Field sources in existing structures or when remodeling are often less controllable than with new construction. Simply following conventional no and low cost measures will preclude the majority of high EMF conditions, but some internal wiring errors producing fields may go undetected. Conducting a gaussmeter survey might detect these fields, but this would depend on the skill of the practitioner. There is presently no licensing or certification process for those who conduct EMF gaussmeter surveys. In addition, some of those who conduct such surveys offer to perform remedial work and so have a potential conflict of interest in the outcome of the surveys. These factors should be considered before embarking on an EMF survey. Nonetheless, the following is a summary of the EMF survey process.
The EMF survey should follow a written protocol agreed upon by both the surveyor and the school district. The measurement of magnetic fields is done with a gaussmeter, which records field levels in milligauss. Three-axis and single-axis meters are useful for different tasks. For the initial survey a three-axis meter is fast and easy to use. 9 A gaussmeter survey can describe the range of magnetic field levels in a school. It can help locate internal and external sources of these fields. With regard to internal sources, the gaussmeter can be used at electric panels to determine which circuits are involved and pinpoint deviations from the NEC. A survey can also be used after changes have been made in order to verify field reductions. The surveyor should specify the equipment being used, how it is calibrated, and its purpose.
A single-axis gaussmeter is useful for finding the source of a magnetic field. Some gaussmeters can correctly measure the 18O Hertz (cycle) field as well as the 60 Hz field. This may be important since this component may represent as much as one-third of the magnetic field from computers and fluorescent lights. Readings are typically taken at three feet above the floor, but they should be taken wherever there is concern about field levels. Readings should also be taken adjacent to building elements which are potential magnetic field sources such as secondary electric transmission lines, transformers, electric panels, heaters, air conditioning equipment, and elevator motors, as well as at equipment such as copiers and computer monitors.
Magnetic field readings of .01 - 1 mG are well within the range of commonly seen levels.11 Fields in the range of 1-10 mG are the subject of much medical controversy. If EMF is to be minimized, these levels indicate the presence of field sources which should be identified. Exposures of 10-100 mG are uncommon, and readings of more than 100 mG are rare. In order to verify these readings in occupied spaces the measurements should be repeated at different times of the day and week. Since EMF is directly proportionate to current flow the measured magnetic field levels originating from power lines will be markedly different for different seasons and times of the day. Because of the operating air conditioners a hot summer afternoon will usually have higher field levels than an autumn morning. A thorough field survey will reveal if these levels are present throughout the building or campus or if they are localized in specific areas.
Consider EMF shielding as a last resort. Two strategies can be considered: shield the occupied space or shield the EMF source. Shielding the occupied space may be the appropriate solution when the fields from such sources as power drops, electrical panels, and busbars are very large. This may be especially true in high-density areas and buildings where it is not possible to separate the internal field source and occupied space. A different situation is posed when the source is much smaller and within the occupied space itself, such as computer monitors in a classroom. In these cases a shielding device can be introduced around the source. It is important to understand that "radiation" shields which attach to the front of a monitor and provide relief from glare are incapable of any beneficial shielding of a magnetic field. These devices can only shield the electric field portion of EMF. At this time there are no practical (much less low cost) shielding techniques for adjacent power lines.
In certain cases where locations of EMF sources in buildings are fixed and nearby facilities are negatively impacted by the resulting field, EMF shielding may be a desirable option. This is especially relevant with additions or changes to existing facilities. Although the cost is usually significant, recent advances in EMF shielding technology have placed this option where it may still be less costly than other options such as relocation or abandonment of the adjacent spaces. When fields result from sources outside of the building such as power lines and engulf the entire structure, shielding will not be a viable field-reduction strategy.
Types of EMF Shielding
There are currently two general types of EMF shielding which, when introduced between the source and the area of concern, can reduce field levels. "Passive" shielding utilizes ferromagnetic and/or conductive materials. "Active" shielding uses an externally energized conductor which is placed in such a way as to generate magnetic fields which oppose or cancel those from the original source. Although technically feasible, active shielding is extremely limited in application and is generally considered to be in the experimental stage of development.
1) Ferromagnetic passive shielding
The traditional method of passively shielding a magnetic field involves special "ferromagnetic" metals (they contain iron and are magnetically permeable). Typical materials include low-carbon steel and Mu metal (an alloy of nickel, steel, and molybdenum). These materials have high magnetic permeability, i.e., they have an extremely high capacity to concentrate magnetic lines of flux (force) through the metal, providing a path of lower resistance to the magnetic flux than air, which is highly-resistant to a magnetic field.
2) Conductive passive shielding
A second passive shielding method involves using conductive materials such as aluminum or steel. When oscillating lines of magnetic flux pass through these conductive materials they cause electrical currents to flow in the metal. These "eddy" currents generate lines of flux which "oppose" (are 180 degrees out of phase with) the original lines of flux. These lines of flux tend to cancel the original field.
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