FAQs on Power-Frequency Fields and Cancer (part 1 of 2)

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Revision notes:
v1.9 (19-Dec-93): Sections added on whether powerlines radiate, how close
one has to live to a powerline to be considered exposed, reducing fields,
and on the impact of powerlines on property values.  The sections on
laboratory studies and on arguments pro and con were broken up into
sections. The sections on confounders, and on application of the Hill
criteria were expanded and broken up into sections.  The sections on new
European epidemiological studies and on standards were updated.
v2.2 (28-Dec-94):  Schreiber study and Ahlbom et al meta-analysis added to
Q18.  First version approved for and posted to *.answers newsgroups.
Converted to two parts.

1) Why is there a concern about powerlines and cancer?

Most of the concern about power lines and cancer stems from epidemiological
studies of people living near powerlines, and epidemiological studies of
people working in electrical occupations.  Some of these epidemiological
studies appear to show a relationship between exposure to power-frequency
magnetic fields and the incidence of cancer.  Laboratory studies have shown
little evidence of a link between power-frequency fields and cancer.

2) What is the difference between the electromagnetic [EM] energy
associated with power lines and other forms of EM energy such as microwaves
or x-rays?

X-rays, ultraviolet (UV) light, visible light, infrared light, microwaves
(MW), radiowaves (RF), and electromagnetic fields from electrical power
systems are all parts of the EM spectrum.   The parts of the EM spectrum
are characterized by their frequency or wavelength.  The frequency and
wavelength are related, and as the frequency rises the wavelength gets
shorter.  The frequency is the rate at which the EM field changes direction
and is usually given in Hertz (Hz), where one Hz is one cycle per second.

Power-frequency fields in the US vary 60 times per second, so they are 60
Hz fields, and have a wavelength of 3000 miles (5000 km).  Power in most of
the rest of the world is at 50 Hz.  The power-frequency fields are often
referred to as extremely low frequencies or ELF.  Broadcast AM radio has a
frequency of around one million Hz and a wavelength of around 1000 ft (300
m). Microwave ovens have a frequency of about 2.5 billion Hz, and a
wavelength of about 5 inches (12 cm).  X-rays and UV light have frequencies
of millions of billions of Hz, and wavelengths of less than a thousandth of
an inch (10 nm or less).

3)  What differences are there in the biological effects of these different
portions of the EM spectrum?

The interaction of biological material with an EM source depends on the
frequency of the source.  We usually talk about the EM spectrum as though
it produced waves of energy.  This is not strictly correct, because
sometimes EM energy acts like particles rather than waves; this is
particularly true at high frequencies.  This double nature of the EM
spectrum is referred to as "wave-particle duality".  The particle nature of
EM energy is important because it is the energy per particle (or photons,
as these particles are called) that determines what biological effects EM
energy will have.

At the very high frequencies characteristic of UV light and X-rays, EM
particles (photons) have sufficient energy to break chemical bonds.  This
breaking of bonds is termed ionization, and this portion of the EM spectrum
is termed ionizing radiation.  At lower frequencies, such as those
characteristic of visible light, RR and MW, the photons do not carry enough
energy to break chemical bonds; but they do carry enough energy to cause
molecules to vibrate, causing heating (thermal effects).  This portion of
the EM spectrum is termed the thermal, non-ionizing portion.  At
frequencies below those used in commercial broadcast radio (such as the
50/60 Hz frequencies generated in the production and distribution of
electricity), the photons have insufficient energy to cause heating, and
this portion of the EM spectrum is termed the non-thermal, non-ionizing
portion.

4)  What is difference between EM radiation and EM fields?

When dealing with fields from an electromagnetic source it is customary to
distinguish between near fields (which do not transmit energy to infinity
from the source) and radiation (which does).  In general, EM sources
produce both radiant energy (radiation) and non-radiant energy (fields).
Radiated energy exists apart from its source, travels away from the source,
and continues to exist even if the source is turned off.  Non-radiant
energy is not projected away into space, and it ceases to exist when the
energy source is turned off.  When a person or object is more than several
wavelengths from an EM source, a condition called far-field, the radiation
component of the EM source dominates.  When a person or object is less than
one wavelength from an EM source, a condition called near-field, the field
effect dominates, and the electrical and magnetic components are unrelated.


For ionizing frequencies where the wavelengths are less than a thousandth
of an inch (less than 10 nm), human exposure is entirely in the far-field,
and only the radiation from the EM source is relevant to health effects.
For MW and RF, where the wavelengths are in inches to a few thousand feet
(a few cm to a km), human exposure can be in both the near- and the
far-field, so that both field and radiation effects can be relevant.  For
power-frequency fields, where the wavelength is thousands of miles
(thousands of km), human exposure is always in the near-field, and only the
field component is relevant to possible health effects.

5)  Do power lines produce electromagnetic radiation?

The fields associated with transmission lines are purely near-field.  While
the lines theoretically might radiate some energy the efficiency of this is
so low that this effect can for all practical purposes be ignored.  To be
an effective radiation source, and antenna must have a length comparable to
its wavelength.  Power-frequency sources are clearly too short compared to
their wavelength (3000 miles, 5000 km) to be effective radiation sources.

This is not to say that there is no loss of power during transmission.
There are many sources of loss in transmission lines that have nothing to
do with "radiation" (in the sense as it is used in electromagnetic theory).
 Loss of energy is a result of resistive heating, not "radiation".  This is
in sharp contrast to radiofrequency antennas, which "lose" energy to space
by radiation.  Likewise, there are many ways of transmitting energy from
point A to point B that do not involve radiation.  Electrical circuits do
it all the time.

The only practical exception to the statement that power-frequency fields
do not radiate is the use of extremely-low-frequency antennas to broadcast
to submerged submarines.  The US Navy runs a power-frequency antenna in
Northern Wisconsin and the Upper Peninsula of Michigan.  To overcome the
inherent inefficiency of the frequency, the antenna is several hundred
kilometers in length.

6)  How do ionizing EM sources cause biological effects?

Ionizing EM radiation carries sufficient energy per photon to break
chemical bonds.  In particular, ionizing radiation is capable of breaking
bonds in the genetic material of the cell, the DNA.  Severe damage to DNA
can kill cells, resulting in tissue damage or death.  Lesser damage to DNA
can result in permanent changes in the cells which may lead to cancer.  If
these changes occur in reproductive cells, they can lead to inherited
changes, a phenomena called mutation.  All of the known hazards from
exposure to the ionizing portion of the EM spectrum are the result of the
breaking of chemical bonds in DNA.  For frequencies below that of UV light,
DNA damage does not occur because the photons do not have enough energy to
break chemical bonds.   Well-accepted safety standards exist to prevent
significant damage to the genetic material of persons exposed to ionizing
EM radiation.

7)  How do the thermal non-ionizing EM sources cause biological effects?

Visible light, MW, and RF can cause molecules to vibrate, causing heating.
This molecular heating can kill cells.  If enough cells are killed, burns
and other forms of long-term, and possibly permanent tissue damage can
occur.  Cells which are not killed by heating gradually return to normal
after the heating ceases; permanent non-lethal cellular damage is not known
to occur.  All of the known hazards from exposure to the thermal
non-ionizing portion of the EM spectrum are the result of heating.  For
frequencies below about the middle of the AM broadcast spectrum, this
heating does not occur because the photons do not have enough energy to
cause molecular vibrations.  Well-accepted safety standards exist to
prevent significant thermal damage to persons exposed to MW and RFs [45]
and also for persons exposed to lasers and UV light.

The molecular vibration caused by MW is how and why a MW oven works -
exposure of the food to the microwaves causes water molecules to vibrate
and get hot.  MW and RF penetrate and heat best when the size of the object
is close to the wavelength.  For the 2450 MHz (2.45 billion Hz) used in
microwave ovens the wavelength is 5 inches (12 cm), a good match for most
of what we cook.

8)  How do the power-frequency EM fields cause biological effects?

The electrical and magnetic fields associated with power-frequency fields
cannot break bonds or cause molecular heating because the energy per photon
is too low. Thus the known mechanisms through which ionizing radiation, MWs
and RFs effect biological material have no relevance for power-frequency
fields.

The electrical fields associated with the power-frequency fields exist
whenever voltage is present, and regardless of whether current is flowing.
These electrical fields have very little ability to penetrate buildings or
even skin.  The magnetic fields associated with power-frequency fields
exist only when current is flowing.  These magnetic fields are difficult to
shield, and easily penetrate buildings and people.   Because
power-frequency electrical fields do not penetrate, any biological effects
from routine exposure to power-frequency fields must be due to the magnetic
component of the field.

Exposure of people to power-frequency magnetic fields results in the
induction of electrical currents in the body.  These currents are similar
to naturally-occurring currents.  It requires a power-frequency magnetic
field in excess of 5 Gauss (500 microT, see Q9 for typical exposures) to
induce electrical currents of a magnitude similar to those that occur
naturally in the body.   Electrical currents that are above those that
occur naturally in the body can cause noticeable effects, including direct
nerve stimulation.  Well-accepted safety standards exist to protect persons
from exposure to power-frequency fields that would induce such currents
(Q25 for safety standards).

9)  What sort of power-frequency magnetic fields are common in residences
and workplaces?

In the US magnetic fields are commonly measured in Gauss (G) or milliGauss
(mG), where 1,000 mG = 1G.  In the rest of the world, they are measured in
Tesla (T), were 10,000 G equals 1 T (1 G = 100 microT; 1 microT = 10 mG).
Power-frequency fields are measured with a calibrated gauss meter.
Measurements must be done in multiple locations over a substantial period
of time because there are large variations in fields over space and time.

Within the right-of-way (ROW) of a high voltage transmission line, fields
can approach 100 mG (0.1 G, 10 microT).  At the edge of a high-voltage
transmission ROW, the field will be 1-10 mG (0.1-1.0 microT).  Ten meters
from a 12 kV (1200 volt) distribution line fields will be 2-10 mG (0.2-1.0
microT).  Actual fields depend on voltage, design and current.

Fields within residences vary from over 1000 mG (100 microT) a few inches
(cm) from certain appliances to less than 0.2 mG (0.02 microT) in the
center of some rooms.  Appliances that have the highest fields are those
with high currents (e.g., toasters, electric blankets) or high-speed
electric motors (e.g., vacuum cleaners, electric clocks, blenders, power
tools).  Appliance fields decrease very rapidly with distance. See
Theriault [24] for further details.

Occupational exposures in excess of 100 mG (10 microT) have been reported
(e.g., in arc welders and electrical cable splicers).  In electrical
occupations mean exposures range from 5 to 40 mG (0.5 to 4 microT).  See
Theriault [24] for further details.

10)  Can power-frequency fields in homes and workplaces be reduced?

There are engineering techniques that can be used to decrease the magnetic
fields produced by power lines, substations, transformers and even
household wiring and appliances.  Once the fields are produced, however,
shielding is very difficult.  Small area can be shielded by the use of Mu
metal, a nickel-iron-copper alloy with high magnetic permeability and low
hysteresis losses.  Mu metal shields are very expensive, and limited to
small volumes.

11)  What is known about the relationship between powerline corridors and
cancer rates?

Some studies have shown that children (but not adults) living near certain
types of powerlines (high current distribution lines and transmission
lines) have higher than average rates of leukemia, lymphomas and brain
cancers [1-3, 38, 45].  The correlations are not strong, and none of the
studies have shown dose-response relationships.  When power-frequency
fields are actually measured, the correlation vanishes.  Several other
studies have shown no correlations between residence near power lines and
cancer risk [4-6, 37].

12)  How big is the cancer risk associated with living next to a
powerline?

The excess cancer found in epidemiological studies is usually quantified in
a number called the relative risk (RR).  This is the risk of an exposed
person getting cancer divided by the risk of an unexposed person getting
cancer.  Since no one is unexposed to power-frequency fields, the
comparison is actually high exposure versus low exposure.  A RR of 1.0
means no effect, a RR of less the 1.0 means a decreased risk in exposed
groups, and a RR of greater than one means an increased risk in exposed
groups.  Relative risks are generally given with 95% confidence intervals.
These 95% confidence intervals are almost never adjusted for multiple
comparisons even when multiple types of cancer and multiple indices of
exposure are studied (see Olsen et al, [38], Fig. 2 for an example of a
multiple comparison adjustment).

An overview of the epidemiology requires that studies be combined using a
technique known as meta-analysis.  Meta-analysis is not easy to do, since
the epidemiological studies of residential exposure use a wide variety of
methods for assessing exposure.  Meta-analysis also gets out-of-date
rapidly in this field.  The following RRs (called summary RRs in
meta-analysis) for the residential exposure studies are adapted from
Hutchison [7] and Doll et al [39] by inclusion of the new European studies
(Q18).  The confidence intervals should be viewed as measures of the
diversity of the data, rather than as strict tests of the statistical
significance of the data.
   childhood leukemia: 1.5 (0.8 - 3.0)  8 studies
   childhood brain cancer:  1.9 (0.9 - 3.0)  6 studies
   childhood lymphoma: 2.5 (0.3 - 40) 2 studies
   all childhood cancer:  1.5 (0.9 - 2.5)  5 studies
   adult leukemia:  1.1 (0.8-1.6)  3 studies
   adult brain cancer:  0.7 (0.4 - 1.3)  1 study
   all adult cancer:  1.1 (0.9-1.3)  3 studies

As a base-line for comparison, the age-adjusted cancer incidence rate for
adults in the United States is 3 per 1,000 per year for all cancer (that
is, 0.3% of the population gets cancer in a given year),and 1 per 10,000
per year for leukemia [26].

13)  How close do you have to be to a power line to be considered exposed
to power-frequency magnetic fields?

The epidemiological studies that show a relationship between cancer and
powerlines do not provide any consistent guidance as to what distance or
exposure level is associated with increased risk.  The studies have used a
wide variety of techniques to measure exposure, and they differ in the type
of lines that are studied.  The US studies have been based predominantly on
neighborhood distribution lines [1-3], whereas the European studies have
been based strictly on high-voltage transmission lines [4-6, 37, 38, 44,
46].

Field measurements:  Several studies have measured power-frequency fields
in the residences [2, 3, 45].  Both one-time (spot), peak, and 24-hour
average measurement have been made; none of the studies using measured
fields have shown a relationship between exposure and cancer risk.

Proximity to lines:  Several studies have used the distance from the power
line corridor to the residence as a measure of power-frequency fields [4-6,
44, 46].  When something we can measure (distance to the line), is used as
an index of what we really want to measure (the magnetic field), it is
called a surrogate (or proxy) measure.  With one exception, studies that
have used distance from power lines as a surrogate measure of exposure have
shown no significant relationship between proximity to lines and risk of
cancer.  The exception is a childhood leukemia study [46] that showed a RR
of 2.9 (1.0-7.3) for residence within 50 m of high-voltage transmission
lines.  This same study showed no elevation of child leukemia risks at
51-100 m, and no increase in childhood brain cancer, overall childhood
cancer, or any types of adult cancer at any distance.

Wire Codes:  The original US powerline studies used a combination of the
type of wiring (distribution vs transmission, number and thickness of
wires) and the distance from the wiring to the residence as a surrogate
measure of exposure [1-3].  This technique is known as wirecoding.  Three
studies using wirecodes [1-3] have shown a relationship between childhood
cancer and high-current configuration wirecodes.  Two of these studies
[2, 3] failed to show a significant relationship between exposure and
cancer when actual measurements were made.  Wirecodes correlate with
measured fields, although the correlation is not very good [47].  The
wirecode scheme was developed for the U.S., and does not appear to be
readily applicable elsewhere.

Calculated Historic Fields:  The recent European studies have used utility
records and maps to calculate what fields would have been produced by
powerlines in the past [37, 38, 44, 46].  Typically, the calculated field
at the time of diagnosis or the average field for a number of years prior
to diagnosis are used as a measure of exposure (Q17).  These calculated
exposures explicitly exclude contributions from other sources such as
distribution lines, household wiring, or appliances.  When the field
calculations are done for contemporary measured fields they correlate
reasonably well [46].  Of course, there is no way to check the accuracy of
the calculated historic fields.

14)  What is known about the relationship between electrical occupations
and cancer rates?

Several studies have shown that people who work in electrical occupations
have higher than average leukemia, lymphoma, and brain cancer rates [8-10,
36].  Most of the cautions listed for the residential studies apply here
also: many negative studies, weak correlations, no dose-response
relationships.  Additionally, these studies are mostly based on job titles,
not on measured exposures.

Meta-analysis of the occupational studies is even more difficult than the
residential studies.  First, a variety of epidemiological techniques are
used, and studies using different techniques should not really be combined.
 Second, a wide range of definitions of electrical occupations are used,
and very few studies actually measured exposure.  The following RRs (Q12)
for the occupational exposure studies are adapted from Hutchison [7] and
Davis et al [40].  Again, the confidence intervals should be viewed as
measures diversity rather than as tests of the statistical significance.
leukemia: 1.15 (1.0-1.3)  28 studies
brain: 1.15 (1.0-1.4)  19 studies
lymphoma: 1.2 (0.9-1.5)  6 studies
all cancer:  1.0 (0.9-1.1)  8 studies

The above relative risks do not take into account the recent European
studies (Q18).  Adding these new studies raises the summary RR for leukemia
to about 1.2, and lowers the summary RRs for brain cancer and lymphomas to
essentially one.  Another new study of cancer in the electrical power
industry [30] shows no significant elevation of leukemia, brain cancer or
lymphoma risks.

15)  What do laboratory studies tell us about power-frequency fields and
cancer?

Carcinogens, agents that cause cancer, are generally of two types:
genotoxins and promoters.  Genotoxic agents (often called initiators)
directly damage the genetic material of cells.  Genotoxins usually effect
all types of cells, and may cause many different types of cancer.
Genotoxins generally do not have thresholds for their effect; in other
words, as the dose of the genotoxin is lowered the risk gets smaller, but
it never goes away.  A promoter (often called an epigenetic agent) is
something that increases the cancer risk in animals already exposed to a
genotoxic carcinogen.  Promoters usually effect only certain types of
cells, and may cause only certain types of cancer.  Promoters generally
have thresholds for their effect; in other words, as the dose of the
promoter is lowered a level is reached in which there is no risk.

Power-frequency fields show none of the classic signs of being genotoxins -
they do not cause DNA damage or chromosome breaks, and they are not
mutagenic [11-15, 31].  No studies have shown that animals exposed to
power-frequency fields have increased cancer rates.

There are agents (for example, promoters) that influence the development of
cancer without directly damaging the genetic material.  It has been
suggested that power-frequency EMFs could promote cancer [17, 18].   Most
promotion studies of power-frequency fields have been negative [14, 19-21];
but recently there was a positive report of promotion of breast cancer in
rats [32].

16)  How do laboratory studies of the effects of power-frequency fields on
cell growth, immune function, and melatonin relate to the question of
cancer risk?

There are other biological effects that might be related to cancer.  There
are substances (called mitogens) that cause non-growing normal cells to
start growing.  Some mitogens appear to be carcinogens.  There have been
numerous studies of the effects of power-frequency fields on cell growth
(proliferation) and tumor growth (progression). Studies of effects on
proliferation and progression have had very mixed results: 75% show no
effect on growth, while the rest are about equally mixed between studies
showing increased growth and studies showing decreased growth [11, 12, 15,
20-22, 33].  With one possible exception [33] there have been no reported
effects on proliferation or progression for fields below 2000 mG (200
microT).

Suppression of the immune system in animals and humans is associated with
increased rates of certain types of cancer, particularly lymphomas [34,
35].  Immune suppression has not been associated with excess leukemia and
brain cancer.  Some studies have shown that power frequency fields can have
effects on cells of the immune system [41), but no studies have shown the
type or magnitude of immunesuppression that is associated with increased
cancer risks.

It has also been suggested that power-frequency EM fields might suppress
the production of the hormone melatonin, and that melatonin has
cancer-preventive activity [42, 43].  This is highly speculative.  There
have been some reports that EM fields effect melatonin production, but
studies using power-frequency magnetic fields have not shown reproducible
effects. In addition, while there is evidence that melatonin has
cancer-preventive activity against breast cancer in rats, there is no
evidence that melatonin effects other types of cancer, or that it has any
effect on breast cancer in humans.

17)  Do power-frequency fields show any effects at all in laboratory
studies?

While the laboratory evidence does not suggest a link between
power-frequency magnetic fields and cancer, numerous studies have reported
that these fields do have bioeffects, particularly at high field strength
[16, 17, 41].  Power-frequency fields intense enough to induce electrical
currents in excess of those that occur naturally (above 5 G, 500 microT,
see Q8) have shown reproducible effects, including effects on humans [16].
Below about 2 G (200 microT) there are few published (and replicated)
reports of bioeffects, although there are unreplicated reports of effects
for fields as low as about 200 mG (20 microT). Even among the scientists
who believe that there may be a connection between power-frequency fields
and cancer, there is no consensus as to mechanisms which would connect
these bioeffects with cancer causation [16, 18].

18)  What about the new Swedish study showing a link between power lines
and cancer?

There are new residential and occupational studies from Sweden [46],
Denmark [36, 38], Finland [37] and the Netherlands [44].  Some of the
Swedish studies are still available only as translations of the unpublished
preliminary reports.  The published studies are considerably more cautious
in there interpretations of the data than were the unpublished preliminary
reports and the earlier press reports.

The authors of the Scandnavian childhood cancer studies [37, 38, 46] have
produced a collaborative meta-analysis of their data [51].   The RRs from
this meta-analysis are shown below in comparison to meta-analysis of the
prior studies [7, 39].
Childhood leukemia, Scandanavian:  2.1 (1.1-4.1)
Childhood leukemia, prior studies:  1.3 (0.8-2.1)
Childhood lymphoma, Scandanavian:  1.0 (0.3-3.7)
Childhood lymphoma, prior studies:  none
Childhood CNS cancer, Scandanavian:  1.5 (0.7-3.2)
Childhood CNS cancer, prior studies:  2.4 (1.7-3.5)
All childhood cancer, Scandanavian:  1.3 (0.9-2.1)
All childhood cancer, prior studies:  1.6 (1.3-1.9)

- Fleychting & Ahlbom [Magnetic fields and cancer in people residing near
Swedish high voltage powerlines].   A case-control study of everyone who
lived within 300 meters of high-voltage powerlines between 1960 and 1985.
For children all types of tumors were analyzed, for adults only leukemia
and brain tumors were studied.  The data on childhood cancer has been
published [46].  Exposure was assessed by spot measurements, calculated
retrospective assessments, and distance from powerlines.  No increased
overall cancer risk was found for either children or adults.  An increased
risk for leukemia (but not other cancers) was found in children for
calculated fields over 2 mG (0.2 microT) at the time of diagnosis, and for
residence within 50 m of the powerline.  The increased relative risk of
leukemia is found only in one-family homes; there is no excess risk in
apartments.  The retrospective fields calculations do not take into account
sources other the transmission lines.  No significantly elevated cancer
risks were found for measured fields.

- Verkasalo et al [37].  Study design similar to Fleychting & Ahlbom
(above).  Cohort study of cancer in children in Finland living within 500 m
of high-voltage lines.  Only calculated retrospective fields were used to
define exposure.  The calculated fields are based only on lines of 110 kV
and above and do not take into account fields from other sources such as
distribution lines, household wiring or appliances. Both average fields and
cumulative fields (microT - years) were used as exposure metrics.  For all
cancers the RR was 1.5 (0.7 - 2.7) for average exposure above 0.20 microT
(2 mG), and 1.4 (0.8 - 2.3) for cumulative exposure above 0.50
microT-years.  A significant excess risk of brain cancer way found in boys,
the excess was due entirely to one exposed boy who developed three
independent brain tumors.  No significantly increased risks were found for
brain tumors in girls or for leukemia, lymphomas or other tumors in
either sex.

- Olsen and Nielson [38].  Case-control study based on all childhood
leukemia, brain tumors and lymphomas diagnosed in Denmark between 1968 and
1986.  Exposure was assessed on the basis of calculated fields over the
period from conception to diagnosis.  No overall increase in cancer risk
was found when 0.25 microT (2.5 mG) was used as the cut-point to define
exposure (as specified in the study design).  After the data were analyzed,
it was found that the risk for all childhood cancer was significantly
elevated if 0.40 microT (4 mG) was used as the cut-point.  For the 0.40
microT cut-point the RR for all cancer (corrected for multiple comparisons)
was 5.6 (1.2 - 30).  No significant increased risk was found for leukemia
or brain cancer for any cut-point.  A significant increase in lymphoma risk
was found for the 0.10 microT cut-point but not for higher cut-points.

- Guenel et al [36].  Case-control study based on all cancer in actively
employed Danes between 1970 and 1987 who were 20-64 years old in 1970.
Each occupation-industry combination was coded on the basis of supposed
50-Hz magnetic field exposure.  No significant increases in risk were seen
for breast cancer, malignant lymphomas or brain tumors.  Leukemia incidence
was elevated among men in the highest exposure category; women in similar
exposure categories showed no excess risk.  For men in the highest
exposure category the RR for leukemia was 1.6 (1.2 - 2.2).

-Floderus et al [Occupational exposure to EM fields in relation to leukemia
and brain tumors].  Case-control study of leukemia and brain tumors of men,
20-64 years of age in 1980.  Exposure calculations were based on the job
held longest during the 10-year period prior to diagnosis.  Many
measurements were taken using a person whose job was most similar to that
of the person in the study. About two-thirds of the subjects in the study
could be assessed in this manner.  A significantly elevated risk was found
for leukemia, but not for brain cancer.

-Schreiber et al [44].  Retrospective cohort study of people in an urban
area in the Netherlands.  People were considered exposed in they lived
within 100 m of transmission equipment (150 kV lines plus a substation).
Fields in the exposed group were 1-11 mG (0.1-1.1 microT), fields in the
unexposed group were 0.2-1.5 mG (0.02-0.15 microT).  For all cancers the
RR (exposed group vs the general Dutch population) was 0.85 (0.63-1.14).
No cases of leukemia or brain cancer were seen in the exposed group.

End:  powerlines-cancer-FAQ/part1



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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 Hill criteria [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 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,
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 high current configurations 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 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 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 al [49] for 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 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 physics"
d) Jacksons [26] and Olsens [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 comparable property 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 study [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
an eyesore, 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
had any knowledge of possible evidence connecting power transmission lines
to certain health risks such as cancer; 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 high
current configuration 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.

-----------------------
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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 residential electric and magnetic fields
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4) MP Coleman et al: Leukemia and residence near electricity transmission
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--------------
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



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