The Facts about Low frequency EM fields

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18 Οκτ 2013 (πριν από 3 χρόνια και 11 μήνες)

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The Facts about Low frequency EM fields
1. Is there something to worry about?
In 1979 Wertheimer and Leeper reported an association between childhood leukaemia and certain features of the
wiring connecting their homes to the electrical distribution lines.
Abstract
An excess of electrical wiring configurations suggestive of high current-flow was noted in Colorado in
1976--1977 near the homes of children who developed cancer, as compared to the homes of control
children. The finding was strongest for children who had spent their entire lives at the same address, and
it appeared to be dose-related. It did not seem to be an artifact of neighborhood, street congestion, social
class, or family structure. The reason for the correlation is uncertain; possible effects of current in the
water pipes or of AC magnetic fields are suggested.
There have been many studies since, but no cause-and-effect link between ELF field exposure and cancer has
been confirmed.
2. Perry Review article
2.1 The question
Are cancers or other health problems linked to exposure to LF
EM fields?After nearly a hundred studies anomalous results
make no clear conclusion possible. Little as yet is known about
possible mechanisms. The question remains, is it necessary to
reduce exposure to LF EM fields or is the commotion just the
birth of another fad?
2.2 EM Frequency Spectrum
See figure right.
2.3 Health effects of EM radiation
ELF fields interact with living tissues only by inducing electric
fields and currents in them.
The magnitude of these induced currents from exposure to ELF
fields at levels normally found in our environment, is less than
the currents occurring naturally in the body.
There is no convincing evidence that exposure to ELF fields
causes direct damage to biological molecules, including DNA.
It is thus unlikely that they could initiate the process of
carcinogenesis.
3. Power lines
Electric and magnetic fields underneath overhead transmission
lines may be as high as 12 kV/m and 30 µT respectively.
Around generating stations and substations, electric fields up to 16 kV/m and magnetic fields up to 270 µT may
be found.
3.1 Electric Field Studies
Available evidence suggests that, apart from stimulation arising from electric charge induced on the surface of
the body, the effects of exposures of up to 20 kV/m are few and innocuous. Electric fields have not been shown
to have any effect on reproduction or development in animals at strengths over 100 kV/m.


3.2 Magnetic fields
Distance at which magnetic fields are reduced to background levels
Field

µT

Earth's magnetic field intensity, at its surface

50

Domestic electrical appliances

0.02 to 7


Source

Microtesla (µT)


At 30 cm

At 1.0 m

television



1.299

0.111

desktop light

3.281

0.121

transmission line

30.0

1.1

3.3 Magnetic Field Studies
There is little confirmed experimental evidence that ELF magnetic fields can affect human physiology and
behaviour at field strengths found in the home or environment.
Exposure of volunteers for several hours to ELF fields up to 5 millitesla
1
4. Ethical questions
had little effect on a number of clinical
and physiological tests, including blood changes, ECG, heart rate, blood pressure, and body temperature.
1. What is the ethical thing to do when designing in a situation where some doubt about safety exists?
2. If there are potential, but not well understood, hazards in building a product, what are the future
consequences of doing nothing?
3. Does the same thing apply to security issues in the design of software? e.g. protection from virus attack,
data encryption
4. When it is a matter of public safety, how much expert evidence is enough?
5. Are warnings to the consumer enough to get the designer off the hook?
6. Must a product be engineered to be totally safe at all costs, even if the user is at fault?
7. In designing equipment for consumers where does the balance between cost, efficiency and safety lie?
8. In light of research findings indicating that there is no hazard associated with low-frequency magnetic
fields, how much time effort and money should be expended when designing products that will emit this
type of radiation?
9. How does this relate to practice in IT?
References
Wertheimer N, Leeper E, ‘Electrical wiring configurations and childhood cancer’, Am J Epidemiol, 1979
Mar;109(3):273-84.
Perry, T.S., Spectrum, IEEE, Dec. 1994, Volume: 31, Issue: 12
Electromagnetic fields from Power lines, Wiring & Appliances, http://www.greenfacts.org/en/power-lines/l-
2/power-lines-1.htm


1
5 mT = 5,000 µT