risk associated with such exposure?
Epidemiological associations are greatly strengthened when there is
laboratory evidence for a risk. When the US Surgeon General first stated
that smoking caused lung cancer, the laboratory evidence was ambiguous. It
was known that cigarette smoke and tobacco contained carcinogens, but no
one had been able to make lab animals get cancer by smoking (mostly because
it is hard to convince animals to smoke). Currently the laboratory
evidence linking cancer and smoking is much stronger.
Power-frequency fields show little evidence of the type effects on cells,
tissues or animals that point towards their being a cause of cancer, or to
their contributing to cancer.
19E) Criterion Five: Are there plausible biological mechanisms that
suggest an association between exposure to power frequency fields and the
risk of cancer?
The fifth Hill criterion is whether there are *plausible biological
mechanisms* that suggest that there should be a risk? When it is
understood how something causes disease, it is much easier to interpret
ambiguous epidemiology. For smoking, while the direct laboratory evidence
connecting smoking and cancer was weak at the time of the Surgeon Generals
report, the association was highly plausible because there were known
cancer-causing agents in tobacco smoke.
>From what is known of power-frequency fields and their effects on
biological systems there is no reason to even suspect that they pose a risk
to people at the exposure levels associated with the generation and
distribution of electricity.
20) If exposure to power-frequency magnetic fields does not explain the
positive residential and occupations studies, what other factors could?
There are basically four factors that can result in false associations in
epidemiological studies: inadequate dose assessment (Q20A), confounders
(Q20B), inappropriate controls (Q20C), and publication bias (Q20D).
20A) 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, of the rate of change of the field. If
we do not know who is really exposed, and who is not, we will usually (but
now always) underestimate the true risk.
20B) 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 RconfounderS 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,
traffic density, and socioeconomic class.
- PCBs: Many transformers contain polychlorinated biphenyls (PCBs) and it
has been suggested that PCB contamination of the power-line corridors might
be the cause of the excess cancer. This is unlikely. First, PCB leakage
is rare. Second, PCB exposure has been linked to lymphomas, not leukemia
or brain cancer.
- Herbicides: It has been suggested that herbicides sprayed on the
powerline corridors might be a cause of cancer. This is an unlikely
explanation, since herbicide spraying would not effect distribution systems
in urban areas (where 3 of 5 positive childhood cancer studies have been
done).
Traffic density: Transmission lines frequently run along major roads,
and the Rhigh current configurationsS associated with excess childhood
leukemia in the US studies [1-3, Q13] 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 real
confounder, since traffic density has been shown to correlate with
childhood leukemia risk [28]. Note that this would explain only the
residential connection, not the occupational connection.
- 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 [29]. This is of particular
concern in the US residential exposure studies that are based on
"wirecoding", since the type of wirecodes that are correlated with
childhood cancer are found predominantly in older (poorer?) neighborhoods,
and/or neighborhoods with a high proportion of rental housing.
20C) 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 RcontrolS group that is identical to the RexposedS 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 socioeconomic
class, race and occupation) that are linked to differences in cancer rates.
See Jones et al [48] for an example of how selection bias could effect a
powerline study.
20D) Could analysis of the epidemiological studies of power lines and
cancer be skewed by publication bias?
It is a known that positive studies in many fields are more likely to be
published than negative studies (see Dickersin et or examples from
cancer clinical trials). This can severely bias meta-analysis studies such
as those discussed in Q12 and Q14. Such publication bias will increase
apparent risks. This is a bigger potential problem for the occupational
studies than the residential ones. 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 (such is human nature!).
Several specific examples of publication bias are known in the studies of
electrical occupations and cancer (see Doll et al [39], page 94). In their
review Coleman and Beral [8] report the results of a Canadian study that
found a RR of 2.4 for leukemia in electrical workers. The British NRPB
review [39] 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.
21) 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 [1-3, 45].
b) The epidemiological studies that show a significant correlation between
work in electrical occupations and cancer, particularly leukemia and brain
cancer [8-10, 36].
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) [17, 18].
d) The one laboratory study that provides evidence that power-frequency
magnetic fields can promote chemically-induced breast cancer [32].
22) 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 (Q19) to the entire body of
epidemiological and laboratory studies [24, 27].
b) The fact that all studies of genotoxicity, and all but one study of
promotion have been negative (Q15).
c) Adair's [25] biophysical analysis that indicates that Rany 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 physics"
d) JacksonUs [26] and OlsenUs [38] epidemiological analysis that shows that
childhood and adult leukemia rates have been stable over a period of time
when per capita power consumption risen dramatically
23) 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 wave-forms).
24) What are some good overview articles?
A good review of the area was published by Oak Ridge Associated
Universities [40]. It is available from National Technical Information
Service (ARAU 92/F-8) and the US Government Printing Office
(029-000-00443-9). If you are in the U.K., the National Radiation
Protection Board has a good review [39]. Two other good review are
Theriault [24] and Bates [27].
25) Are there exposure standards for power-frequency fields?
Yes, a number of governmental and professional organizations have developed
exposure standards. These standards are based on keeping the body currents
induced by power-frequency EM fields to a level below the naturally
occurring fields (Q8). The most generally relevant are:
- Board statement on restriction on human exposures to static and time
varying EM fields and radiation, National Radiation Protection Board,
Chilton, 1993.
50 Hz electrical field: 12 kV/m
60 Hz electrical field: 10 kV/m
50 Hz magnetic field: 1.6 mT (16 G)
60 Hz magnetic field: 1.33 mT (13.3 G)
- Sub-radiofrequency (30 KHz and below) magnetic fields, In: Documentation
of the threshold limit values, American Committee of Government and
Industrial Hygienists, pp. 55-64,1992.
At 60 Hz: 1 mT (10 G); 0.1 mT (1 G) for pacemaker wearers
- HP Jammet et al: Interim guidelines on limits of exposure to 50/60 Hz
electric and magnetic fields. Health Physics 58:113-122, 1990.
*H-field (rms)
24 hr general public: 0.1 mT = 1 G
Short-term general public: 1 mT = 10 G
Occupational continuous: 0.5 mT = 5 G
Occupational short-term: 5 mT = 50 G
*E-field (rms)
24 hr general public: 5 kV/m
Short-term general public: 10 kV/m
Occupational continuous: 10 kV/m
Occupational short-term: 30 kV/m
26) 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 Rcomparable propertyS studies, but I would argue that any studies done
prior to about 1991 (when London et al [3] was published) would be
irrelevant. So far I have found one recent RstudyS [50]. The first part
of the study was a survey of homeowners in Tennessee who lived adjacent to
high voltage transmission lines. Of these owners, 53% considered the lines
Ran eyesoreS, but none considered the lines a health hazard. Of owners who
thought the towers were eyesores, 28% said that the presence of the lines
adversely affected then price they were willing to pay. None of the owners
Rhad any knowledge of possible evidence connecting power transmission lines
to certain health risks such as cancerS; but 87% said that if they had
known of potential health risks, it would have adversely affected then
price they were willing to pay. In the second part of the studies, the
values of comparable houses adjacent to, and not adjacent to, the
powerlines were found to have sold for the same price.
It appears possible that the presence of obvious transmission lines or
substations will adversely affect property values if there has been recent
local publicity. It would appear less unlikely that the presence of Rhigh
current configurationS distribution lines of the type correlated with
childhood cancer in the US studies [1-3] would affect property values,
since few people would recognized their existence.
-----------------------
References:
1) N Wertheimer & E Leeper: Electrical wiring configurations and childhood
cancer. Amer J Epidemiol 109:273-284, 1979.
2) DA Savitz et al: Case-control study of childhood cancer and exposure to
60-Hz magnetic fields. Amer J Epidemiol 128:21-38, 1988.
3) SJ London et al: Exposure to residentlectric and magnetic fields
and risk of childhood leukemia. Amer J Epidemiol 134:923-937, 1991.
4) MP Coleman et al: Leukemia and residence near electricity transmission
equipment: a case-control study. Br J Cancer 60:793-798, 1989.
5) ME McDowall: Mortality of persons resident in the vicinity of electrical
transmission facilities. Br J Cancer 53:271-279, 1986.
6) A Myers et al: Childhood cancer and overhead powerlines: a case-control
study. Br J Cancer 62:1008-1014, 1990.
7) G.B. Hutchison: Cancer and exposure to electric power. Health Environ
Digest 6:1-4, 1992.
8) M Coleman & V Beral: A review of epidemiological studies of the health
effects of living near or working with electrical generation and
transmission equipment. Int J Epidemiol 17:1-13, 1988.
9) JR Jauchem & JH Merritt: The epidemiology of exposure to EM fields: an
overview of the recent literature. J Clin Epidemiol 44:895-906, 1991.
10) DA Savitz & EE Calle: Leukemia and occupational exposure to EM fields:
Review of epidemiological studies. J Occup Med 29:47-51, 1987.
11) GK Livingston et al: Reproductive integrity of mammalian cells exposed
to power frequency EM fields. Environ Molec Mutat 17:49-58, 1991.
12) M Rosenthal & G Obe: Effects of 50-Hertz EM fields on proliferation and
on chromosomal aberrations in human peripheral lymphocytes untreated and
pretreated with chemical mutagens. Mutat Res 210:329-335, 1989.
13) J. Nafziger et al: DNA mutations and 50 Hz EM fields. Bioelec Bioenerg
30:133-141, 1993.
14) A. Rannug et al: A study on skin tumor formation in mice with 50 Hz
magnetic field exposure. Carcinogenesis 14:573-578, 1993.
15) R. Zwingelberg et al: Exposure of rats of a 50-Hz, 30-mT magnetic
field influences neither the frequencies of sister-chromatid exchanges nor
proliferation characteristics of cultured peripheral lymphocytes. Mutat
Res 302:39-44, 1993.
16) TS Tenforde: Biological interactions and potential health effects of
extremely-low-frequency magnetic fields from power lines and other common
sources. Ann Rev Publ Health 13:173-196, 1992.
17) R Goodman & A Shirley-Henderson: Transcription and translation in cells
exposed to extremely low frequency EM fields. Bioelec Bioenerg 25:335-355,
1991.
18) RB Goldberg & WA Creasey: A review of cancer induction by extremely low
frequency EM fields. Is there a plausible mechanism? Medical Hypoth
35:265-274, 1991.
19) A Rannug et al: Rat liver foci study on coexposure with 50 Hz magnetic
fields and known carcinogens. Bioelectromag 14:17-27, 1993.
20) MA Stuchly et al: Modification of tumor promotion in the mouse skin by
exposure to an alternating magnetic field. Cancer Letters 65:1-7, 1992.
21) JRN McLean et al: Cancer promotion in a mouse-skin model by a 60-Hz
magnetic field: II. Tumor development and immune response. Bioelectromag
12:273-287, 1991.
22) S Baumann et al: Lack of effects from 2000-Hz magnetic fields on
mammary adenocarcinoma and reproductive hormones in rats. Bioelectromag
10:329-333, 1989.
23) AB Hill: The environment and disease: Association or causation? Proc
Royal Soc Med 58:295-300, 1965.
24) G Theriault: Cancer risks due to exposure to electromagnetic fields.
Rec. Results Cancer Res. 120:166-180; 1990.
25) RK Adair: Constraints on biological effects of weak
extremely-low-frequency electromagnetic fields, Phys Rev A 43:1039-1048,
1991.
26) J.D. Jackson: Are the stray 60-Hz electromagnetic fields associated
with the distribution and use of electric power a significant cause of
cancer? Proc Nat Acad Sci USA 89:3508-3510, 1992.
27) MN Bates: Extremely low frequency electromagnetic fields and cancer:
the epidemiologic evidence, Environ Health Perspec 95:147-156, 1991.
28) DA Savitz & L Feingold: Association of childhood leukemia with
residential traffic density. Scan J Work Environ Health 15:360-363, 1989.
29) JM Peters et al: Exposure to residential electric and magnetic fields
and risk of childhood leukemia. Rad Res 133:131-132, 1993.
30) JD Sahl et al: Cohort and nested case-control studies of hematopoietic
cancers and brain cancer among electric utility workers. Epidemiology
4:104-114, 1993.
31) J McCann et al: A critical review of the genotoxic potential of
electric and magnetic fields. Mut Res 297:61-95, 1993.
32) W Loscher et al: Tumor promotion in a breast cancer model by exposure
to a weak alternating magnetic field. Cancer Letters 71:75-81, 1993.
33) AR Liboff et al: Time-varying magnetic fields: Effects on DNA
synthesis. Science 223:818-820, 1984.
34) I Penn: Why do immunosuppressed patients develop cancer? Crit Rev
Oncogen 1:27-52, 1989.
35) GR Krueger: Abnormal variation of the immune system as related to
cancer. Cancer Growth Prog 4:139-161, 1989.
36) P Guenel et al: Incidence of cancer in persons with occupational
exposure to electromagnetic fields in Denmark. Br J Indust Med 50:758-764,
1993.
37) PJ Verkasalo et al: Risk of cancer in Finnish children living close to
power lines. BMJ 307:895-899, 1993.
38) JH Olsen et al: Residence near high voltage facilities and risk of
cancer in children. BMJ 307:891-895, 1993.
39) R Doll et al, Electromagnetic Fields and the Risk of Cancer, NRPB,
Chilton, 1992.
40) JG Davis et al: Health Effects of Low-Frequency Electric and Magnetic
Fields. Oak Ridge Associated Universities, 1992.
41) J Walleczek: Electromagnetic field effects on cells of the immune
system: the role of calcium signaling. FASEB J 6:3177-3185, 1992.
42) RG Stevens et al: Electric power, pineal function, and the risk of
breast cancer. FASEB J 6:853-860, 1992.
43) RJ Reiter & BA Richardson: Magnetic field effects on pineal
indoleamine metabolism and possible biological consequences. FASEB J
6:2283-2287, 1992.
44) GH Schreiber et al: Cancer mortality and residence near electricity
transmission equipment: A retrospective cohort study. Int J Epidem
22:9-15, 1993.
45) RC Petersen: Radiofrequency/microwave protection guides. Health Phys
61:59-67, 1991.
46) M Feychting & A Ahlbom: Magnetic fields and cancer in children
residing near Swedish high-voltage Power Lines. Amer J Epidem 7:467-481,
1993.
47) WT Kaune et al: Residential magnetic and electric fields.
Bioelectromag 8:315-335, 1987.
48) TL Jones et al: Selection bias from differential residential mobility
as an explanation for associations of wire codes with childhood cancer. J
Clin Epidemiol 46:545-548; 1993.
49) K Dickersin et al: Publication bias and randomized controlled trials.
Cont Clin Trials 8:343-353; 1987.
50) H Kung & CF Seagle: Impact of power transmission lines on property
values: A case study. Appraisal J 60:413-418, 1992.
51) A Ahlbom et al: Electromagnetic fields and childhood cancer. Lancet
343:1295-1296, 1993.
--------------
Acknowledgments: This FAQ sheet owes much to the many readers of
sci.med.physics show have sent me comments and suggestions, including:
kfoster@eniac.seas.upenn.edu (from whom I stole most of Q5)
gary%ke4zv.uucp@mathcs.emory.edu (who suggested adding a quantum approach)
aa2h@virginia.edu (suggestions on thermal effects and confounders)
p.farrell@trl.oz.au (SI units, suggesting the pro/con arguments section)
drchambe@tekig5.pen.tek.com (a start on the property value question)
John Moulder (jmoulder@its.mcw.edu) Voice: 414-266-4670
Radiation Biology Group FAX: 414-257-2466
Medical College of Wisconsin, Milwaukee