- How far do I have to be from a
power line to avoid a high magnetic field?
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.
- What is the difference between
electric and magnetic fields?
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
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
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.
- What types of power lines can present
an EMF problem?
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.
- The magnetic field readings in my home,
school, or office building are higher than I would like because of a power line
nearby. Can anything be done about this?
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.
- How did this whole issue of EMF
exposure concern get started, and how real is it?
This article (although dated) may help answer your questions:
History and Status of the Issue
- Are transformers a problem?
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
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.
- Homes and office buildings are
full of electrical wiring. Why are fields not high everywhere?
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.
- I haven't seen anything on this page about
EMF from cell phones and TV or FM broadcast towers. Why?
RF measurements and cell
tower surveys are discussed in depth on another page of this site.
- I heard on the news (read on the
Internet) that the latest research on EMF says__________ . Is this true?
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
- Would you let your kids
live in this house? or Would you work in this office?