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What are
Electromagnetic
fields
(EMF) or E-Smog?
As
seen lately on Carte Blanche and other major
TV
channels.
The
unseen, invisible, cannot touch, feel, smell, sense enemy to the human being.
Electromagnetic
radiation or E-Smog has been around since the birth of the universe; light is its most
familiar form. Electric and magnetic fields are part of the spectrum of
electromagnetic radiation which extends from static electric and magnetic
fields, through radiofrequency and infrared radiation, to X-rays.
Comprehensive information is provided hereafter
on what electromagnetic fields are, their impact on health, as well as the
current exposure standards and recommended precautions
Damaging effect of a cell phone call. Typical effect of
EMF.
Definitions and sources
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Table
of contents
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Electric fields are created by differences in
voltage: the higher the voltage, the stronger will be the resultant field. Magnetic
fields are created when electric current flows: the greater the current, the
stronger the magnetic field. An electric field will exist even when there is no
current flowing. If current does flow, the strength of the magnetic field will
vary with power consumption but the electric field strength will be constant.
(Extract from Electromagnetic fields
published by the WHO Regional Office for Europe in 1999 (Local authorities,
health and environment briefing pamphlet series; 32).
Natural
sources of electromagnetic fields
Electromagnetic
fields are present everywhere in our environment but are invisible to the human
eye. Electric fields are produced by the local build-up of electric charges in
the atmosphere associated with thunderstorms. The earth's magnetic field causes
a compass needle to orient in a North-South direction and is used by birds and
fish for navigation.
Human-made
sources of electromagnetic fields
Besides
natural sources the electromagnetic spectrum also includes fields generated by
human-made sources: X-rays are employed to diagnose a broken limb after a sport
accident. The electricity that comes out of every power socket has associated
low frequency electromagnetic fields. And various kinds of higher frequency radio waves
are used to transmit information – whether via TV antennas, radio
stations or mobile phone base stations.
The
basics of wavelength and frequency
What makes the various forms of electromagnetic
fields so different?
One of the main characteristics which defines an electromagnetic field (EMF) is
its frequency or its corresponding wavelength. Fields of different frequencies
interact with the body in different ways. One can imagine electromagnetic waves
as series of very regular waves that travel at an enormous speed, the speed of
light. The frequency simply describes the number of oscillations or cycles per
second, while the term wavelength describes the distance between one wave and
the next. Hence wavelength and frequency are inseparably intertwined: the higher
the frequency the shorter the wavelength.
A
simple analogy should help to illustrate the concept: Tie a long rope to a door
handle and keep hold of the free end. Moving it up and then down slowly will
generate a single big wave; more rapid motion will generate a whole series of
small waves. The length of the rope remains constant, therefore, the more waves
you generate (higher frequency) the smaller will be the distance between them
(shorter wavelength).
What is the difference between non-ionizing
electromagnetic fields and ionizing
radiation?
Wavelength and frequency determine another important characteristic of
electromagnetic fields: Electromagnetic waves are carried by particles called
quanta. Quanta of higher frequency (shorter wavelength) waves carry more energy
than lower frequency (longer wavelength) fields. Some electromagnetic waves
carry so much energy per quantum that they have the ability to break bonds
between molecules. In the electromagnetic spectrum, gamma rays given off by
radioactive materials, cosmic rays and X-rays carry this property and are called
'ionizing radiation'. Fields whose quanta are insufficient to break molecular
bonds are called 'non-ionizing radiation'. Man-made sources of electromagnetic
fields that form a major part of industrialized life - electricity, microwaves
and radiofrequency fields – are found at the relatively long wavelength and
low frequency end of the electromagnetic spectrum and their quanta are unable to
break chemical bonds.
Electromagnetic
fields at low frequencies
Electric
fields exist whenever a positive or negative electrical charge is present. They
exert forces on other charges within the field. The strength of the electric
field is measured in volts per meter (V/m). Any electrical wire that is charged
will produce an associated electric field. This field exists even when there is
no current flowing. The higher the voltage, the stronger the electric field at a
given distance from the wire.
Electric
fields are strongest close to a charge or charged conductor, and their strength
rapidly diminishes with distance from it. Conductors such as metal shield them
very effectively. Other materials, such as building materials and trees, provide
some shielding capability. Therefore, the electric fields from power lines
outside the house are reduced by walls, buildings, and trees. When power lines
are buried in the ground, the electric fields at the surface are hardly
detectable.
Magnetic
fields arise from the motion of electric charges. The strength of the magnetic
field is measured in amperes per meter (A/m); more commonly in electromagnetic
field research, scientists specify a related quantity, the flux density (in
microtesla, µT) instead. In contrast to electric fields, a magnetic field is
only produced once a device is switched on and current flows. The higher the
current, the greater the strength of the magnetic field.
Like
electric fields, magnetic fields are strongest close to their origin and rapidly
decrease at greater distances from the source. Magnetic fields are not blocked
by common materials such as the walls of buildings.
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Electric fields
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Magnetic fields
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- Electric
fields arise from voltage.
- Their
strength is measured in Volts per meter (V/m)
- An
electric field can be present even when a device is switched off.
- Field
strength decreases with distance from the source.
- Most
building materials shield electric fields to some extent.
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- Magnetic
fields arise from current flows.
- Their
strength is measured in amperes per meter (A/m). Commonly, EMF
investigators use a related measure, flux density (in microtesla (µT)
or millitesla (mT) instead.
- Magnetic
fields exist as soon as a device is switched on and current flows.
- Field
strength decreases with distance from the source.
- Magnetic
fields are not attenuated by most materials.
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With kind permission from the National
Radiological Protection Board, UK
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Electric fields
Plugging a wire into an outlet creates electric fields in the air surrounding
the appliance. The higher the voltage the stronger the field produced. Since the
voltage can exist even when no current is flowing, the appliance does not have
to be turned on for an electric field to exist in the room surrounding it.
Magnetic fields
Magnetic fields are created only when the electric current flows. Magnetic
fields and electric fields then exist together in the room environment. The
greater the current the stronger the magnetic field. High voltages are used for
the transmission and distribution of electricity whereas relatively low voltages
are used in the home. The voltages used by power transmission equipment vary
little from day to day, currents through a transmission line vary with power
consumption.
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With kind permission from the National
Radiological Protection Board, UK
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Electric
fields around the wire to an appliance only cease to exist when the appliance is
unplugged or switched off at the wall. They will still exist around the cable
behind the wall.
How do static fields differ from time-varying
fields?
A static field does not vary over time. A direct current (DC) is an electric
current flowing in one direction only. In any battery-powered appliance the
current flows from the battery to the appliance and then back to the battery. It
will create a static magnetic field. The earth's magnetic field is also a static
field. So is the magnetic field around a bar magnet which can be visualized by
observing the pattern that is formed when iron filings are sprinkled around it.
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With kind permission from the National
Radiological Protection Board, UK
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In
contrast, time-varying electromagnetic fields are produced by alternating
currents (AC). Alternating currents reverse their direction at regular
intervals. In most European countries electricity changes direction with a
frequency of 50 cycles per second or 50 Hertz. Equally, the associated
electromagnetic field changes its orientation 50 times every second. North
American electricity has a frequency of 60 Hertz.
What are the main sources of low, intermediate and
high frequency fields?
The time-varying electromagnetic fields produced by electrical appliances are an
example of extremely low frequency (ELF)
fields. ELF fields generally have frequencies up to 300 Hz. Other
technologies produce intermediate
frequency (IF) fields with frequencies from 300 Hz to 10 MHz and radiofrequency
(RF) fields with frequencies of 10 MHz to 300 GHz. The effects of
electromagnetic fields on the human body depend not only on their field level
but on their frequency and energy. Our electricity power supply and all
appliances using electricity are the main sources of ELF fields; computer
screens, anti-theft devices and security systems are the main sources of IF
fields; and radio, television, radar and cellular telephone antennas, and
microwave ovens are the main sources of RF fields. These fields induce currents
within the human body, which if sufficient can produce a range of effects such
as heating and electrical shock, depending on their amplitude and frequency
range. (However, to produce such effects, the fields outside the body would have
to be very strong, far stronger than present in normal environments.)
Electromagnetic
fields at high frequencies
Mobile
telephones, television and radio transmitters and radar produce RF fields. These
fields are used to transmit information over long distances and form the basis
of telecommunications as well as radio and television broadcasting all over the
world. Microwaves are RF fields at high frequencies in the GHz range. In
microwaves ovens, we use them to quickly heat food.
At
radio frequencies, electric and magnetic fields are closely interrelated and we
typically measure their levels as power densities in watts per square meter (W/m2).
Key
points:
 | The electromagnetic
spectrum encompasses both natural and human-made sources of electromagnetic
fields.
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| Frequency and
wavelength characterize an electromagnetic field. In an electromagnetic
wave, these two characteristics are directly related to each other: the
higher the frequency the shorter the wavelength.
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| Ionizing radiation
such as X-ray and gamma-rays consists of photons which carry sufficient
energy to break molecular bonds. Photons of electromagnetic waves at power
and radio frequencies have much lower energy that do not have this ability.
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| Electric fields
exist whenever charge is present and are measured in volts per meter (V/m).
Magnetic fields arise from current flow. Their flux densities are measured
in microtesla (µT) or millitesla (mT).
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| At radio and
microwave frequencies, electric and magnetic fields are considered together
as the two components of an electromagnetic wave. Power density, measured in
watts per square meter (W/m2), describes the intensity of these
fields.
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| Low frequency and
high frequency electromagnetic waves affect the human body in different
ways.
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| Electrical power
supplies and appliances are the most common sources of low frequency
electric and magnetic fields in our living environment. Everyday sources of
radiofrequency electromagnetic fields are telecommunications, broadcasting
antennas and microwave ovens.
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