United States electric power system operates at 60 cycles
a second or hertz (Hz). This means that the electric charges
(current) flowing in the system change direction 60 times
a second. This changing of direction of current is called
alternating current or AC. AC fields induce weak electrical
currents in conducting objects, including humans, and have
been the focus for research on how electric and magnetic fields
could affect human health.
and magnetic fields have different properties. Electric fields
are easily shielded or weakened by conducting objects (trees,
buildings, etc.) but magnetic fields are not. Both electric
and magnetic fields diminish with increasing distance from
the source. Magnetic fields are typically measured in gauss,
a unit of magnetic field strength or magnetic flux density.
1,000 milligauss (mG) = 1 gauss. Recent interest and research
have focused on the potential health effects of magnetic fields.
Some epidemiological studies have suggested that a link may
exist between exposure to these fields and certain types of
cancer, primarily leukemia and brain cancer. Other studies
have found no such link. Laboratory researchers are studying
how such an association is biologically possible. At this
point, there is no scientific consensus about this issue,
except a general agreement that there is a cause for concern
and that more information is needed. A national research effort
is under way, and major study results are expected in the
next few years.
is not know at this point whether exposure to magnetic fields
from power frequency sources constitutes a health hazard.
Therefore, it can not be determined what levels of exposure
are "safe" or "unsafe". Some studies have
shown that exposure to higher levels of this radiation is
not necessarily worse than exposure to lower levels. More
research is required to identify dose-response relationships.
There is some evidence from laboratory studies to suggest
that there may be "windows" for effects. This means
that biological effects are observed at some frequencies and
intensities but not at others. Also, it is not known if continuous
exposure to a given field intensity causes a biological effect,
or if repeatedly entering and exiting of the field causes
effects. In light of all this uncertainty, it is impossible
to say what is a "safe" distance from any magnetic
field source or what is a "safe" exposure. The only
thing possible at this point in time is to make comparisons.
For instance, the typical home has a background magnetic field
level (away from appliances) that ranges from 0.1 to 4.0 milligauss.
Although some experiments with cells have reported effects
at field levels as low as 2 milligauss, there is no laboratory
evidence for adverse human health effects at this level.
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and distribution lines can be collectively referred
to as power lines. Magnetic field levels from power
lines will be determined by the amount of current flowing
through the line, the arrangement and proximity of the
lines themselves with respect to each other, the height
of the line above the ground, and the proximity of the
lines to other power lines.
lines carry electricity over long distances and usually
operate at voltages of 100 kilovolts and above. For
any transmission line in New Jersey, at a perpendicular
distance of 400 feet from the center of the line configuration,
the magnetic field level on the ground from the line
will be approximately 1 milligauss or less. At distances
closer than 400 feet, it is difficult to predict what
the magnetic field level will be as each situation becomes
unique to that particular line. Some transmission lines
carry very little current and expose people to lower
magnetic field levels than what they would encounter
from a distribution line. Measurements made by Department
staff under transmission lines in New Jersey have ranged
between 8 - 130 milligauss. In general, fields from
both transmission lines and distribution lines will
vary, depending on the time of day, the day of the week,
the time of year and the ambient temperature. However,
for transmission lines, magnetic fields will rarely
vary by more than a factor of two.
lines operate at lower voltages and bring power from
substations to businesses and homes. Distribution lines
may expose people to magnetic field levels as high or
higher than transmission lines. This is because they
are physically closer to the ground than transmission
lines. For this same reason, distribution lines that
are buried underground can sometimes expose one to a
higher magnetic field if one is standing directly over
top of them than what one would receive from the same
line mounted overhead on a pole. The Department has
received information from electrical utilities in New
Jersey that some underground distribution lines operating
at a voltage of 69 kilovolts may produce magnetic field
levels as high as 55 milligauss directly above the line.
50 feet from the center of the line, this level drops
to 1 milligauss. In general, most magnetic fields from
distribution lines will be a lot lower and may even
be as low as 1 milligauss, directly above or below the
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substations serve many functions in controlling and
transferring power on an electrical system. Substations
may utilize transmission lines, distribution lines or
a combination of both. In general, the strongest magnetic
fields around the outside of the substation comes from
the power lines entering and leaving the station. While
transformers inside the substation can produce high
magnetic fields, the fields remain localized around
the transformers. Beyond the substation fence, the magnetic
fields produced by the equipment within the station
are typically indistinguishable from background levels.
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are electrical devices used to adjust the voltage-current
relationship of an electrical power circuit for best
efficiency during transmission and distribution use.
There are electric and magnetic fields near a transformer
and around the lines that connect to them. But the fields
tend to drop off rapidly as one moves away from the
transformer. Utilities use a variety of transformers
throughout their systems. Step-up transformers are used
at the power generating station to raise the voltage
so the power can be economically delivered over transmission
lines. The magnetic fields from these types of transformers
are high but localized and to do not travel beyond the
bounds of the substation. Step-down transformers are
used to reduce line voltages.
(pole-mounted) transformers are used where distribution
lines are overhead and surface (pad-mounted) transformers
are used where distribution lines are underground. Frequently
in urban situations, transformers can be located within
buildings. If the transformer is what is referred to
as a network transformer, which can supply power to
an entire block, magnetic fields on the floor directly
above the transformer can be as high as 700 milligauss.
Since magnetic fields remain localized around the transformer
itself, a pole mounted transformer will have very little
impact on ground level magnetic fields, which will be
dominated by the overhead distribution lines coming
in and going out of the transformer.
mounted transformers have magnetic fields similar in
intensity to kitchen appliances. The magnetic fields
near this type of transformer are elevated close to
the surface of the transformer. A few feet away, the
levels drop off to background.
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that operate either on batteries or by plugging into
the household wiring usually come equipped with an AC/DC
switch. If DC is chosen, current flows one way from
the batteries to the appliance. DC fields, unlike AC
fields, do not induce electrical currents in humans
unless the DC field changes in space or time relative
to the person in the field. In most situations, a battery
operated appliance is unlikely to induce electrical
current in the person using the appliance.
general, appliances using AC have potentially high,
localized magnetic fields that decrease rapidly with
distance. Magnetic fields from appliances are often
stronger than the fields directly beneath power lines.
The intensity of the magnetic field from an appliance
appears to be related to product function and design.
Here are some examples of average magnetic field levels
6 inches away from certain appliances:
dryer - 300 milligauss
shaver - 100 milligauss
- 70 milligauss
opener - 600 milligauss
maker - 7 milligauss
oven - 200 milligauss
TV (1 foot away) - 7 milligauss
example of how rapidly magnetic fields from appliances
drop over distance:
6 inches - 600 milligauss
1 foot - 150 milligauss
2 feet - 20 milligauss
3 feet - 2 milligauss
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trains run on AC while others use DC. Some trains that
use AC operate at 25 or 16.75 hertz. Very little is
known about the biological effects from 25 or 16.75
hertz AC or DC. Areas of strong AC magnetic fields have
been measured close to the floor on some DC trains.
Magnetic fields measured in trains powered by 60 hertz
AC have been reported to be as high as 500 milligauss
in the passenger areas at seat height. Department staff
have not made any measurements on train lines.
MAGNETIC AND ELECTRIC FIELDS (To
measurement of electric and magnetic fields from nonionizing
radiation sources is a complex task. The Department
does not have a certification program for testing or
measurement firms and therefore, cannot endorse any
such companies. The best way to obtain accurate readings
of 60 hertz electric and magnetic fields is to
contact the owner (electric utility) of the power lines
in question and request that measurements be made. Please
be aware that the utility supplying power to the house
may not be the same utility that owns the high voltage
transmission lines running by the property of interest.
Utility personnel have been trained in this area of
expertise and will probably provide the most accurate
readings. They are usually reluctant to interpret any
readings although they may try to put them into perspective.
Anyone having any questions regarding measurements should
contact the Department. As a quick rule of thumb, typical
magnetic field levels found in homes range from 0.1
- 4.0 milligauss. Any readings above that are not necessarily
hazardous, but higher. It is not necessary to obtain
readings if a power line is more than 400 feet away
from the home or area of interest.
more information on this topic, please visit the following
Internet areas: Please
read this Disclaimer prior to
connecting to these websites.
Lines and Cancer: FAQ's from John E. Moulder, PH.D. of
the Medical College of Wisconsin
OET Bulletin No. 56 (Q&A about Biological
Effects and Potential Hazards of Radiofrequency Radiation)
to Nonionizing Radiaiton Sections Main Page