06-10-2007, 09:23
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חבר מתאריך: 23.01.07
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במקור נכתב על ידי האזרח
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Electric & magnetic fields (EMF) from electric power: is there a health hazard?
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Introduction
What are electric and magnetic fields (EMF)?
How are we exposed to EMF?
Is there a health hazard?
Typical field strengths from EMF sources
Reference
Further web reading
Introduction Just over 100 years ago, human exposure to electric and magnetic fields was limited to those fields arising naturally. Within the last 50 years there has been a dramatic growth in the use of electric power and valid questions are now being raised concerning safe levels of exposure to man-made electric and magnetic fields.
What are electric and magnetic fields (EMF)? A small piece of iron held near a magnet will move towards and attach itself to the magnet. The iron is attracted to the magnet because of the magnetic field which surrounds it. The earth itself is a huge magnet, and it is the earth's magnetic field which, for example, controls the direction of the needle on a compass.
Electricity also produces fields:
- A field called the electric field occurs when an electric charge or voltage is present and results from the force electric charges exert on each other.
- A magnetic field is produced by the movement of the electric charge, that is, by the flow of electric current. The magnetic field effect is used in electromagnets and electric motors.
Plug an ordinary lamp into an electric outlet; the voltage on the lamp cord produces an electric field. Switch the lamp on and the current flow through the cord produces a magnetic field. The higher the voltage, the stronger the electric field. The higher the current, the stronger the magnetic field.
Unlike natural fields, which remain relatively constant, fields produced by electrical equipment usually change rapidly with time. Electricity in Australia is supplied as alternating current at a frequency of 50 cycles per second. This means that the electric and magnetic fields reverse their direction 50 times every second.
How are we exposed to EMF? Electric and magnetic fields (EMF) come from anything which produces, carries or is powered by electricity. This means there are fields associated with large and small powerlines, wiring in homes and places of work, and all electrical appliances. In the table overleaf several sources of EMF are listed along with typical strengths of the fields to allow comparisons.
Is there a health hazard? The main hazard to be avoided with electricity is fatal electric shock from direct contact with conductors.
In addition, if you stand beneath a high-voltage powerline you may experience nuisance spark discharges between your body and an insulated or poorly earthed conducting object such as a metal fence or shed.
The above effects are well-established and well-understood. However, in recent times concern has been expressed by some scientists and members of the community that EMF may be responsible for certain long-term health hazards, especially cancer.
Hundreds of scientific studies have been conducted on this topic throughout the world over the last 25 years. The studies have been of two types: research on human populations; and laboratory based research on animals, isolated cells and tissues.
In general, the results of the laboratory studies show no good evidence that exposure to 50 Hz EMFs cause or promote cancer, nor do human studies in general. However, in 2001, after reviewing all the relevant research, both the International Agency for Research on Cancer (IARC) and a UK expert group on non-ionising radiation concluded that exposure to prolonged and relatively high levels of magnetic fields may increase the risk of leukaemia in children. Specifically, children with average daily exposures of more than 0.3 to 0.4 uT were found to have a leukaemia risk of 2 in 20,000 per year, whereas children with lower exposures had a risk of 1 in 20,000. On the basis of the research findings, IARC classified magnetic fields as 'possibly carcinogenic', a classification shared with coffee, pickled vegetables, bitumen, engine exhaust and hundreds of other substances.
The World Health Organisation is advocating the use of precautionary measures to reduce the individual's exposure to magnetic fields during this time of scientific uncertainty. This is consistent with the way other 'possibly carcinogenic' substances are managed. Individuals may reduce their exposure by minimising the use of certain electrical appliances, or increasing the distance to sources of high magnetic fields.
The Radiation Protection division has two magnetic field meters available for hire by South Australian residents. Please contact the division if you wish to hire a meter.
Typical field strengths from EMF sources Source Electric field
strength* Magnetic field
strength+ 275,000 volt transmission line - directly underneath 3
0.5 to 2.5
- edge of easement 0.3
0.1 to 0.5
66,000 volt distribution line - directly underneath 0.4
0.5 to 2.0
- edge of easement 0.04
0.1 to 0.3
Various appliances - Electric blanket at surface 2
1.0 to 3.0
- Vacuum cleaner at 1 metre 0.002
0.1 to 2.0
- Hairdryer at 30 cm 0.04
0.1 to 7.0
- Iron at 30 cm 0.06
1.2 to 3.0
- Toaster at 30 cm 0.04
0.6 to 7.0
- Video display unit (VDU) at 50 cm in front 0.01
0.05 to 0.4
* measured in kilovolts per metre (kV/m) + measured in microTesla (uT)
Reference National Health and Medical Research Council (NHMRC): 'Interim Guidelines on Limits of Exposure to 50/60 Hz Electric and Magnetic Fields (1989)'.
Further web reading GreenFacts
www.greenfacts.org/power-lines/index.htm
IARC
http://monographs.iarc.fr/ENG/Monog...80/volume80.pdf
Report from the UK
http://www.hpa.org.uk/radiation/pub...d12-1.htm#concs
World Health Organization
www.who.int/docstore/peh-emf/publications/facts_press/efact/efs263.html
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נראה לי שזה הופך למשהו אישי במקצת.... אין לי דבר נגדך.
האתר הזה של EPA שהבאת מסביר מה היא קרינה אלקטרומגנטית. שנינו יודעים מה זה.
מה שצריך למצוא הוא מה שנקרא "מנה מזיקה" שזה שילוב של זמן חשיפה ורמת קרינה.
אבל, זה הפך דו-שיח ונראה לי שאני פורש.
מבחינתי שנינו חותרים לאותה מטרה - מזעור נזקים סביבתיים, ככה ש no harm done . . .
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