DC electric fields having
potential gradients of less than 200 V/cm have not been found
to produce any biological effects. Indirect effects, however,
can occur. DC electric fields could cause acceleration of
charged airborne particles against the face of the VDT
operators. The particles could be responsible for the skin
rashes and eye problems reported by some VDT operators.
It is not yet possible to
determine whether pulsed EMFs emitted from VDTs represent a
health hazard. Most published standards apply from about
10-300 kHz to the MHz range and do not cover the ELF or VLF
bands of electromagnetic radiation. The recommendation of the
American Conference of Governmental Industrial Hygienists
should be followed and that all radiofrequency radiation
exposures be kept as low as reasonably possible.
Experiments conducted at the
Canadian Center for Occupational Health and Safety led to a
practical method for reducing pulsed electric field emissions
from VDTs. This consisted of installing an enclosure made of
plywood that was lined with copper foil around the sides and
back of the unit. The copper foil had a grounding wire
soldered to it which could be connected to a grounded metal
screw on the modem cable. This shield effectively reduced
emissions through the sides and top to zero. An alternative
method for reducing exposures to VDT emissions is to redesign
the workplace in such a way that no one can sit or stand close
to the side of a VDT or behind it. A working distance of 1-1.5
m was recommended. VDTs should not be located back to front
(as in a line) since individual operators could be exposed to
emissions from nearby terminals.
In one case study, where VDTs
were flickering from the fields produced by power cables in a
room below, it was found that 3-mm thick iron or 250-um thick
mu-metal would provide the required shielding. For any other
given set of circumstances, the thickness of the material
required would depend on the strength of the underlying
fields.
Fabrics with
electromagnetic shielding properties
There are many materials that
have been developed for shielding people from EMFs. One,
for example, that has been reasonably well- studied, and
actually used clinically, is Farabloc.
Farabloc, a fabric with
electromagnetic shielding properties, can reduce the symptoms,
signs, and muscular strength deficits secondary to
delayed-onset quadriceps muscle soreness caused by eccentric
exercise in humans. A randomized, single-blind,
placebo-controlled, crossover trial found that double layers
of Farabloc fabric wrapped around the thigh reduces pain and
strength loss and serum levels of malondialdehyde, creatine
phosphokinase, myoglobin, leukocytes, and neutrophils.
Farabloc shields high-frequency electromagnetic fields. This
material has also been found to reduce phantom pain.
This site has many fabrics for
shielding electric fields and one suggestion for magnetic
fields:
www.lessemf.com
Power line
AC field reduction strategies
Concern
about exposure to EMFs is primarily focused on power frequency
EMFs, which falls within the frequency range of 3 to 3,000 Hz,
designated the extremely low-frequency (ELF) band. In the ELF
range, wavelengths are extremely long, from 100 to 100,000 km,
which means that for practical purposes, one is always in the
near field. The electric and magnetic fields from power
frequency sources are considered independent of one other. For
a given system or source, electric fields are determined by
voltages and the magnetic fields are determined by currents.
Electric fields are produced
whenever a potential or voltage exists between 2 objects.
Magnetic fields are produced by moving electric charges, which
generally implies an electric current. Any wire that carries
an electric current, therefore, is a source of magnetic
fields. Ground currents can be important sources of
residential magnetic fields.
The physical layout of electric
wires is critical for reducing both magnetic and electric
fields. The most basic reduction approach is to locate sources
at a distance from a critical region, such as an office or
sensitive piece of electronic equipment. Minimum spacing of
"hot" and neutral wires will result in lower
magnetic field exposures. Conducting objects are effective in
shielding electric fields. For example, placing a grounded
conductive enclosure around a space will eliminate the
electric field in it. For ELF electric fields, the conductive
enclosure can be as simple and inexpensive as a wire mesh
screen.
Magnetic fields, especially ELF
magnetic fields, are much harder to shield than electric
fields because they readily penetrate most materials. Magnetic
field shielding can be active or passive. Passive shielding is
done by changing the magnetic field in a region of space.
Active shielding is done by creating additional sources to
produce an opposing canceling field, and so altering the
intensity from the source.
Passive shielding is
accomplished by placing a material shield between the magnetic
field source and the region to be shielded. The shields can be
made of ferromagnetic materials that alter the structure of
the magnetic field by providing a preferred path for the
magnetic flux lines, a process known as flux shunting. Shields
can also be made of conductive materials in which the magnetic
field source induces electric currents that tend to oppose or
cancel the original magnetic field, a process known as induced
current shielding. Shielding in which induced currents flow in
conductive loops of wire is being investigated as a means of
reducing magnetic fields near power lines. Only a small number
of materials, which can be expensive, are suitable for
reducing the magnetic fields. Design and cost considerations
must, therefore, be part of the shielding process.
Active shielding involves the
use of coils that carry current oriented in such a way that it
will reduce or cancel the magnetic field to be avoided.
Controlling alternate current pathways that produce large
current loops is also an important approach.
These are examples of sites
that help with shielding in industrial settings:
www.emfshielding.com
www.magnetic-shield.com
These sites have a useful
glossary of magnetic and shielding terms:
www.tech-etch.com
www.magnetic-shield.com
There
are commercially available glare and antistatic fields which
will effectively eliminate frontal fields from computer
monitors. However, they do not to shield from the magnetic
fields produced by these monitors. Magnetic fields are much
more difficult to shield against. Both static and alternating
magnetic fields easily penetrate most common materials,
including steel, concrete, and lead. Magnetic fields can be
altered (and reduced) by high-permeability metal alloys, which
are typically very expensive. Some so called low-radiation
monitors use such alloys to shield the deflection coil which
helps reduce magnetic field emissions and use other types of
shielding to reduce electric field emissions.
By
conducting a survey around the home or workplace,
high-exposure points can be identified. "Avoidance"
action can be as simple as moving furniture or the EMF source,
or both. It is known that the backs of monitors and microwave
ovens can produce especially strong EMFs. There are many
reports of individuals being sleepy or tired when sitting
behind one of these sources and other symptoms dramatically
improve when their positions are changed away from the source.
