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The questions addressed here are representative of those we often hear from our clients. They deal primarily with practical issues related to measurement surveys and field management. Absent are questions relating to the possible health effects of specific types of exposure. Those questions require a more substantive and context-sensitive answer than could be provided here.
This question is more difficult than it appears for a number of reasons. First, what constitutes a high field is open to interpretation. For the purpose of this discussion, we will consider it to be one that is higher than normally occurring background fields in a home or office that has no other high magnetic field sources. Second, the strength of the power line field depends upon the type of line, its specific design characteristics, and how heavily it is being used at any point in time. Given the number of variables, a definitive answer could not be provided that applies under all conditions. It is possible, however, to offer a range of distances that generally apply. In regard to transmission lines (large cross-country lines, often on metal or concrete towers), it is extremely unlikely that any substantial field elevation would be noted beyond 500 feet, and often not beyond 300 feet. At distances less than 100 feet, it is almost certain that the field will be increased to some extent. It is within this range of 100 feet to 500 feet that field measurements are necessary to determine the influence of the power line.
Here's the simple answer. Electric and magnetic fields are present wherever electricity is in use. At power line frequencies, they exist as separate forces with unique properties, but are often collectively (and inaccurately) referred to as electromagnetic fields.
An electric field is present wherever electricity is present, even though no current is flowing (no electricity is in use). Its strength is proportional to the voltage of the electrical supply, and is measured in volts per meter. An electric field is blocked or substantially reduced by intervening structures, including trees and most common building materials.
A magnetic field is present only when electric current is flowing (electricity is in use). Its strength is proportional to the amount of current flowing, and is measured in a number of different units, of which milligauss is the most common in the United States. A magnetic field is not substantially reduced by most materials.
The magnetic field is the component most closely identified with the potential for adverse bioeffects, and is also the component responsible for equipment interference problems.
Any type of line can be a problem if it is close enough and has enough current flowing in it. Although large transmission lines elicit the most concern, more people actually experience elevated fields from overhead distribution lines that run through neighborhoods and down city streets, or from underground residential distribution lines (URD lines). That's because these lines are often so close that the field, however limited, sometimes impinges upon living or working areas. Also, distribution lines are usually operating with some degree of net imbalance, which causes the magnetic field to drop off more slowly with distance.
Possibly. Requesting that the utility move or modify their lines is usually not within the realm of reality, and conventional shielding with metal plates is impractical for a number of reasons. Active magnetic shielding may be an effective solution.
This article (although dated) may help answer your questions: History and Status of the Issue
In regard to transformers on poles and transformers on the ground in residential neighborhoods, probably not. The magnetic field is very high close to these units, but drops off rapidly as one moves away. At 4 to 5 feet the field is usually down to background levels, and people are seldom that close for a significant period of time. On the other hand, the wires feeding into and out of the transformer produce fields that may not drop off as quickly. The real question is: How close are these wires (underground or overhead) and how high is their field?
In regard to transformers in vaults associated with large commercial buildings, or on the other side of a wall in a commercial building, yes they are a problem. Extremely high fields can be created in occupied areas adjacent to these units. The same applies to other components of the electrical system connected to the transformers.
Most of the wires that pass through walls, floors, and ceilings are actually cables containing two or more current carrying conductors. At any point in time, the current is flowing in one direction on one wire, and in the opposite direction on the other wire. Since these wires are very close together inside the cable jacket, the magnetic field around one wire is cancelled by the opposite magnetic field around the other wire. The field is negligible a few inches from the wire. EMF problems occur when this balance is destroyed by electrical wiring errors.
RF measurements and cell tower surveys are discussed in depth on another page of this site.
News coverage of an issue as controversial and complex as EMF health effects is more likely to reflect the views of whichever interest group has been the most successful at capturing the attention (or securing the support) of the media. Journalists are, for the most part, generalists who are required to cover a wide range of issues, and who often lack the background to understand the information in a press release. This makes them vulnerable to the influence of any "specialists" to whom they turn for assistance. If you are really interested in something, at least find and read the press release of the organization who performed the work or issued the report.
As for what you find on the Internet...., just be sure to read multiple sources. Beware of sites which prey upon your fears, or encourage adversarial actions. Misinformation is rampant. Also watch out for EMF mail lists and newsgroups populated by pseudo-experts who are all too eager to dispense questionable advice. Quacks
This is what I call the universal question. It may be phrased differently, but the underlying request is always the same: "Tell me if this is a safe place to live or work." In surveys of homes near power lines, or in the evaluation of an office environment, the client is not always sure how to respond to measured field levels that are somewhere between very low and very high.
With many chemical or physical agents (lead, radon, etc.) that are thought to represent an environmental health risk, recognized standards exist for limiting exposure. Unfortunately, this is not yet the case for power frequency electric and magnetic fields. Therefore, any attempt to definitively classify an environment as safe or unsafe would not be possible for at least two reasons:
What we can do is ensure that you have access to authoritative, unbiased information designed to inform the general public about electric and magnetic field health effects research. You will also be provided data on average field levels that have been shown to exist in comparable environments through nationwide studies, and through our own 21 years of performing measurement surveys. Following this, you should be in a position to make a responsible and informed decision that is consistent with your priorities, and not those of the EMF consultant, the electric utility, or any other organization.
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