Extremely Low Frequency Radiation/Power Lines
Kelly Classic, Certified Medical Physicist
Everyone is exposed to a complex mix of electromagnetic fields (EMF)
of different frequencies that permeate our environment. Exposures to
many EMF frequencies are increasing significantly as technology
advances unabated and new applications are found. While the enormous
benefits of using electricity in everyday life and health care are
unquestioned, during the past 20 years the general public has become
increasingly concerned about potential adverse health effects of
exposure to electric and magnetic fields at extremely low frequencies
(ELF). Such exposures arise mainly from the transmission and use of
electrical energy at the power frequencies of 50/60 Hz.
Electromagnetic fields consist of electric (E) and magnetic (H)
waves traveling together. They travel at the speed of light and are
characterized by a frequency and a wavelength. The frequency is simply
the number of oscillations in the wave per unit time, measured in units
of hertz (1 Hz = 1 cycle per second), and the wavelength is the
distance travelled by the wave in one oscillation (or cycle).
ELF fields are defined as those having frequencies up to 300 Hz. At
frequencies this low, the wavelengths in air are very long (6,000 km at
50 Hz and 5,000 km at 60 Hz) and, in practical situations, the electric
and magnetic fields act independently of one another and are measured
separately.
Electric fields arise from electric charges. They govern the motion
of other charges situated in them. Their strength is measured in units
of volt per meter (V/m) or kilovolt per meter (kV/m). When charges
accumulate on an object they create a tendency for like or opposite
charges to be repelled or attracted, respectively. The strength of that
tendency is characterized by the voltage and is measured in units of
volt (V). Any device connected to an electrical outlet, even if the
device is not switched on, will have an associated electric field that
is proportional to the voltage of the source to which it is connected.
Electric fields are strongest close to the device and diminish with
distance. Common materials, such as wood and metal, shield against them.
Magnetic fields come from the motion of electric charges, that is, a
current. Their strength is measured in units of ampere per metre (A/m)
but is usually expressed in terms of the corresponding magnetic
induction measured in units of tesla (T) or millitesla (mT). In some
countries another unit, called the gauss (G), is commonly used for
measuring magnetic induction (10,000 G = 1 T). Any device connected to
an electrical outlet, when the device is switched on and a current is
flowing, will have an associated magnetic field, the strength of which
is directly related to the current drawn from the source. Magnetic
fields are strongest close to the device and get lower with distance.
Most common materials do not shield them.
Naturally occurring 50/60 Hz electric and magnetic field levels are
extremely low, on the order of 0.0001 V/m and 0.00001 microtesla (μT),
respectively. Human exposure to ELF fields is primarily associated with
the generation, transmission, and use of electrical energy, for
example, power lines. Electrical energy from power-generating stations
is distributed to communities via high-voltage transmission lines.
Transformers are used to lower the voltage for connections to
residential distribution lines that deliver the energy to homes.
Electric and magnetic fields underneath overhead transmission lines may
be as high as 12 kV/m and 30 μT respectively. Around generating
stations and substations, electric fields up to 16 kV/m and magnetic
fields up to 270 μT may be found.
Electric and magnetic fields in homes depend on many factors,
including the distance from local power lines, the number and type of
electrical appliances in use in the home, and the configuration and
position of household electrical wiring. Electric fields around most
household appliances and equipment typically do not exceed 500 V/m and
magnetic fields typically do not exceed 150 μT. In both cases, field
levels may be substantially greater at small distances but they do
decrease rapidly with distance.
In the workplace, electric and magnetic fields exist around
electrical equipment and wiring throughout industry. Workers whose job
it is to maintain transmission and distribution lines may be exposed to
very large electric and magnetic fields. Within generating stations and
substations, electric fields in excess of 25 kV/m and magnetic fields in
excess of 2 mT may be found. Welders can be subjected to magnetic-field
exposures as high as 130 mT. Near induction furnaces and industrial
electrolytic cells magnetic fields can be as high as 50 mT. Office
workers are exposed to very much smaller fields when using equipment
such as photocopying machines and video display terminals.
The only practical way that ELF fields interact with living tissues
is by inducing electric fields and currents in them. However, the
magnitude of these induced currents from exposure to ELF fields at
levels normally found in our environment is less than the currents
occurring naturally in the body. Available evidence on the health
effects of electric fields suggests that the effects of exposures of up
to 20 kV/m are few and not of any health consequences. Electric fields
have not been shown to have any effect on reproduction or development
in animals at strengths over 100 kV/m.
There is little confirmed experimental evidence that ELF magnetic
fields can affect human physiology and behavior at field strengths
found in the home or environment. Exposure of volunteers for several
hours to ELF fields up to 5 mT had little effect on a number of
clinical and physiological tests, including blood changes, ECG, heart
rate, blood pressure, and body temperature. Some investigators have
reported that ELF field exposure may suppress secretion of melatonin, a
hormone connected with our day-night rhythms. It has been suggested
that melatonin might be protective against breast cancer so that such
suppression might contribute to an increased incidence of breast cancer
already initiated by other agents. While there is some evidence for
melatonin effects in laboratory animals, volunteer studies have not
confirmed such changes in humans.
There is no convincing evidence that exposure to ELF fields causes
direct damage to biological molecules, including DNA. It is thus
unlikely that they could initiate the process of carcinogenesis.
However, studies are still underway to determine if ELF exposure can
influence cancer promotion or copromotion. Recent animal studies have
not found evidence that ELF field exposure affects cancer incidence. In
1979 Wertheimer and Leeper reported an association between childhood
leukemia and certain features of the wiring connecting their homes to
the electrical distribution lines. Since then, a large number of
studies have been conducted to follow up this important result.
Analysis of these papers by the U.S. National Academy of Sciences in 1996
suggested that residence near power lines was associated with an
elevated risk of childhood leukemia (relative risk RR=1.5), but not
with other cancers. A similar association between cancer and
residential exposure of adults was not seen from these studies.
Many studies published during the last decade on occupational
exposure to ELF fields have exhibited a number of inconsistencies. They
suggest there may be a small elevation in the risk of leukemia among
electrical workers. However, confounding factors, such as possible
exposures to chemicals in the work environment, have not been
adequately taken into account in many of them. Assessment of ELF field
exposure has not correlated well with the cancer risk among exposed
subjects. Therefore, a cause-and-effect link between ELF field exposure
and cancer has not been confirmed.
The International Commission on Non-Ionizing Radiation Protection
(ICNIRP) has published guidelines on exposure limits for all EMF. The
guidelines provide adequate protection against known health effects and
those that can occur when touching charged objects in an external
electric field. Since current scientific information is only weakly
suggestive and does not establish that exposure to ELF fields at levels
normally encountered in our living environment might cause adverse
health effects, there is no need for any specific protective measures
for members of the general public. Where there are sources of high ELF
field exposure, access by the public will generally be restricted by
fences or barriers so that no additional protective measures will be
needed.
Protection from 50/60 Hz electric-field exposure can be relatively
easily achieved using shielding materials. This is only necessary for
workers in very high field areas. More commonly, where electric fields
are very large, access of personnel is restricted. There is no
practical, economical way to shield against ELF magnetic fields. Where
magnetic fields are very strong, the only practical protective method
available is to limit exposure of personnel.
Strong ELF fields cause electromagnetic interference (EMI) in
cardiac pacemakers or other implanted electromedical devices.
Individuals using these devices should contact their doctor to
determine their susceptibility to these effects. Office workers may see
image movement on the screen of their computer terminal. ELF magnetic
fields around the terminal greater than about 1 μT (10 mG) can cause
interference with the image on the screen. A simple solution to this
problem is to relocate the computer to another part of the room where
the magnetic fields are below 1 μT. These magnetic fields are found
near cables that provide electric power to office or apartment
buildings or around transformers associated with power supplies to
buildings. The fields from these sources are generally well below the
levels that cause any health concern.
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