Electromagnetic Fields in a Transformer
8.05.5 Undisturbed Electric Field Strength
by High-voltage Overhead Lines (50 Hz) in Vertical Profile
8.05.6 Influence of Trees
on the Spread of Electric Fields
beneath Overhead Lines (50 Hz)
Electromagnetic Field Strength as a Function of
Distance from Several EMF Sources[3]
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Definition for: electromagnetic field
A
field of force, produced by electric charges and currents,
which has both an electric and a magnetic component and
contains electromagnetic energy. The properties of
electromagnetic fields were outlined by Scottish physicist
James Clerk Maxwell in 1865.
Are Electromagnetic
Fields Hazardous To Your Health?
EMF fields are known to interact with tissues by inducing
electric fields and currents in them. This is the only
established mechanism of action of these fields. However,
the electric currents induced by EMF fields commonly found
in our environment are normally much lower than the
strongest electric currents naturally occurring in the body
such as those that control the beating of the heart.
Many studies
show that sources that emit electromagnetic fields, give off
radiation at low frequencies well below the safe level.
There have been no instances in which people have been
directly affected by cell phones, alarm clocks, pagers, etc,
with the exception of tanning beds that emit ultraviolet
radiation. People have very little risk (if any at all) of
developing cancer from these sources.
How are humans exposed?
Pinpointing a single source of exposure is
difficult because humans may be exposed to a multitude of
possible sources. Electromagnetic fields are found naturally
in the environment, such as in lightning and in the Earth's
magnetic field, which causes a compass needle to point
north. The natural electromagnetic fields in the human body
allow messages to flow through the nervous system.
Electric and magnetic fields are produced by
distribution or power lines, the electrical wires inside the
walls of homes, and a wide assortment of electrical
appliances. While electric fields do not pass through
building materials, magnetic field do. Magnetic fields can
also be found near buried electric lines.
Electromagnetic fields are commonly found around:
- high voltage power lines
- neighborhood transmission lines
- grounding systems that protect
residents from lightning
- grounding systems that protect
residents from electric shock that can result from
faulty appliances
- the operation of common electrical
appliances, including microwave ovens
- ovens, electric ranges, aquariums,
table fans, electric space heaters
- computer monitors, electric
clocks, clock radios, heated waterbeds
- electric blankets, hair dryers,
cellular phones
Electromagnetic fields are divided into several categories:
- appliances and power
lines--extremely low frequencies (ELF)
- AM radio transmission--high and
low frequencies (HF and LF)
- TVs and video display
terminals--very low frequencies (VLF)
- TV and FM radio
transmissions--very high frequencies (VHF)
- microwaves--super high frequencies
(SHF)
Recommended safety levels range from 0.5 mG to 2.5 mG as the
maximum exposure -
with 1.0 mG as a preferred standard. Adverse biological
effects have been found at 2.5 mG.
How much are we
exposed?
The amount of exposure is difficult to
determine. Exposure to forces produced by external sources
is generally constant, but the structure of a building
determines how much of the final field strength can be found
inside the building. EMFs produced inside a home can widely
vary depending upon the wiring system and appliance usage.
These variables cloud efforts to determine the relative
importance of local power lines as opposed to the use of
appliances or to internal house wiring.
What are the effects of
exposure?
Most of the research--especially research
focusing on the relationship between exposure to EMFs and
cancer--has focused on public health issues. In most cases,
the findings are difficult to trust because of the
difficulty in controlling the amount of exposure and other
variables, including socioeconomic status, nutrition,
lifestyles, genetics, occupation, and exposure to other
cancer-causing factors. Many study designs lack validity,
which causes further doubt about their value.
Given these limitations and shortcomings, the
research nevertheless does suggest the need for further
investigation of the association between EMFs and some
cancers. These include childhood brain cancer and female
breast cancer. Occupational concerns related to EMFs include
adult brain cancer, adult leukemia, and male breast cancer.
A review of occupational studies examined the
occurrence of malignant melanoma, colon cancer, lung cancer,
bladder cancer, and non-Hodgkin's lymphoma and found
questionable evidence linking those cancers with EMFs or
with working in an "electrical" job.
A number of studies examined the relationship
between parental "electrical" occupation and childhood
central nervous system cancers and found little to suggest a
link. The evidence for adult central nervous system cancers,
male breast cancer, and leukemia is also very weak. Many
respected scientists critique the studies as being flawed.
In spite of limited evidence concerning the
link between EMFs and cancer, a weak connection between the
two appears to exist. Further, valid studies are needed to
determine the specific nature of this link.
Are there any
measurable negative effects?
Currently, the only known measurable negative
effects of EMFs are related to quality-of-life issues. Local
power lines, poles, substations, and high voltage
transmission lines are aesthetically displeasing; a possible
reduction in property values and a deterioration of
community may occur as families choose to relocate to avoid
EMFs. Those who live in areas selected for new EMF
facilities may be resentful and resistant to them being
built..
In woodlands and farmlands, several rare but
easily explained occurrences result from electromagnetic
fields. Damaged conifer needles growing close to power lines
serve much the same function as a lightning rod in
discharging electricity build-up in the air; power lines
sometimes create stray voltage effects that cause constant
low-level electric shock. Decreased milk production in dairy
cows and other minor problems can be avoided by properly
grounding equipment and buildings.
The placement of distribution towers that
produce EMFs into recreational areas and natural areas are
said to have a negative effect, although no quantifiable
data are available and there appears to be no noticeable
decrease in the use of those areas.
Generally, scientists on both sides say they
are dealing at most with rare diseases and an increased risk
that is almost infinitesimal, especially compared with
life's other everyday risks.
What can someone do?
In spite of the absence of valid evidence
based on solid scientific research, the effects of EMFs on
the health of individuals and environments may still be of
concern to some people. Until research data suggests a need
for more extreme action, those who want to do something may
wish to consider the following suggestions:
- Stay away from the EMF sources.
The strength of a magnetic field drops quickly just a
few feet from the source. Do not sleep or sit for long
periods of time near electrical devices, especially
those with motors.
- Stay a minimum of 18 inches away
from a video display terminal (VDT). Turn it off when
not in use. Don't sit close to the back or sides of the
monitor even when it is in another room or behind a
partition.
- Where feasible, turn off or unplug
waterbed heaters and electric blankets before going to
bed. Unplugging and turning these appliances off removes
the electric field.
- Stay several feet away from all
sides of a television including TVs located in another
room against a wall.
- Don't place beds or cribs against
wall with major appliances nearby or on the other side
of the wall.
- Move electric clocks, clock
radios, and telephone answering machines at least 4 feet
away from the head of the bed.
- Fluorescent bulbs generate
stronger fields than incandescent lamps. Estimate 1 foot
for incandescent bulbs and 3 feet for fluorescent lamps.
- Avoid using electric blankets and
hair blow dryers.
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