מיגון קרינה

EMF/ELF Radiation Shielding

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Exposure to electromagnetic fields, or EMF, has become an issue of concern for a great many people and is an active area of biophysical research. Discussion over the possible biological effects of electromagnetic fields first began to surface in the late 1960s following the introduction of new, higher voltage electric power transmission lines. An argument can be made that initial speculation regarding possible detrimental health effects of these lines arose among property owners who objected to their presence due to esthetic factors and the resulting loss of property values. In association with environmental action groups, who opposed construction of the lines on the basis of physical destruction and segmentation of habitat, an alliance was formed which worked to bring the issue into public awareness.
The first scientific study to attract serious interest in the issue came in 1979 following the work of epidemiologist Nancy Wertheimer, who was looking for possible causes for a number of childhood leukemia cases in the Denver metropolitan area. Her research, performed with physicist Ed Leeper, found that children with leukemia were more than twice as likely to have lived in homes near high current power lines, where the electromagnetic fields were stronger. Research on the issue has accelerated since that time, with mixed results, and will be discussed in greater detail later in this article.
Physical Science Concepts
The understanding of a few simple physical concepts is important to the discussion of any interaction between external physical agents and biological systems. Surrounding any wire or conductor that carries electricity, there exist both electric and magnetic fields, collectively referred to as electromagnetic fields, or EMF. These fields often extend for considerable distances around the wire. Although the early health effects studies looked primarily at the effects of large cross-country power transmission lines, and to some extent the public still associates EMF with these lines, it has become clear that anywhere electricity is in use, electric and magnetic fields will be present, often at significant intensities. This includes overhead and underground power distribution lines running throughout residential and commercial neighborhoods, certain types of interior structural wiring, as well as many common electrical devices. If detrimental bioeffects were to be confirmed, the ubiquitous nature of electricity in modern society could represent widespread public exposure to a potentially harmful physical agent.
The types of field that we are concerned about from a health effects standpoint are alternating current, or time-varying, fields whose strength and direction change regularly with time. They arise exclusively from man-made sources, specifically electric power and communications systems, and have been present in our environment for only about the past century. The earth’s strong, steady-state magnetic field is often cited as a point of comparison with these fields, but this comparison is not especially meaningful since the influence on matter can be quite different between time-varying fields and static (non-time-varying) fields. It should be noted that naturally occurring time-varying fields, associated with geological and meteorological phenomena, do exist but are not considered detrimental. For the purposes of this article we will look at only a small part of the electromagnetic spectrum, the extremely low frequency, or ELF portion. Electric power distribution in the United States is at a frequency of 60 Hz, and falls within this region. This is the part of the spectrum where most of the research has been concentrated, although substantial work has also been done in relation to radio frequency and microwave fields.
Electromagnetic waves at these low frequencies contain relatively small amounts of energy and are often referred to as non-ionizing radiation. An important distinction must be drawn between this and the ionizing radiation with which most of us are familiar. Ionizing radiation, represented by X-rays, gamma rays, cosmic rays, and alpha and beta particle emissions from radioactive materials, has dramatic and well documented detrimental effects on living things. These high frequency waves or particles have enough energy to eject electrons from molecules, and can damage the structure of cells (including DNA) directly, or through the creation of highly reactive free radicals within cells. Low frequency, non-ionizing radiation does not react with matter in this way. It also differs from radiation in the microwave portion of the spectrum in that it lacks the energy to damage cells by thermal effects. For these reasons, well characterized interaction models which examine the effect of physical or chemical agents have proven inadequate for studying the effects of low frequency electromagnetic fields, and researchers have been presented with a new challenge in identifying biophysical mechanisms of interaction.