Protection
Humans and animals exposed to AC irradiation (such as that from electrical appliances) for long periods of time, and especially to strong fields, can sometimes develop unexpected reactions. The issue of cancer being produced by these kinds of field exposures remains unsettled. With the exception of therapeutic applications, most of these field exposures should be limited as much as possible. Therapeutic fields are typically quite different from the normal environmental fields we are exposed to regularly. Besides hair dryers and electric blankets, some of the strongest and most prolonged exposures we experience include riding on electric trains, using video monitors, and talking on non-hands-free cell phones. Additionally, all the risk factors for the combination of environmental fields and other potential toxic exposures, such as radon or smoking, are unknown. Since these exposures are not always entirely avoidable, are there ways we can reduce our risks?
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Typical emissions
Electric fields in V/m range from:
< 1 at 30 cm from a hair dryer,
0-10 in a typical house, and
0-50 in an urban environment,
to 30 at a distance of 30 cm from a television screen,
6 at 30 cm from a refrigerator, and
5,000 under a 380 kV power line.
Magnetic fields in uT (20 uT=0.2 gauss) are in the range of:
0-0.1 in an urban environment,
0-1 in a typical house,
0.1-1 at 30 cm from a refrigerator,
20 under a 380 kV power line,
100-500 at a distance of 30 cm from a television, and
1,000-2,500 at 30 cm from a hair dryer.
You can actually see how much EMF a hairdryer and a TV produce!
More data is available at these sites: www.wave-guide.org
Effective technologies exist for shielding people from electric fields
Commercially available anti-glare and anti-static filters can effectively eliminate the harmful frontal fields computer monitors emit. However, they do not shield from the magnetic fields produced by these monitors; magnetic fields are notably more difficult to shield against. Both static and alternating magnetic fields can easily penetrate 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 can help reduce magnetic field emissions.
Detection and avoidance
One of the most effective methods of reducing your negative EMF exposure is through detection and avoidance. By knowing the position and strength of fields emitted from a monitor, for example, you can establish an imaginary circle of safety around it. Appropriate measuring instruments can be used to determine this circle of safety. You may take a similar approach around the home or place of work, where long-term exposure is possible from other, sometimes unexpected sources of EMFs. By conducting a survey around the home or workplace, high-exposure points can be identified. Avoidance action can be as simple as repositioning furniture, moving the EMF source, or both. It is known that the backs of monitors and microwave ovens can produce especially strong EMFs. There are numerous reports of individuals being sleepy or tired while sitting behind one of these sources, and other symptoms dramatically improve when their positions are relocated away from the source.
Emissions from video display terminals or VDTs
Four types of fields are emitted from monitors (video display terminals or VDTs) at levels significantly above background EMF:
· DC electric fields which result from electrostatic charging of the screen,
· ELF magnetic fields, and
· VLF magnetic and
· VLF electric fields.
- VLF pulsed electric fields are emitted from the flyback transformer and ELF and VLF magnetic fields are produced by the horizontal and vertical deflection coils.
Pulsed electric fields in the vicinity of VDTs occur as follows fro surveyed machines: - The strongest, 300+ V/m, can be measured 20 cm from the right side of the unit where the flyback transformer is located. These decrease to about 50 V/m at a distance of 40 cm.
- At the front they average around 50 V/m, but decrease rapidly to negligible values at 25 to 30 cm.
- Through the top of the unit they average 280 V/m measured 10 cm from the top and decrease to about 30 V/m at a distance of 30 cm.
- With a printer located on top of the VDT, emissions through the front and top are significantly decreased – since the printer acts as an effective shield.
DC electric fields having potential gradients of less than 200 V/cm have not been found to produce any biological effects, though indirect effects can occur. DC electric fields may cause an acceleration of charged airborne particles against the face of the VDT operator. The particles may 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 between 10 and 300 kHz to the MHz range, and do not cover the ELF or VLF bands of electromagnetic radiation. The American Conference of Governmental Industrial Hygienists recommendations should be taken seriously, and all radiofrequency radiation exposures should be kept as low as reasonably possible.
Experiments conducted at the Canadian Center for Occupational Health and Safety have led to a practical method for reducing pulsed electric field emissions from VDTs. The method calls for the installation of a plywood enclosure lined with copper foil. A grounding wire soldered to the copper foil connects to a grounded metal screw on the modem cable. This shield effectively removed emissions through the sides and top of the VDT. Another way to reduce exposures to VDT emissions is to reposition the workplace so that no one may sit or stand near the side or rear of a VDT. A working distance of 1-1.5m is recommended. Keep in mind that VDTs should not be positioned in a line (back to front), as individual operators may be exposed to emissions from nearby terminals.
One case study, which examined VDTs that were flickering from the fields produced by power cables in a room below, found that 3mm-thick iron or 250um-thick metal could provide the required shielding. For any other given set of circumstances, the required thickness of the material would depend on the strength of the underlying fields.
Fabrics with electromagnetic shielding properties
Many materials have been developed for shielding people from negative EMFs. One reasonably well-studied example which has been used clinically is Farabloc, a fabric with electromagnetic shielding properties.
Farabloc can reduce a person’s signs, symptoms, and muscular strength deficits resulting from delayed-onset quadriceps soreness caused by eccentric exercise. A randomized, single-blind, placebo-controlled crossover trial found that double layers of Farabloc fabric wrapped around the thigh reduces pain, reverses strength loss, and lowers malondialdehyde, creatine phosphokinase, myoglobin, leukocyte, and meutrophils serum levels. Farabloc shields high-frequency EMFs. 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 focused primarily on power frequency EMFs, which fall within a frequency range of between 3 and 3,000Hz. This range is designated as an extremely low frequency (ELF) band. In the ELF range, wavelengths are extremely long, between 100 and 100,000km, which for practical purposes means that we are almost always in the field. The electric and magnetic fields from power frequency sources are considered independent of one another. 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 of voltage exists between two objects. Magnetic fields are produced by moving electric charges, which generally implies an electric current. Therefore, any wire that carries an electric current is a source of magnetic fields. Ground currents can be important sources of residential magnetic fields.
The physical layout of electric wires is critical in reducing both magnetic and electric fields. The most basic reduction approach calls for relocating sources at a distance from a critical region, such as an office or piece of sensitive 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 that space. 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. Shielding can be active or passive. Active shielding is done by creating additional sources to produce an opposing canceling field, thereby altering the intensity from the source. Active shielding involves the use of coils that carry a 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 a valuable approach.
Passive shielding is done by changing the magnetic field in a region. It is accomplished by placing a material shield between the magnetic field source and the region to be shielded. Shields can be made of ferromagnetic materials that alter the structure of the magnetic field by providing a preferred path the for the magnetic flux lines. This process is known as flux shunting. Shields may also be made conductive materials in which the magnetic field source induces electric currents that tend to oppose or cancel the original magnetic field. This process is 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 are suitable for reducing magnetic fields, and these materials can be expensive. Design and cost considerations must therefore be part of the shielding process.
These are examples of sites that help with shielding in industrial settings:
These sites have a useful glossary of magnetic and shielding terms:
www.tech-etch.com
www.magnetic-shield.com
