Xref: helios.physics.utoronto.ca sci.med.physics:2458 sci.answers:1477 news.answers:27581
From: firstname.lastname@example.org (John Moulder)
Subject: Powerlines & Cancer FAQs 2/6: FAQ 1
Date: Mon, 15 Aug 1994 16:26:48 -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.
Keywords: powerlines, magnetic fields, cancer, EMF, non-ionizing
FAQs on Power-Frequency Fields and Cancer (Q&A, Part 1 of 3)
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 (IR),
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) Why do different types of EM sources produce different biological
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. 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 part of the EM
spectrum is termed ionizing. The well-known biological effects of X-rays
are associated with the ionization of molecules. At lower frequencies,
such as those characteristic of visible light, RF, and MW, the energy of
a photon is very much (by a factor of thousands or more) below those
needed to disrupt chemical bonds. This part of the EM spectrum is
termed non-ionizing. Because non-ionizing EM energy cannot break
chemical there is no analogy between the biological effects of ionizing
and nonionizing EM energy.
Non-ionizing EM sources can still produce biological effects. One
mechanism is by inducing electrical currents in tissues, which cause
heating. The efficiency with which an EM source can induce electrical
currents, and thus produce heating, depends on the frequency of the
source, and the size and orientation of the object being heated. At
frequencies below that used for broadcast AM radio, EM sources couple
poorly with the bodies of humans and animals, and thus are very
inefficient at inducing electrical currents and causing heating.
Thus in terms of potential biological effects the EM spectrum can be
divided into three portions:
1) The ionizing portion, where direct chemical damage can occur (X-rays,
2) The portion of the non-ionizing spectrum in which the wavelength is
smaller than that of the body, where heating can occur (visible light,
IR, MW and RF).
3) The portion of the non-ionizing spectrum in which the wavelength is
much larger than that of the body, where heating seldom occurs (power
4) What is difference between EM radiation and EM fields?
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. Fields are not projected away into space, and cease 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.
For ionizing frequencies, where the wavelengths are less than one
hundred thousandths of an inch (less than 300 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 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 EM radiation?
To be an effective radiation source an 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. While the lines theoretically might radiate some
energy, the efficiency is so low that this effect can for all practical
purposes be ignored.
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 EM theory). Much
of the loss of energy is a result of resistive heating; this is in sharp
contrast to RF antennas, which "lose" energy to space by radiation.
Likewise, there are many ways of transmitting energy 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 ELF 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 enough energy per photon to break 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 also lead to
inherited changes, a phenomenon called mutation. All of the known human
health 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 [M3].
7) How do RF, MW, visible light, and IR light sources cause biological
A principal mechanism by which RF, MW, visible light, and IR light
sources cause biological effects is by heating (thermal effects). This
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. At the whole-animal level, tissue injury and other thermally-
induced effects can be expected when the amount of power absorbed by the
animal is similar to or exceeds the amount of heat generated by normal
body processes. Some of these thermal effects are very subtle, and do
not represent biological hazards.
It is possible to produce thermal effects even with very low levels of
absorbed power. One example is the "microwave hearing" phenomenon;
these are auditory sensations that a person experiences when his head is
exposed to pulsed microwaves such as those produced by radar. The
³microwave hearing² effects is a thermal effect, but it can be observed
at very low average power levels.
Since thermal effects are produced by induced currents, not by the
electric or magnetic fields directly, they can be produced by fields at
many different frequencies. Well-accepted safety standards exist to
prevent significant thermal damage to persons exposed to MW and RFs [M2]
and also for persons exposed to lasers, IR and UV light [M4].
8) How do the power-frequency EM fields cause biological effects?
The electrical and magnetic fields associated with power-frequency
fields cannot break bonds because the energy per photon is too low. The
magnetic field intensities to which people are exposed in residential
settings and in the vast majority of occupational settings cannot cause
heating because the induced electrical currents are 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. If these currents are
sufficiently intense, they can cause heating, nerve excitation and other
effects [F2,K1]. At power frequencies, the body is poorly coupled to
external fields, and the induced currents are usually too small to
produce obvious effects. Shocks, and other obvious effects usually
require that the body actually touch conductive objects, allowing
current to pass directly into the body.
It requires a power-frequency magnetic field in excess of 5 G (500
microT, see Question 10 for typical exposures) to induce electrical
currents of a magnitude similar to those that occur naturally in the
body. Well-accepted safety standards exist to protect persons from
exposure to power-frequency fields that would induce such currents
9) Do non-ionizing EM sources cause non-thermal as well as thermal
One distinction that is often made in discussions of the biological
effects of non-ionizing EM sources is between "nonthermal" and "thermal"
effects. This refers to the mechanism for the effect, non-thermal
effects being a result of a direct interaction between the field and the
organism, and thermal effects being a result of heating. Microwave
burns are an obvious thermal effect, and electrical shocks are an
obvious nonthermal effect. There are many reported biological effects
of non-ionizing EM sources (some of which have not been reproduced)
whose mechanisms are totally unknown, and it is difficult (and not very
useful) to try to draw a distinction between "thermal" and "nonthermal"
mechanisms for such effects.
10) What sort of power-frequency fields are common in residences and
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,
magnetic fields are measured in Tesla (T), were 10,000 G equals 1 T (1 G
= 100 microT; 1 microT = 10 mG). Electrical fields are measured in
volts/meter (v/m). Measurement techniques are discussed in Questions 29
Within the right-of-way (ROW) of a high-voltage (115-765 kV, 115,000-
765,000 volt) transmission line, fields can approach 100 mG (10 microT)
and 10,000 v/m. At the edge of a high-voltage transmission ROW, the
fields will be 1-10 mG (0.1-1.0 microT) and 100-1,000 v/m. Ten meters
from a 12 kV (12,000 volt) distribution line fields will be 2-10 mG
(0.2-1.0 microT) and 2-20 v/m. Actual magnetic fields depend on
voltage, design and current; actual electrical fields are affected only
by voltage and design [F5].
Fields within residences vary from over 1000 mG (100 microT) and 200 v/m
a few inches (cm) from certain appliances to less than 0.2 mG (0.02
microT) and 2 v/m in the center of some rooms. Appliances that have the
highest magnetic 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 [F5].
Occupational exposures in excess of 1000 mG (100 microT) and 5000 v/m
have been reported (e.g., in arc welders and electrical cable splicers).
In "electrical" occupations typical mean exposures range from 5 to 40 mG
(0.5 to 4 microT) and 100-2000 v/m [F5,F7].
11) 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 areas 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.
Increasing the height of towers, and thus the height of the conductors
above the ground, will reduce the field intensity at the edge of the
ROW. The size, spacing and configuration of conductors can be modified
to reduce magnetic fields, but this approach is limited by electrical
safety considerations. Placing multiple circuits on the same set of
towers can also lower the field intensity at the edge of the ROW,
although it generally requires higher towers. Replacing lower voltage
lines with higher voltage ones can also lower the magnetic fields.
Burying transmission lines greatly reduces their magnetic fields. The
reduction occurs because the underground lines use rubber, plastic or
oil for insulation rather than air. This allows the conductors to be
placed much closer together and allows greater phase cancellation.
Placing high voltage lines underground is very expensive, adding costs
that are measured in hundreds of thousands of US dollars per mile.
12) What is known about the relationship between powerline corridors and
Some studies have shown that children living near certain types of
powerlines (high current distribution and transmission lines) have
higher than average rates of leukemia [C1,C6,C10,C18], brain cancers
[C1,C6] and/or overall cancer [C5,C16]. 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 [C6,C10,C18]. Several other studies have shown no correlations
between residence near power lines and risks of childhood leukemia
[C3,C5,C7,C8,C9,C15,C16], childhood brain cancer [C5,C8,C15,C16,C18], or
overall childhood cancer [C15,C18]. With one exception [C2] all studies
of correlations between adult cancer and residence near power lines have
been negative [C4,C8,C9,C11,C13,C17].
13) How big is the "cancer risk" associated with living next to a
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,
[C16], 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 [B3] and Doll et al [B4] by inclusion of the new European
studies (Question 19). The confidence intervals should be viewed as
measures of the diversity of the data, rather than as strict tests of
the statistical significance.
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.
14) How close do you have to be to a power line to be considered exposed
to power-frequency magnetic fields?
The 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 cancer incidence. 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, whereas the European
studies have been based strictly on high-voltage transmission lines
Field measurements: Several studies have measured power-frequency fields
in residences [C6,C7,C10,C13,C18]. Both one-time (spot), peak, and 24-
hour average measurements have been made; none of the studies using
measured fields have shown a relationship between exposure and cancer.
Proximity to lines: Several studies have used the distance from the
power line corridor to the residence as a measure of power-frequency
fields [C4,C5,C8,C9, C8,C9,C11,C13,C18]. 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 two exceptions [C5,C18], studies that have used distance
from power lines as a surrogate measure of exposure have shown no
significant relationship between proximity to lines and the incidence of
cancer. The major exception is a childhood leukemia study [C18] that
showed a significant increase in leukemia incidence for residence within
50 m (150 ft) of high-voltage transmission lines. This same study
[C13,C18] showed no elevation of child leukemia rates at 51-100 m (150-
300 ft), and no increase in childhood brain cancer, overall childhood
cancer, or any types of adult cancer at any distance.
Wirecodes: 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 [C1,C2,C3,C6,C7,C10]. This technique is known as
"wirecoding". Three studies using wirecodes [C1,C6,C10] have shown a
relationship between childhood cancer and "high-current configuration"
wirecodes. Two of these studies [C6,C10] failed to show a significant
relationship between exposure and cancer when actual measurements were
made. Wirecodes are stable over time [F4], but correlate poorly with
measured fields [F4,F5,F6]. The wirecode scheme was developed for the
U.S., and is not 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 power lines in the past [C13,C15,C16,C18]. 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
(Question 19). These calculated exposures explicitly exclude
contributions from other sources such as distribution lines, household
wiring, or appliances. There is no way to check the accuracy of the
calculated historic fields.
15) 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 cancer rates. The original studies
[D1,D2] were only of leukemia. Some later studies also implicated
brain, lymphoma and/or breast cancer [B2,B3,B4]. 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
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 be combined.
Second, a wide range of definitions of "electrical occupations" are
used, and very few studies actually measured exposure. The following
RRs (Question 13) for the occupational exposure studies are adapted from
Hutchison [B3] and Davis et al [A2]. Again, the confidence intervals
should be viewed as measures of diversity rather than as tests of the
leukemia: 1.15 (1.0-1.3) 28 studies
brain: 1.15 (1.0-1.4) 19 studies
lymphoma: 1.20 (0.9-1.5) 6 studies
all cancer: 1.00 (0.9-1.1) 8 studies
The above meta-analysis is out-of-date, and does not take into account
more recent studies (see Question 19B & 19C). Some of the new
occupational exposure studies [D9,D10,D12,D14] shows small but
statistically significant increases in leukemia, but others
[D8,D11,D13,D15,D16] do not. None of the new studies of electrical
occupations show significant elevation of any types of cancer other than
leukemia and breast cancer (specifically brain cancer or lymphoma)
[D10,D12,D13,D14,D15,D16]. Adding these eight new studies raises the
summary RR for leukemia slightly, and lowers the summary RRs for brain
cancer and lymphomas to essentially one.
Copyright (C) by John Moulder